Effective December 6, 2006, this report has been made publicly available in accordance with Section 734.3(b)(3) and published in accordance with Section 734.7 of the U.S. Export Administration Regulations. As a result of this publication, this report is subject to only copyright protection and does not require any license agreement from EPRI. This notice supersedes the export control restrictions and any proprietary licensed material notices embedded in the document prior to publication.
Outline of Guide for Application of Transmission Line Surge Arresters—42 to 765 kV Extended Outline 1012313
Outline of Guide for Application of Transmission Line Surge Arresters—42 to 765 kV Extended Outline 1012313 Technical Update, October 2006
EPRI Project Manager A. Phillips
ELECTRIC POWER RESEARCH INSTITUTE 3420 Hillview Avenue, Palo Alto, California 94304-1338 . PO Box 10412, Palo Alto, California 94303-0813 . USA 800.313.3774 . 650.855.2121 .
[email protected] . www.epri.com
DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM: (A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS DOCUMENT IS SUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR (B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDING ANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THIS DOCUMENT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT. ORGANIZATION(S) THAT PREPARED THIS DOCUMENT Kinectrics North America, Inc.
NOTE For further information about EPRI, call the EPRI Customer Assistance Center at 800.313.3774 or e-mail
[email protected]. Electric Power Research Institute and EPRI are registered service marks of the Electric Power Research Institute, Inc. Copyright © 2006 Electric Power Research Institute, Inc. All rights reserved.
CITATIONS This report was prepared by Kinectrics North America Inc. 800 Kipling Avenue Toronto, Ontario, Canada Principal Investigator W.A. Chisholm This report describes research sponsored by the Electric Power Research Institute (EPRI). This publication is a corporate document that should be cited in the literature in the following manner: Outline of Guide for Application of Transmission Line Surge Arresters—42 to 765 kV: Extended Outline. EPRI, Palo Alto, CA: 2006. 1012313.
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PRODUCT DESCRIPTION
Lightning flashovers are the most frequent cause of transmission line outages. Transmission line surge arresters (TLSA) limit lightning overvoltages between phase conductors and towers, and thus eliminate most outages on protected structures. This guide provides a tutorial on the relevant lightning phenomena, with an in-depth look at the operation, application, and placement of TLSA to maximize flashover protection and minimize capital investment. The guide also describes ways to improve tower grounding for better performance of overhead groundwires. Results and Findings The guide contains an in-depth description of the following areas: •
The parameters that influence transmission line lightning performance / parameters. Lightning incidence scales performance in all regions. With overhead groundwires, lightning currents act against local soil resistivity to create insulator stress. Adding arresters reduces the influence of grounding. When OHGW are removed, leaving only arresters, the lightning charge replaces peak current as a dominant stress. The section also discusses other transmission line features that affect line lightning performance, including line, tower, insulator and arrester air gap geometries; tower impedance; and nonlinear corona effects.
•
TLSA selection/specification. Before selecting a TLSA, utility engineers should consider a number of design questions concerning arrester operating characteristics and rating, temporary overvoltages, arrester protective levels and insulation coordination, TLSA energy capability, arrester failures, TLSA housings, and TLSA installation and handling. Selecting an arrester system (possibly including a series gap or insulator) for a particular transmission line is the process of simultaneously satisfying these concerns with a single arrester type.
•
Placement of arresters for improved lightning performance. The efficient application of TLSA to improve line performance requires the investigation of all available mitigation options and weighing of the performance benefits against real cost. Estimating the effects of changes in tower structure and design, shielding, grounding, and arresters on the lightning performance of transmission lines is crucial to this process. This section discusses backflashover protection, unshielded applications, and transmission lines over varying terrain.
Challenges and Objectives One difficulty in focusing this report is the wide range of technical backgrounds of the readers. Electrical engineers will be most interested in insulation coordination and risk management. Civil engineers will be more interested in what will be gained and lost if a new line is designed without overhead groundwires (OHGW) and with compact insulation, protected by TLSA. While v
these readers will find what interests them, the main focus of this report is a utility project manager facing a decision to replace existing overhead groundwires (OHGW), the fastestdecaying transmission line component, with a typical life of 25 to 55 years. What has changed in ten years is that the decision to put up TLSA in place of OHGW is commercially and technically viable in many areas. Having this new alternative, with its reduced visual impact and peak-load loss reduction, can help the utility bottom line, especially considering that the OHGW conductors represent 4% of the total line investment. Applications, Values, and Use New lines with reduced visual impact are already taking advantage of TLSA to replace overhead groundwires. So far, these applications have been made in areas of difficult grounding and low lightning incidence. However, the alternative of buried transmission cable looms like the sword of Damocles, motivating overhead line engineers to deliver more reliability with fewer resources. EPRI Perspective Lightning causes power outages that cost utilities more than $1 billion per year directly, in damaged or destroyed equipment. The indirect damage to customers from all power quality problems is estimated to exceed $100 billion per year, with more than half of these disturbances having lightning as a root cause. This guide presents TLSA theory and design information to enable utilities to minimize the number outages due to lightning. EPRI developed this guide with the understanding that users may not be familiar with either TLSA or the current standards that do, or should, apply to them. The guide is tutorial in nature and does not anticipate every situation or utility need. In general, however, experience has shown that properly designed, installed, inspected, and maintained hardware such as TLSA, counterpoise, and overhead groundwires can significantly improve system reliability and power quality. Approach In 1997, EPRI delivered a TLSA application guide (TR-108913), consolidating literature with results of a survey of 31 EPRI-member utilities. EPRI also supported arrester energy and mechanical tests at the EPRI Power Delivery Center-Lenox. The state of the art of TLSA has advanced considerably since the last EPRI guide was published. Line arresters have proved themselves as technically and economically feasible for improving performance of conventional lines with overhead groundwires. TLSA have also been used on 230-kV and 400-kV lines without overhead groundwires, where the extra surge duties raise new electrical reliability concerns. Most of the difficulties found in applying TLSA relate to the spotty reliability of mechanical components. This can be addressed by ensuring that TLSA components meet the same high reliability standards that apply to other line components. This document is an extended outline that will be built upon and refined over the new few years to develop a completed guide Keywords Reliability Lightning and weather impacts Power quality Transmission lines vi
ABSTRACT In most areas, flashovers from lightning are by far the most frequent cause of transmission line outages. Transmission line surge arresters (TLSA) limit the lightning over-voltages between phase conductors and the tower structure. This prevents flashovers and insulation damage on that structure. This guide provides a tutorial of the relevant lightning phenomena, in general, and offers an in-depth look at the operation, application, and placement of TLSA to maximize flashover protection and minimize capital investment. The guide also considers other mitigation measures, including improved tower grounding and the application of overhead groundwires. EPRI developed this guide with the understanding that users may not be familiar with either TLSA or the current standards that do or should apply to them. The guide, therefore, is tutorial and does not anticipate every situation or utility need. Overall, however, experience has shown that properly designed, installed, inspected, and maintained hardware such as TLSA, counterpoise, and overhead groundwires can significantly improve system reliability and power quality. This document is an extended outline that will be built upon and refined over the new few years to develop a completed guide
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CONTENTS
1 PURPOSE ..............................................................................................................................1-1 2 DEFINITIONS .........................................................................................................................2-1 3 WHY PROTECT TRANSMISSION LINES FROM LIGHTNING .............................................3-1 Economic Impact of Power Quality Problems .......................................................................3-1 Classification of Power Quality Problems..............................................................................3-1 Lightning as a Root Cause of Short-Duration Faults.............................................................3-3 Typical Power Line Lightning Mitigation Options...................................................................3-4 Utility Investment in Lightning Protection using Overhead Groundwires...............................3-4 Utility Investment in Other Lightning Protection Methods......................................................3-5 4 TRANSMISSION LINE LIGHTNING PERFORMANCE PARAMETERS ...............................4-1 Introduction ...........................................................................................................................4-1 Lightning Incidence Parameters............................................................................................4-2 Ground Flash Density (GFD)............................................................................................4-2 Lightning Incidence to Lines .............................................................................................4-4 Lightning Current Parameters ...............................................................................................4-8 Stroke Current Peak Magnitudes .....................................................................................4-9 Stroke Current Rate of Rise ...........................................................................................4-11 Stroke Current Waveshapes ..........................................................................................4-12 Total Charge Delivered...................................................................................................4-13 Number of Strokes in a Flash .........................................................................................4-14 Transmission Line Parameters............................................................................................4-14 Line Conductor Geometries............................................................................................4-15 Tower Geometries ..........................................................................................................4-15 Insulator / Air Gap Geometries .......................................................................................4-16 Volt-Time Curve Penetration Method.........................................................................4-18
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Disruptive Index Method ............................................................................................4-18 Leader Progression Method.......................................................................................4-19 Tower Ground Characteristics........................................................................................4-19 Buried Tower Grillage ................................................................................................4-20 Driven Ground Rods ..................................................................................................4-20 Counterpoise .............................................................................................................4-20 Transmission Line Surge Arresters (TLSA)....................................................................4-21 Nonlinear Corona Effects ...............................................................................................4-21 5 TLSA SELECTION AND SPECIFICATION ...........................................................................5-1 Introduction ...........................................................................................................................5-1 Transmission Line Arresters..................................................................................................5-1 Arrester Operating Characteristics ........................................................................................5-5 Arrester Rating and MCOV ...................................................................................................5-7 Temporary Overvoltages.......................................................................................................5-7 Arrester Protective Levels and Insulation Coordination ........................................................5-8 Lightning Insulation Coordination .....................................................................................5-9 Switching Surge Insulation Coordination........................................................................5-11 Energy Capability of TLSA ..................................................................................................5-12 Lightning Energy.............................................................................................................5-12 Switching Energy............................................................................................................5-13 Arrester Failures..................................................................................................................5-15 Electrical Failure Modes .................................................................................................5-15 Arrester Disconnects ......................................................................................................5-16 TLSA Housings ...................................................................................................................5-18 TLSA Installation and Handling ...........................................................................................5-19 Handling and Installation Recommendations ......................................................................5-20 Handling .........................................................................................................................5-20 Installation and Maintenance..........................................................................................5-20 Arrester Markings ...........................................................................................................5-20 6 PLACEMENT OF ARRESTERS FOR IMPROVED LIGHTNING PERFORMANCE..............6-1 Economics.............................................................................................................................6-1 Backflashover Protection.......................................................................................................6-1 Phase Location of TLSA...................................................................................................6-1
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Coupling to Overhead Groundwires.............................................................................6-1 Crossarm Voltage ........................................................................................................6-3 TLSA Location for High Tower Footing Resistance..........................................................6-4 Unshielded Applications ........................................................................................................6-5 Arrester Energy ................................................................................................................6-6 Vertical Circuits.................................................................................................................6-6 Horizontal Circuits ............................................................................................................6-8 Transmission Lines over Unchanging Terrain.......................................................................6-9 Compact Transmission Lines ..............................................................................................6-11 Compact Transmission Lines with Overhead Groundwires and TLSA...........................6-11 Compact Unshielded Transmission Lines with TLSA .....................................................6-13 7 APPLICATION SOFTWARE ..................................................................................................7-1 Introduction ...........................................................................................................................7-1 Lightning Performance Design Workstation (LPDW) ............................................................7-1 TFLASH Overview.................................................................................................................7-1 Building a TFLASH Model .....................................................................................................7-2 TFLASH Capabilities ........................................................................................................7-2 General Procedure for Constructing a Line Model ...........................................................7-2 Analyzing a TFLASH Model ..................................................................................................7-5 The Classical Solution - The Average Performance of the Line.......................................7-5 Oscillographs - Line Behavior for a User-Specified Stroke ..............................................7-6 General Procedure / Sample Application .........................................................................7-7 8 CASE STUDIES .....................................................................................................................8-1 44 kV Case Studies...............................................................................................................8-1 Comparison of OHGW versus TLSA using Customer Momentary Disturbance Benchmark .......................................................................................................................8-1 Application Experience with 44-kV Arrester Application...................................................8-3 46 kV Case Studies...............................................................................................................8-4 66 kV Case Study .................................................................................................................8-4 69 kV Case Study .................................................................................................................8-4 115 kV Vertical Line Case Study...........................................................................................8-5 115 kV Horizontal H-Frame Line Case Study .......................................................................8-7 115 kV Horizontal Steel Lattice Line Case Study..................................................................8-8
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115-kV Horizontal Line: Predicted Outage Rate...............................................................8-8 115 kV Horizontal Line: Outage Rates with Partial TLSA Treatments .............................8-8 115 kV Horizontal Steel Lattice Line: Lessons Learned ...................................................8-9 138 kV Case Studies...........................................................................................................8-10 154 kV Case Study .............................................................................................................8-10 161 kV Case Studies...........................................................................................................8-10 230 kV Case Studies...........................................................................................................8-10 230 kV Horizontal Line: Application Experience.............................................................8-10 230 kV Circuit with 35-kV Underbuild .............................................................................8-11 275 kV Case Studies...........................................................................................................8-11 400 kV Case Study .............................................................................................................8-11 500 kV Case Studies...........................................................................................................8-12 765 kV Case Study .............................................................................................................8-13 9 REFERENCES .......................................................................................................................9-1 A TLSA MECHANICAL PERFORMANCE TESTS .................................................................. A-1 Line Disconnector Testing.................................................................................................... A-1 Arrester Body Testing........................................................................................................... A-2 Evaluation of Different Arrester Installation Configurations.................................................. A-2 B TLSA ENERGY WITHSTAND TEST DATA ......................................................................... B-1 Definition of Withstand Criteria............................................................................................. B-1 Tests of 63-mm TLSA to Destruction ................................................................................... B-1 Tests of 8.4-kV MCOV Samples to Thermal Runaway ........................................................ B-1 C TRANSMISSION LINE LIGHTNING PERFORMANCE CASE STUDIES ............................ C-1 D MECHANICAL FORCE ANALYSIS FOR GAPLESS TLSA INSTALLATIONS................... D-1
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LIST OF FIGURES Figure 3-1 Power Quality Acceptability Curves. Left: Computer Business Equipment Manufacturers Association (CBEMA) 1996; Right: Information Technology Industry Council (2000)....................................................................................................................3-2 Figure 3-2 Origins of Power Quality Disturbances < Substitute EPRI figure here > [Plata 2002] ..................................................................................................................................3-2 Figure 3-3 Power System Areas where Transmission Faults will Cause 50% and 70% Sags .........................................................................................3-3 Figure 4-1 Lightning Ground Flash Density for Continental USA, 1989-1998 [Orville Huffines] .............................................................................................................................4-3 Figure 4-2 Lightning Ground Flash Density from NALDN, 2000 to 2003 (Vaisala, need permission).........................................................................................................................4-3 Figure 4-3 Optical Transient Density Map from (NASA 2006) and Estimate of Ground Flash Density .....................................................................................................................4-4 Figure 4-4: Relation between Lateral Attractive Distance Da of Horizontal Conductor and Average Conductor Height h . Curve 1: Eriksson; Curve 2: D=2h; Curve 3: Rizk.............4-5 Figure 4-5 Striking Distances from Ground and Conductor to a Downward Leader ..................4-6 Figure 4-6: Modeling of Lightning Shielding Failures using L2 Applet [Red 2005] for Peak Stroke Currents of 5, 15 and 25 kA....................................................................................4-7 Figure 4-7 Relation between Lightning Leader Potential and Stroke Charge [Mazur 2001] ......4-8 Figure 4-8 Lightning to Instrumented Rods on Tokyo Electric Transmission Towers [Takami 2005] ..................................................................................................................4-10 Figure 4-9 Relation between Maximum Rate of Rise and Peak Amplitude of Lightning to Tokyo Electric Transmission Towers [Takami 2006]........................................................4-11 Figure 4-10 Relation between “Virtual Front Time” and First Peak Amplitude for Lightning to Tokyo Electric Towers [Takami 2006] ..........................................................................4-12 Figure 4-11 Percentage of Positive Cloud-to-Ground Lightning Flashes (Left) and Density of Large-Amplitude Positive Flashes (Right) in USA [Boccippio et al .................4-14 Figure 4-12 Flashover paths for a V-String Configuration .......................................................4-17 Figure 5-1 Internal Construction of Silicon Carbide Lightning Surge Arrester ...........................5-3 Figure 5-2 General Electric 138-kV Gapped MOV TLSA in Virginia [Koch 1985, Zed 2004] ..................................................................................................................................5-4 Figure 5-3 Classification of Arrester Design Features [Richter et al 2004] ................................5-5 Figure 5-4 TLSA Volt-Amp Curve < substitute a modern version > ...........................................5-6 Figure 5-5 Typical Lightning Current Distribution on an Unshielded Transmission Line with a Top-Phase Arrester having R=20 at 40 kA. .......................................................5-11
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Figure 5-6 Effect of Small Variation in Reference Voltage on Discharge Current....................5-14 Figure 5-7 Comparison of 63-mm MOV Block Charge Absorption at Destruction with Firing Level for DLSA disconnect [Lat CEA Guide, CEATI 3312A, permission or substitute needed]............................................................................................................5-17 Figure 6-1 Plot of Voltage versus Time at Various Points on a Double-Circuit Tower using L-5 Applet and Step Waveshape (final to use CIGRE concave) ..............................6-3 Figure 6-2 Plot of Voltage versus Time at Various Points on Double-Circuit Tower, Taking Into Account Relative Coupling from Shield Wires (final to use CIGRE concave).............................................................................................................................6-3 Figure 6-3 Schematic of Traveling Waves Propagating Towards a Structure with Low Footing Resistance: If Tower 3 has no arrester, it may flashover. .....................................6-5 Figure 6-4 A Schematic showing the Shielding Angle on an Unshielded Transmission Line with the Top Phase protected by TLSA ......................................................................6-7 Figure 6-5 Options for Improving Compact Line Lightning Performance .................................6-13 Figure 6-6 Typical 115-kV Compact Line Geometry from 1980, using Polymer Post and Semiconductive Glaze Bell Insulators [Ontario Hydro 1980]............................................6-15 Figure 7-1 Tower Modeling Screen from EPRI TFLASH (dummy) ............................................7-3 Figure 7-2 Conductor Information Screen from TFLASH (dummy)............................................7-4 Figure 8-1 Probability of Flashover on 44-kV Line in Delta Configuration with Overhead Groundwire.........................................................................................................................8-2 Figure 8-2 69-kV Line Configurations considered by TU Electric for Improved Lightning Performance.......................................................................................................................8-4 Figure 8-3 Application of TLSA on TU Electric 69-kV Lines [Sanders and Newman 1992] .......8-5 Figure 8-4 Voltage across 115-kV or 138-kV class TLSA compared to Insulator Flashover Levels ................................................................................................................8-5 Figure 8-5 Strategy for Mounting TLSA with Suitable Mechanical Rating to Restrain Conductor...........................................................................................................................8-6 Figure 8-6 Mounting of TLSA with Insufficient Horizontal Clearance.........................................8-7 Figure 8-7 Single Circuit 115-kV Structure with Single OHGW Lightning Protection [Tarasiewicz 2000] .............................................................................................................8-8 Figure 8-8 Typical 400-kV Line Geometries at Statnett (Norway)............................................8-12 Figure 8-9 Compact 400-kV Unshielded Design with Top-Phase TLSA..................................8-12 Figure 8-10 Traveling Waves near Open Terminal..................................................................8-13 Figure 8-11 TLSA on AEP 765-kV Line for Switching Surge Control ......................................8-14 Figure A-1 Shear and Tension Tests on TLSA Disconnects .................................................... A-1 Figure A-2 Arrester Body Bending Test Setup.......................................................................... A-2 Figure D-1 Example of Typical Conductor to Pole Suspension ................................................ D-1 Figure D-2 Example of Typical Conductorto Tower Mast Suspension ..................................... D-2
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LIST OF TABLES Table 3-1 Typical Power Line Lightning Mitigation Options .......................................................3-4 Table 4-1 Geometric and Contact Resistance for Typical Surface Electrodes .......................4-19 Table 5-1 External Gap or MCOV Requirements for TLSA .....................................................5-21 Table 6-1 Footing Resistance at Steel Lattice Towers along Hypothetical 138-kV Transmission Line ..............................................................................................................6-4 Table 6-2 Flash Incidence for 161 km (100 miles) of a Horizontal Circuit, 18 m (60 feet) 2 above Flat Terrain with Ground Flash Density of 3.9 per km per year..............................6-8 Table 6-3 Flashover Data for 161 km (100 miles) of Unshielded Transmission Line for Various TLSA Installations ...............................................................................................6-10 Table 6-4 Flashover Data for 161 km (100 miles) of Vertical Circuit, Steel Pole, Shielded Transmission Line for Various TLSA Installations............................................................6-11 Table 6-5 Effects of Compact Line Insulation and Phase Spacing on Lightning Performance (Outages per 100 km per year)...................................................................6-12 Table 6-6 Options for Improving Compact Line Lightning Performance (Outages per 100 km per year) .....................................................................................................................6-13 Table 8-1 Comparison of Costs for 44-kV Lightning Protection Options ...................................8-3 Table 8-2 Reduction in Total Lightning Outages for Nine Treatment Options ...........................8-9 Table A-1 Observed TLSA Failure Loads ................................................................................. A-2
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1 PURPOSE
The purpose of this application guide is to help utilities use transmission line surge arresters (TLSA) for the reduction or prevention of outages caused by lightning on transmission lines that operate at system voltages up to and including 765 kV. Other mitigation measures, such as improved tower grounding and the application of overhead groundwires, are considered in this guide. The guide was developed with the understanding that users may not be familiar with either TLSA or the current standards that do or should apply to them. While it has the penalty of making this a long document, considerable tutorial material is included. Even so, the guide cannot anticipate every situation or utility need. The user must consider the particular requirements of a given application and select those criteria that fit that application. In certain situations the user may want to develop additional criteria to address a particular use. (Description of target audience – I want to help an electrical engineer to ask the right questions and make the right decisions when she receives a report that the overhead groundwires on a line have reached their end-of-life. This help includes the technical vocabulary and basic understanding of mechanical issues such as vibration, wind load and conductor restraint force needed to work efficiently with mechanical engineers doing tower head layouts.)
1-1
2 DEFINITIONS
Arc
A continuous luminous discharge of electricity across an insulating medium, usually accompanied by the partial volatilization of the electrodes
Backflashover
A flashover of insulation resulting from a lightning stroke to part of a network or electric installation that is normally at ground potential
Basic Lightning Impulse Insulation Level (BIL)
For self-restoring insulation such as air. The crest value of a standard 1.2/50 µs lightning impulse for which the insulation exhibits a 10% chance of flashover under standard conditions.
Basic Switching Impulse Insulation Level (BSL)
For self-restoring insulation such as air. The crest value of a standard 250/2500 µs switching impulse for which the insulation exhibits a 10% chance of flashover under standard conditions.
Chopped-Wave Lightning Impulse
A prospective full lightning impulse during which any type of discharge causes a rapid collapse of the voltage.
Chopped-Wave Lightning Impulse Withstand
The insulation strength necessary to withstand a surge exceeding the BIL, but chopped at 2 or 3 µs. Transformers and circuit breakers must have a chopped-wave withstand of at least 1.15 times the BIL.
CIGRÉ
Conference Internationale des Grands Reseaux Electriques a haute tension (International Conference on Large High Voltage Electric Systems). CIGRE is an international technical organization, similar to the IEEE Power Engineering Society, which focuses primarily on transmission voltage systems. CIGRE holds a general conference in Paris every two years. The various study committees meet more frequently at other locations. CIGRE'S official publication is "Electra."
Continuing Current
A small-amplitude (100 A) long-duration (tens to hundreds of milliseconds) current that flows between strokes of a lightning flash, with moderate return-stroke channel luminosity and significant transfer of charge from cloud to ground.
2-1
Definitions
Core
The internal insulating part of an arrester that carries the mechanical loads and restrains the metal oxide valve blocks.
Corona
A luminous discharge due to ionization of the air surrounding a conductor caused by a voltage gradient exceeding a certain critical value
Critical Current
The lightning current at a specific waveshape that will produce a 50% probability of flashover when applied to a particular conductor and location
Critical Flashover Voltage (CFO)
The amplitude of the voltage of a given waveshape that, under specified conditions, causes flashover through the surrounding medium on 50% of the voltage applications
Disconnector
(Arrester) A device that disconnects a failed surge arrester to prevent a permanent fault on the circuit. It also provides a visual indication of a failed arrester
Electrogeometric Model
A model for the “reach” of the final jump of a downward lightning leader to lines, objects or ground based on electrostatic estimates of relations among leader potential, charge and current. Striking distances in this model become functions of the peak magnitude of the first stroke current.
Electromagnetic Transients Program (EMTP)
A large "industry-standard" computer program that simulates transient overvoltages on power systems using pi-section circuit approximations to distributed lines.
Electrostatics
The branch of science treating electrical phenomena associated with electric charges at rest (with no time variation) in the frame of reference
Erosion
The loss of material by leakage current or corona discharge. Erosion is nonconductive and can be uniform, localized or treeshaped. Shallow surface traces can occur on insulator surfaces after arcing.
A base polymer previously used in rubber housings for insulators Ethylene Propylene Diene Monomer (EPDM) and arrester housings, now alloyed with silicone for hydrophobicity External insulation
2-2
The external insulating surfaces and the surrounding air. The dielectric strength of external insulation depends on atmospheric conditions.
Definitions
Flashover
A disruptive discharge through air around or over the surface of solid or liquid insulation, between parts of different potential or polarity, produced by the application of voltage wherein the breakdown path becomes sufficiently ionized to maintain an electric arc.
Magnetic Flux
A condition in a medium produced by magnetomotive force such that, when altered in magnitude, a voltage is induced in an electric circuit linked with the flux.
Front Time (Rise Time)
The time-to-peak of an impulse that is estimated by drawing a straight line through two points on the front of the wave. One point is located at 90% of the crest value; the other point is either 30% or 10% of the crest value. The front time is the first number in the description of a wave shape, i.e., 8 in a wave shape described as 8/20 µs.
Ground
A conducting connection, whether intentional or accidental, by which an electric circuit or equipment is connected to the earth or to some conducting body of relatively large extent that serves in place of the earth.
Ground Flash Density (GFD)
The average number of lightning flashes to ground per square kilometer per year.
Housing
The external covering of a TLSA that protects the core from the weather and may be equipped with weather sheds. In some designs the housing may also include insulating materials between the weather sheds ad the core.
Hydrolysis
A chemical process involving the reaction of a material with water (in liquid or vapor form) that can lead to electrical and mechanical degradation.
Induction Field (Magnetostatic Field)
The electric and magnetic fields created by a constant current.
Insulator
A device intended to give flexible or rigid support to electrical conductors or equipment and to insulate these conductors or equipment from ground or from other conductors or equipment. An insulator comprises one or more insulating parts to which connecting devices (metal fittings) are often permanently attached.
Internal Insulation
Insulation inside a sealed container, often holding gas or oil media. Internal insulation is usually not self-restoring.
2-3
Definitions
Insulator Arcing Horn / Insulator Arcing Ring
A metal part, usually shaped like a (Horn / Ring), placed at one or both ends of an insulator or a string of insulators to establish an arcover path, thereby reducing or eliminating damage by arc-over to the insulator or conductor or both.
Keraunic Level (TD)
The number of days in a year during which thunder was heard, expressed in days per year (TD). Used in the past to estimate lightning ground flash density but no longer recommended for this purpose, as the relation between KL and GFD varies with location.
Lightning flash
The complete lightning discharge, most often composed of leaders from a cloud followed by one or more return strokes.
Lightning impulse protective level
The maximum lightning impulse voltage expected at the terminals of a surge protective device under specified conditions of operation. The lightning impulse protective levels are given by: a) Front-ofwave impulse sparkover or discharge voltage, and b) the higher of either a 1.2/50 impulse sparkover voltage or the discharge voltage for a specified current magnitude and waveshape.
Lightning first stroke
A lightning discharge to ground initiated when the tip of a downward stepped leader meets an upward leader from the earth.
Maximum Continuous Operating Voltage (MCOV)
The maximum line-to-ground power frequency voltage (RMS) that is specified by the manufacturer.
Metal Oxide Varistor (MOV)
Zinc Oxide, sintered with a number of other metal elements to give a highly non-linear semiconducting material used in modern surge arresters. In contrast to the older silicon carbide arresters, MOV arresters do not require spark gaps but can benefit from them.
North American Lightning Detection Network (NALDN)
A system of broadband receivers (20-400 kHz and Global Positioning System (GPS) time) that provides ground flash density, peak radiated field and rise time data for North America.
Overhead Groundwire
Grounded wire or wires placed above phase conductors for the purpose of intercepting direct strokes in order to project the phase conductors from direct strokes. They may be grounded directly or indirectly through short gaps.
Reflection Coefficient, ρ
The portion of an incident wave that, on reaching a change in surge impedance, travels back down the line towards the source. The voltage reflection coefficient is ρv = (Zb-Za)/(Zb+Za) where Za is the initial impedance and Zb is the new impedance encountered.
2-4
Definitions
Refraction Coefficient
The portion of an incident wave that, upon reaching a change in surge impedance, travels on in the new impedance. The refraction coefficient for voltage is (2Zb)/(Zb+Za) where Za is the initial impedance and Zb is the new impedance encountered.
Return Stroke
The luminescent, high-current discharge that is initiated after the stepped leader and pilot streamer have established a highly-ionized path between charge centers. Lightning current flow removes the charge deposited by the stepped leader along the stroke channel.
Self-Restoring Insulation
Insulation, such as porcelain or non-ceramic insulators, that is not damaged by flashovers and soon regains all or most of its insulation strength after a flashover event.
Spark Gap
An air gap between one conducting electrode that is connected to the transmission line and another conducting electrode that is connected to ground, or to the high-voltage terminal of a line surge arrester. Spark gaps were the first lightning protection devices used on transmission lines and active spark gaps, with series nonlinear elements, are still an important surge proactive device technology.
Standard Impulse Voltage Wave
A voltage waveshape with a front time of 1.2µs and a time to half value of 50 µs that is used to test insulation in the laboratory. Testing standards such as IEEE Standard 4 describe the allowable tolerances on this waveshape.
Stepped Leader
Static discharge that propagates from a cloud into the air. Current magnitudes that are associated with stepped leaders are small (on the order of 100 A) in comparison with the return stroke current. The stepped leaders progress in a random direction in discrete time steps of 10 to 80 m in length. Their most frequent velocity of propagation is 0.05% of the speed of light. It is not until the stepped leader is within the striking distance of the point to be struck that the stepped leader is positively directed toward this point.
Stroke
A highly luminous discharge component of the lightning flash. Strokes typically last less than 100 µs. Each component stroke of a flash is separated by several tens of milliseconds. In many cases, a small continuing current flows between strokes.
Surge Arrester
A nonlinear device used to limit transient electrical overvoltages
2-5
Definitions
Surge Impedance
The intrinsic ratio of voltage to current in a conductor. Neglecting losses, Z = √(L/c)w, here L and C are the inductance and capacitance per unit length for a transmission line or cable. For lumped circuits, L and C are the total equivalent inductance and capacitance.
Temporary Fault
A flashover of line insulation that will clear itself after the circuit is momentarily de-energized.
Temporary Overvoltage (TOV)
An oscillatory overvoltage, associated with switching or faults (for example load rejection, single-phase faults) and/or nonlinearities (ferroresonance effects, harmonics) of relatively long duration, which is undamped or slightly damped.
Tower or Pylon
A structure that supports overhead transmission line conductors.
Transmission Line Surge Arresters designed specifically for application on transmission lines to prevent line insulation flashovers. They may be gapped or Arrester (TLSA) gapless. Upward Leader
A stepped leader that travels up toward the cloud from a tall object, such as a transmission tower, skyscraper, or mountain-top. Most stepped leaders are downward traveling. Downward leaders tend to have higher potential and charge, both of which lead to higher first return stroke peak current magnitudes.
Volt-Time Curve
The curve relating the disruptive discharge voltage of a test object to the time of chopping, which may occur on the front, at the crest or on the tail. The curve is obtained by applying impulse voltages of constant shape, but with different peak values.
Wave Velocity
The speed at which voltage and current disturbances travel through a circuit. Neglecting losses, v = √(1/LC), where L and C are the inductance and capacitance per unit length for a transmission line or cable.
Weathershed
The part of a housing that increases the distance measured along insulating surfaces (leakage distance) between the conductive parts of an insulator. Weathersheds also provide protected bottom surfaces that tend to stay dry in wet weather, further improving electrical flashover performance.
2-6
3 WHY PROTECT TRANSMISSION LINES FROM LIGHTNING
Economic Impact of Power Quality Problems Citation from 2001 EPRI report, noting “power disturbance problems cost the US economy between $119B and $188B per year”. [IEEE Spectrum Jan 2006]
Classification of Power Quality Problems Standards have been developed by several interested parties to negotiate what constitutes acceptable power, through the use of time-duration curves of disturbances. Important standards have been recommended by: •
Computer Business Equipment Manufacturing Association (CBEMA)
•
Information Technology Information Council (ITIC)
•
Semiconductor Equipment and Materials International (SEMI-47)
•
American National Standards Institute (ANSI) C84.1
Customer computer equipment used to meet the CBEMA standard for power quality, shown in Figure 3-1. Ride-through for short-duration sags (70% voltage dip for 100 ms) in the 1970s was provided by rotating machines or other locally stored energy for mainframe computers. Linear power supplies with large electrolytic capacitors provided this function in electronic equipment of the period. Revisions to the CBEMA curve were made by its replacement, the Information Technology Industry Council (ITIC), starting in 1996 and adopted in 2000. Figure 3-1 shows that the general nature is the same, but there are differences in detail. The ITIC: •
Raised the tolerance level for short-duration overvoltages, because these are easy to eliminate with surge protective devices inside the equipment
•
Reduced the tolerance level for short-duration voltage sags, because providing additional energy storage in typical switching power supplies adds cost, consumes more power
3-1
Why Protect Transmission Lines from Lightning 250
250
200
200
OVERVOLTAGE CONDITIONS
OVERVOLTAGE CONDITIONS
0
RATED
ACCEPTABLE POWER
VOLTAGE
-50
100
50
+-- 10% 0
RATED
ACCEPTABLE POWER
VOLTAGE 8.33 ms
PERCENT CHANGE IN BUS VOLTAGE
0.5 CYCLE
50
8.33 ms
PERCENT CHANGE IN BUS VOLTAGE
100
0.5 CYCLE
150
150
-50
UNDERVOLTAGE CONDITIONS
UNDERVOLTAGE CONDITIONS
-100
-100 0.0001
0.001
0.01
0.1
1
10
100
1000
0.0001
0.001
TIME IN SECONDS
0.01
0.1
1
10
TIME IN SECONDS
Figure 3-1 Power Quality Acceptability Curves. Left: Computer Business Equipment Manufacturers Association (CBEMA) 1996; Right: Information Technology Industry Council (2000)
Figure 3-2 classifies typical voltage dip – duration curves for six different power quality disturbance root causes. Nearby Distribution Faults are classed as under-voltage conditions in both the CBEMA and ITIC curves. Fuse Operations sit right on the CBEMA curve of acceptable power, but are classed as unacceptable in the ITIC graph. Transmission faults lead to short-duration voltage dips of three to ten ac cycles (50 to 200 ms) that affect a large number of customers at once. For the Transmission Faults in Figure 3-2: •
15% are classified as under-voltage conditions in the CBEMA curve
•
35% are classified as under-voltage conditions in the ITIC curve
Figure 3-2 Origins of Power Quality Disturbances < Substitute EPRI figure here > [Plata 2002]
3-2
100
1000
Why Protect Transmission Lines from Lightning
Lightning as a Root Cause of Short-Duration Faults EPRI has made extensive simulations of power system disturbances, leading for example to the area map in Figure 3-3 where transmission faults will cause unacceptable 70% voltage dips.
Figure 3-3 Power System Areas where Transmission Faults will Cause 50% and 70% Sags
In the area of 70% vulnerability of Figure 3-3, there are about 190 km (120 miles) of transmission lines. In the USA, with its average level of lightning risk, about 120 flashes would strike these average transmission lines every year. If the transmission lines are completely exposed to lightning, like distribution lines, then every flash would cause a flashover. Protective relaying would operate each time, leading to 120 unacceptable voltage dips at the customer load every year. These would not be spread evenly over the year, however – about 80 of the 120 would occur in July and August. Normal practice would be to protect the transmission lines with overhead groundwires (OHGW). These will steer lightning away from the phase conductors and lead it safely to ground. The combination of OHGW, high insulation strength and good grounding from wide-base towers is highly effective, especially in the central US where the soil resistivity is low. With OHGW and good grounding, the lightning outage rate for a typical transmission line will be less than 1 tripout per 100 km per year. For the sensitive area in Figure 3-3, this works out to (190 km x 1 tripout per 100 km per year) or an average of 1.9 voltage dips every year. Put into other terms, 2 of the 120 flashes will cause faults on the protected line, an efficiency of 98.3%.
3-3
Why Protect Transmission Lines from Lightning
Typical Power Line Lightning Mitigation Options The consequences of a lightning fault depend to a large extent on the operating voltage of the line. Figure 3-3 has shown that there will be a region around the fault where customers may record an unacceptable voltage dip. The number of customers in the affected area will scale somewhere between linearly and quadratically with the system voltage, as higher-voltage lines tend to use larger conductors or bundles. The efficiency of various types of lightning protection also varies with operating voltage. High system voltage usually calls for larger electrical clearances, and lightning impulse strength scales linearly at about 500-540 kV per meter of clearance. As an additional factor, however, the insulation requirements for uniform lightning performance also vary with local soil type. Table 3-1 Typical Power Line Lightning Mitigation Options Flashover Type
Influence of Poor* Soil
System Voltage 42 to 100 kV
System Voltage of 100 to 230 kV
System Voltage of 275 to 765 kV
Typical Impulse Insulation Strength (CFO)
No effect on strength
150 to 550 kV
550 to 1100 kV
1100 to 2200 kV
0.3 to 1 m
1 to 2 m
2 to 4 m
Induced Overvoltages
30% extra stress
Add TLSA**, or 300-400 kV CFO
Induced flashovers not likely
No effect
Add TLSA to top and poorly protected phases
Add second overhead groundwire, and/or move them outboard
Add TLSA to top phase(s)
Improve grounding
Improve grounding
Add TLSA to bottom phase(s)
Add TLSA on bottom phases
Add gapped TLSA
Shielding Failures
x
Backflashover
9 more stress on tower with poor soil
Add TLSA to poorly protected phases
* Poor soil = 1000 Ωm where 100 Ωm is typical; ** TLSA = Transmission Line Surge Arrester
Utility Investment in Lightning Protection using Overhead Groundwires Overhead groundwires (OHGW) in Table 3-1 are used for systems operating above 100 kV to provide effective lightning protection over a long service life. Simple visual cues such as broken strands or missing wire signal the end of OHGW service life. However, they do not carry any power – in fact, they dissipate power at peak loads because nearby phase conductors induce circulating currents. One evaluation of the value of OHGW protection was carried out [Red 2006] using a benchmark of the cost per avoided customer momentary disturbance. Using the example in Figure 3-3, this benchmark is: The cost of avoiding 118 of the 120 unacceptable voltage sags using OHGW (The number of affected customers in the area) times (118 avoided disturbances) 3-4
Why Protect Transmission Lines from Lightning
A rough guide is that the OHGW protection adds a total of 10% to the line cost. This breaks down as 4% for the extra conductors (there are usually two), 3% for stronger towers and 3% to make up generation capacity for the peak-power losses. Using typical transmission line cost of $US 100k per mile, the utility capital investment in OHGW in the sensitive area of Figure 3-3 works out to $1.2 million. With about 100,000 customers in the area, the benchmark works out to about 10¢ per avoided customer momentary disturbance. Depending on the system voltage, this value was found to range from 1¢ to 14¢ in (Red 2006). The value is lower for areas of dense lightning and for EHV lines that take advantage of lowreactance phasing to limit induced-current power consumption at peak loads.
Utility Investment in Other Lightning Protection Methods Utilities have other options to OHGW, and have used them with moderate success in areas of minimal lightning like California, British Columbia, New Brunswick, Newfoundland and Quebec. Some utilities reduce their sensitivity to single-phase to ground flashovers by providing single-pole reclosing. Others provide redundant transmission paths. However, high rates of multiple-pole flashover from lightning have forced some of these utilities to retrofit protective measures, such as transmission line surge arresters (TLSA). This guide covers two important applications of TLSA as lightning protection: 1. To improve the efficiency of OHGW protection. If the OHGW is reaching 98.3%, there is not much more to be gained. However, if the ground resistivity is high, OHGW efficiency can fall off to 30-40%. Spot treatment of high-resistance towers (and their neighbors) is also a feasible option. 2. To replace the OHGW completely. The protection budget above $1.2 M would cover an investment of $2k per tower (at five towers per mile). Again, depending on ground resistivity and insulation level, this investment may be better spent on TLSA than on OHGW.
3-5
4 TRANSMISSION LINE LIGHTNING PERFORMANCE PARAMETERS
Introduction The parameters that govern transmission line lightning performance fall into two broad categories: 1. Those that govern lightning incidence to a line 2. Those that govern the development of insulator and air gap voltages when lightning hits a line or hits the earth near a line The designer can substantially improve line lightning performance by paying careful attention to the parameters in both categories. It should be recognized that lightning flashovers are meteorological phenomena that can vary widely from year to year. As such, it is easy to understand that 100% accuracy in forecasting flashover rates is no more possible than 100% accuracy in long-range weather forecasting [r1], except for the situation where TLSA suppress nearly all flashovers that would otherwise normally occur. It should also be noted that the number of lightning flashovers can vary widely from one year to the next. One should not be surprised at a variation of 3:1, and sometimes substantially more than that. Lightning performance calculations are useful for the comparison of line designs in the same meteorological environment. The designer can compare design options, understanding that while the absolute performance of the line may be uncertain, the design is the best option evaluated based on performance requirements, economic considerations, soil conditions and terrain, and the estimated average ground flash density (GFD) where the line is to be located. It is possible to perform some limited lightning performance calculations manually, but the evaluation of multiple design options generally requires the use of a high-speed computer. Transmission line lightning computer programs forecast an average expected flashover rate by first assuming a prescribed average lightning incidence to a line that is based on average or median thunderstorm weather records. Some programs also provide estimates of return periods, i.e., probabilities that flashover rate X will be exceeded only once in Y years, based on statistical distributions of lightning currents and GFD statistics. Transmission line lightning programs 4-1
Transmission Line Lightning Performance Parameters
perform these calculations using concepts outlined in this chapter. Application software is discussed in more detail in Section 7.
Lightning Incidence Parameters "Lightning incidence" is a generic term relating to the likelihood of lightning hitting a line over a designated period of time. To assess line performance, engineers usually refer to the number of flashes to a line per 100 miles, or 100 kilometers, per year. With this information they can use modern analytical procedures to determine the number of flashes to each overhead groundwire and phase conductor. These values are fixed by GFD and the "electrogeometric" twists and contortions of each lightning leader as it approaches the line or the earth. Ground Flash Density (GFD) Every transmission line is located in a specific meteorological environment. Therefore, a key lightning incidence parameter is the average number of flashes to earth per square kilometer, per year along the line corridor. This parameter, called the ground flash density (GFD), is determined by averaging years of ground flash counts recorded by electronic locating systems. This technology has been operated for fifteen years or more, giving satisfactory average values of GFD even in areas of North America where thunderstorms vary substantially from year to year and the density is low. Figure 4-1 shows a ten-year average GFD for the USA.
4-2
Transmission Line Lightning Performance Parameters Figure 4-1 Lightning Ground Flash Density for Continental USA, 1989-1998 [Orville Huffines]
Figure 4-2 Lightning Ground Flash Density from NALDN, 2000 to 2003 (Vaisala, need permission)
For those regions where the GFD is unknown or of dubious accuracy, engineers ten years ago used the local "keraunic level" as an alternative means of estimating the GFD. The keraunic level is defined as the average number of days per year at a specific location on the ground that someone can expect to hear thunder. Weather bureaus in many countries keep yearly records of the keraunic level at their meteorological observation points. From these records, national or international isokeraunic maps can be prepared to show contours of constant keraunic level. A rough relationship between local GFD and keraunic level is given by: GFD = 0.04Td1 / 25 = 0.054Th1.1 Equation 4-1: Ground Flash Density from Annual Thunder-Days or Thunder-Hours
Where: GFD = average flashes to earth/km2/year, Td = average thunder days per year (keraunic level) and Th = average thunder hours per year (keraunic level). This relationship, recommended by IEEE [2] and CIGRE [3] ten years ago, assumes that the ratio of the number of cloud-to-cloud flashes to the number of cloud-to-ground flashes is the same in both tropical and temperate zones. There is considerable evidence that this is not the 4-3
Transmission Line Lightning Performance Parameters
case, but Equation 4-1 should be roughly correct for temperate zones. Other possible relationships have been tabulated in [Red Book 1981]. In tropical areas with more than 100 thunder-days per year, estimates of GFD using Equation 4-1 for moderate climates give unreasonably high results. Even for temperate climates, there is factor-of-two variation: the relation between GFD and TH in Canada [CEA 179 T 372] has half of the slope of the relation in Equation 4-1. Starting in 1995, this problem was addressed with measurements of optical transient density (OTD), carried out by satellite. Comparison of OTD with GFD values suggests that there are roughly three optical flashes for every ground flash. An up-to-date map of OTD is given by [Bocc NASA]
Figure 4-3 Optical Transient Density Map from (NASA 2006) and Estimate of Ground Flash Density
Lightning Incidence to Lines From the regional value of GFD, the approximate number of flashes per year collected by a transmission line in the same region is given by Equation 4-2 [2,3]: Ns =
(
GFD 0 .6 28ht + b 10
)
Equation 4-2: Eriksson Expression for Number of Flashes to Transmission Line
Where Ns = number of flashes to a line 100 km/yr, GFD = ground flash density (flashes/km2/yr), ht = overhead groundwire height at the tower (m) and b = horizontal separation distance between overhead groundwires (m). 4-4
Transmission Line Lightning Performance Parameters
Eriksson’s expression in Equation 4-2 uses the overhead groundwire height at the tower, ht, which can be confusing. Most other expressions use the average height of the wire over ground h given by this height at the tower minus two-thirds of the sag, assuming that the terrain is flat. Other equations for h have been proposed for rolling and mountainous terrain. If no overhead groundwires exist, h in Equation 4-2 becomes the average height of conductor attachment point at the tower, and b is the distance between the outmost phases. If only one shield wire exists, b is also zero. A preferred expression for average flash incidence as a function of first peak return stroke current I and average conductor height h is: Ns =
(
GFD 3.14 ⋅ I 0.69 ⋅ h 0.45 + b 10
)
Equation 4-3: Preferred Expression for Lightning Flash Incidence to Power Lines
This expression gives similar numerical estimates to the Equation 4-2, as shown in Figure 4-4, and has a much stronger basis in the physics of switching-surge gap flashover.
Figure 4-4: Relation between Lateral Attractive Distance Da of Horizontal Conductor and Average Conductor Height h . Curve 1: Eriksson; Curve 2: D=2h; Curve 3: Rizk
Flashes to a line terminate either on one, of the overhead groundwires (if any exist) or on one of the phases. A strike to a phase, known as a "shielding failure”, will usually cause flashover of the insulation at one or more towers which may involve one or more phases. Even if the line has very high insulation strength, and the first stroke is weak, one of the subsequent strokes is likely
4-5
Transmission Line Lightning Performance Parameters
to have enough energy to cause flashover. This means that every shielding failure can be considered to cause a shielding failure flashover. The "electrogeometric theory" of shielding failures is reviewed in [Red Books, IEEE 1243]. The model is based on an assumed perfect correlation between leader charge and flash current. As a stepped downward leader of a flash approaches a line, the last step has a choice of striking the earth or jumping to a shield wire or phase wire (Figure 4-2). The "striking distance" to a shield wire or phase wire is approximated by Equation 4.3 rc = 10I 0.65 Equation 4-4: Recommended Striking Distance from Lines to Vertical Leader (IEEE 1243)
Where: r = striking distance, leader tip to nearest conductor (m) and I = stroke current (kA). The striking distance, rg from the same leader tip to the earth is given by: rg = [3.6 + 1.7 ln(43 − h)]I 0.65 Equation 4-5: Recommended Striking Distance from Earth to Vertical Leader (IEEE 1243)
where: rg= striking distance to earth (m) and h = average conductor height, < 40 m. The shortest striking distance from the leader to overhead groundwire, phase or ground is considered to be the only one that will develop from streamers and leaders into a full return stroke.
Figure 4-5 Striking Distances from Ground and Conductor to a Downward Leader
4-6
Transmission Line Lightning Performance Parameters
Further research has adjusted some of the electrogeometric relationships, with a view towards developing a unified model that predicts both the flash incidence and the shielding failure rate accurately. The expression in Equation 4-3 is one model that accomplishes this. The flash termination point depends not only on the location of its tip as it approaches the line but also on the current, I, that will be delivered by the first stroke in the flash. This current is proportional to the leader charge near the leader tip, and it is this charge that determines the striking distances rc and rg The final breakdown of the air is assumed to occur over the shortest distance and this determines the anchoring point for the stroke current. Electrogeometric theory, as outlined in [Red Books] or [Rizk 1990], can be applied over the expected wide range of lighting amplitudes (from 3 to 200 kA), and the results integrated to estimate the number of expected hits to each shield or phase wire. Alternately, numerical simulations can be performed.An applet (LI2) provided with this guide can be used to explore the various models of shielding failure. Figure 4-6 illustrates the occurrence of a few shielding failures, even for a wellshielded double circuit line, when some randomness is introduced into the leader propagation.
P(I≤5 kA) = 1%
4-7
Transmission Line Lightning Performance Parameters
P(I≤15 kA) = 13%
P(I≤25 kA) = 36% Figure 4-6: Modeling of Lightning Shielding Failures using L2 Applet [Red 2005] for Peak Stroke Currents of 5, 15 and 25 kA
Lightning Current Parameters For computational purposes, a lightning flash to a line is idealized as a vertical, infiniteimpedance, surge current source. The actual surge impedance of a flash channel is still a matter of discussion after a century of lightning research. Values from 300 Ω to infinity have been used. In principle, the flash acts like a single-conductor lossy transmission line that has been lowered from an overhead thundercloud into the vicinity of a line, charged to extremely high voltage and then suddenly connected to the line. Estimates of the leader potential V can be plotted against leader charge Q, both obtained from multiple-point synchronized measurements of electric fields at ground level, as shown in Figure 4-7. The relation CLeader=Q/V then gives an estimate that these vertical charged rods with their corona envelopes have capacitances of about (13.2 MV per coulomb) or 75 nF in the area where [Mazur 2001] carried out these studies.
4-8
Transmission Line Lightning Performance Parameters
Figure 4-7 Relation between Lightning Leader Potential and Stroke Charge [Mazur 2001]
The return stroke is a traveling wave of current that rushes upward, discharging the stored charge on the leader channel and the corona envelope around it. The return stroke current wave of zero potential moves upward at roughly one-third of the speed of light. Charge conservation forces a high current transient out the base of the channel, passing through the stricken point. When the flash discharges a charge pocket in the cloud overhead, it may upset the voltage distribution in the cloud and cause other charge pockets to flash to the channel, creating multiple current transients that can be measured on the ground as a series of high current peaks. The return stroke current, traveling up the stroke channel, creates a powerful electromagnetic field that is well modeled by a simple “transmission line model” [Uman Maclain Krider 1975]. This creates a burst of static, familiar to people using AM radio in the summer, that can be detected with broadband receivers at distances of more than 600 km. Calibrated, remote measurements of EM fields can be inverted to estimate the strength of the return stroke current and, in some terrain, its rise and fall times. This physics is what makes lightning location networks practical and effective for studying lightning parameters. The lightning parameters important in establishing line performance are: 3. Stroke current peak magnitude 4. Stroke current rate-of-rise 5. Stroke current waveshapes 6. Total charge delivered 7. Number of strokes in a flash
4-9
Transmission Line Lightning Performance Parameters
Stroke Current Peak Magnitudes When a lightning flash terminates on a shield or phase wire, the likelihood of a line flashover depends to a great extent on the peak magnitudes of all the strokes in the flash and, for each stroke, on the time interval that each stroke current requires to rise from near zero to its crest value, i.e. the "front time." The IEEE Guide [2] and [I] suggest a simple probability equation (Equation 4.6) to describe expected peak currents in any first stroke in a flash: Pf =
1 ⎛ I ⎞ 1+ ⎜ ⎟ ⎝ 31 ⎠
2.6
Equation 4-6: Probability Distribution for First Return Stroke Peak Current
where: Pf = probability that any first stroke in a flash will equal or exceed stroke peak current, I and I = first stroke peak current magnitude (kA). Field observations have shown that subsequent strokes in a flash are often (but not always) weaker in peak amplitude than first strokes. A cumulative probability equation is given in [IEEE 1243] for subsequent stroke peak magnitudes as: Ps =
1 ⎛I ⎞ 1+ ⎜ s ⎟ ⎝ 12 ⎠
2.7
Equation 4-7: Probability Distribution for Subsequent Return Stroke Peak Current
Where: Ps = probability that any subsequent stroke in a flash will equal or exceed peak current magnitude, Is and Is = subsequent stroke peak magnitude (kA). Lightning location systems give good estimates of the product of (peak stroke current) times (return stroke velocity). If, using wishful thinking, the first return stroke velocity v is fixed (and v=0.3c is a common value) then the current can be established with an error of less than 10%. The log-normal statistical distribution of peak currents will then generate a log-normal distribution of remote peak radiated fields. With more than 107 remote measurements per year, the remote peak-field distributions in wide-area networks have proved to be log normal. Using pessimistic thinking, the return stroke velocity and peak stroke current are perfectly correlated, a condition needed to satisfy charge conservation (Wagner 1963). If an empirical model for perfect correlation is adopted, as for example by (Chowdhuri et al 2004), then the lognormal distribution of peak stroke current should generate remote radiated field statistics that have high skew and kurtosis. They do not (Chisholm and Cummins 2005). Data tend instead to support a hypothesis that charge and peak stroke current are only moderately correlated, like many other variables in lightning parameters. Berger found that the Pearson 4-10
Transmission Line Lightning Performance Parameters
correlation coefficient R between first-stroke peak current and impulse charge was moderate (R=0.77). The correlation observed by [Mazur 2001, Red 2005] recently between the stroke charge and the peak radiated field was fairly similar, at R=0.6, with the relation between stroke charge and leader potential in Figure 4-7 being much stronger (R2=0.61 or R=0.78). This will be a difficult issue to sort out in future research. Measurements of stroke currents to towers can be helpful. The enhancement of the radiated field from speed-of-light propagation in tall towers is well established: For example 9-kA flashes to the CN Tower in Toronto should and do read out as 30-kA flashes in the NALDN lightning detection network, using the fixed return stroke velocity of v=0.3c. This unfortunately means that measurements on short towers will be more meaningful – unfortunate because short towers receive few flashes per year, leading to a long-term research commitment. One ten-year commitment was reported by [Takami, Okabe 2005] for Tokyo Electric Company. Sixty transmission towers were instrumented, leading to 120 flashes for analysis, including those shown in Figure 4-8. Rogowski coils were used to record impulse currents above 9 kA with good high-frequency response.
Figure 4-8 Lightning to Instrumented Rods on Tokyo Electric Transmission Towers [Takami 2005]
Stroke Current Rate of Rise Line flashovers are determined not only by stroke current magnitudes but also by stroke current rate of rise (time derivative). The overall voltage rise with a series R-L circuit is given by V = RI + L dI/dt, where R is the resistance of the path and L is the inductance. For transmission lines, L is the tower or down-lead inductance and the R is the resistance of the grounding system. The faster the current changes through the tower inductance, the greater the voltage across that inductance and, hence, the greater the tower top voltage and the voltage at the tower cross arms. A significant component of any insulator voltage created by a lightning flash to a tower or 4-11
Transmission Line Lightning Performance Parameters
overhead groundwire will be the inductive voltage drop created by stroke currents flowing in the towers. This component is described in [Red Books] in more detail. If a flash hits the earth near a line, the voltage coupled into the line is also dependent on the rise time. A cumulative probability equation for the maximum rate of rise of negative first strokes in a flash is given in [Chowdhuri IEEE, Red 2005] as: PSm =
1 ⎛S ⎞ 1 + ⎜ max ⎟ ⎝ 24 ⎠
4
Equation 4-8: Probability Distribution for First Return Stroke Maximum Steepness Smax
Where: PSm = probability that the rate of rise in a first stroke will equal or exceed Smax, expressed in (kA/µs). A median value of Smax =18.9 kA/µs is given by [Takami 2006] for transmission towers as an alternative to Berger’s median value of Smax=24 kA/µs on standalone masts and chimneys.
Figure 4-9 Relation between Maximum Rate of Rise and Peak Amplitude of Lightning to Tokyo Electric Transmission Towers [Takami 2006]
Takami found that the maximum steepness of lightning flashes to transmission towers was highly correlated with peak current, with R=0.82 and poorly correlated with front time (R=0.26). This supports evaluation of the circuit response with a fixed 2-µs virtual front time, as in the IEEE FLASH program, as shown in Figure 4-10.
4-12
Transmission Line Lightning Performance Parameters
2 µs Ramp
Figure 4-10 Relation between “Virtual Front Time” and First Peak Amplitude for Lightning to Tokyo Electric Towers [Takami 2006]
Typical first return strokes have a concave shape that ensures the maximum rate of rise occurs nearly at the same time as the peak of maximum current. In contrast, laboratory test waves with 1.2/50 µs waveshape have Smax at t=0 and zero slope (dI/dt=0) at the peak of current wave. This means that calculations of insulator voltages using double-exponential current waves in EMTP tend to understate the importance of component inductance. The contribution of tower inductive voltages to insulator voltages depends very much on the stroke wave shape. Stroke Current Waveshapes No two stroke current wave shapes are exactly alike, and the variations in wave shape are substantial. Some examples- are given in [Red Books, Narita 2000 etc]. Computer programs calculate line lightning performance by assuming one of the following: 1.
A linear-rising front (such as 2 µs in the IEEE FLASH program [4])
2. A double exponential wave shape such as the Heidler wave in the EPRI L4 applet 3. A concave front, such as in the CIGRE wave in the L4 applet provided with this guide For line performance where resistive terms are important, all models are acceptable, depending on what is done with it within the program. However, the linear-rising front and concave front both have Smax at the peak of current wave, and they are both superior to the double exponential waveshape in this important respect.
4-13
Transmission Line Lightning Performance Parameters
Total Charge Delivered Most of the charge by a stroke current is delivered after crest current is reached. If one integrates the stroke current wave to obtain the charge delivered, it becomes obvious that the charge delivered is governed by the tail time. It is the charge in a lightning flash that determines the energy fed into TLSA, and it is also the charge that causes pitting, burning, and ablation of overhead groundwires at contact points. It should be noted that, between some stroke current peaks and at the current decay at the end of lightning flashes, a low, continuing current of hundreds of amps can flow for hundreds of milliseconds. The product of 100 A and 100 ms is a charge of 10 coulomb, and this means that the charge delivered by continuing currents can greatly exceed the charge from the current peaks in a flash. These low currents, because of their longer duration, act somewhat like an arc welder and in fact plasma cutting torches make realistic test apparatus when simulating this damage on optical-fiber groundwires. Berger [5] integrated the current records of downward flashes to Mount San Salvatore in Switzerland to determine the charges delivered. The results can be approximated by: PC − =
1 1.7
⎛Q⎞ 1+ ⎜ ⎟ ⎝7⎠
Equation 4-9: Probability Distribution for Negative-Flash Charge
PC + =
1 ⎛Q⎞ 1+ ⎜ ⎟ ⎝ 85 ⎠
2
Equation 4-10: Probability Distribution for Positive-Flash Charge to 2 ms
Where: PC- is the probability that a negative flash will deliver a total charge equal or greater than Q and PC+ probability that a positive flash will deliver a total charge equal or greater than Q, with Q expressed in coulomb. Berger reported that in one case a positive charge reached 300 coulombs. Note that positive flashes tend to deliver almost 10 times as much charge as negative flashes. Positive flashes are infrequent but there is some evidence in Figure 4-11 that the central US is an area where there can be a large number of high-amplitude positive flashes, associated with severe thunderstorms there.
4-14
Transmission Line Lightning Performance Parameters
Figure 4-11 Percentage of Positive Cloud-to-Ground Lightning Flashes (Left) and Density of LargeAmplitude Positive Flashes (Right) in USA [Boccippio et al
Number of Strokes in a Flash The previous section provided two equations that describe the probabilities of observing magnitudes of first and subsequent strokes in a flash. More than half of all observed lightning flashes contain more than one stroke, and the average number of strokes in a flash is roughly three. The subsequent strokes in a flash tend to have a much faster rise time than first strokes, but they tend to be lower in amplitude. A number of studies [I] have concluded that, in calculating line lightning performance, one can assume that the first stroke in a flash is generally at least as severe as subsequent strokes, so the latter can be ignored. This certainly does not hold true in every case. When evaluating circuit breaker reliability, for instance, a subsequent transient can arrive just as breaker contacts are opening and cause a breaker restrike. Also, as mentioned previously, a weak first stroke causing a shielding failure to a phase conductor is likely to be followed up with one or more subsequent strokes, each with a median 12-kA current.
Transmission Line Parameters There are many transmission line parameters that govern line lightning performance. The six fundamental categories are: 1. Line conductor geometries 2. Tower geometries 3. Insulator /air gap geometries 4. Tower ground characteristics 5. Transmission line surge arresters 6. Nonlinear corona effects
4-15
Transmission Line Lightning Performance Parameters
Because these parameters can vary from one tower to the next, calculating line lightning performance in a rigorous way is very difficult, requiring a lot of program input data and computational effort. Line Conductor Geometries Line conductor geometry is a dominant factor in line lightning performance. The size and location in space of the line conductors determines which conductor is hit and how often. The spacing between conductors and their height above the ground determines how they couple electromagnetically to one another and to the lightning stroke. The height, sizes and bundling of conductors establishes the "surge impedance," an important variable that in turn determines the voltage on a conductor for a given stroke current flowing through it. Conductor resistance is normally ignored in lightning surge calculations, since far more distortion of voltage and current wave shapes is created by corona than by the resistance of the metal. Because of the short distances of propagation (usually only a few spans), even the difference between steel in the overhead groundwires and aluminum in the phase conductors is usually ignored, unless induced power losses in overhead groundwires also have to be calculated. The high frequencies involved in lightning surge currents cause a substantial skin effect, with practically all currents flowing on the outer surfaces of the line conductors. Very little current flows inside, where the steel cores of aluminum conductor steel reinforced (ACSR) are located. In a similar fashion, it is assumed that the skin effect causes earth currents to flow on the surface of the earth, except around the ground electrodes at each tower. Tower Geometries Tower geometry is an important parameter in the insulator voltage development process. A typical transmission tower might have an equivalent inductance on the order of 20µH, and a lightning surge current flowing down that tower, changing at a rate of 75 kA/µs, would create a tower top voltage of 1500 kV with respect to ground. A substantial part of this voltage appears at the tower crossarms, and consequently, across the line insulators connected to the crossarm. Tower shapes and heights vary widely, from simple wood poles with one ground wire to tall lattice river crossing structures. Some towers have guy wires stretching out to earth anchors that further complicate the analysis of their electromagnetic contributions to insulator voltage. Whatever the tower geometry, the following two fundamental parameters are needed for analysis: 1. Tower Height - The tower height determines the travel time of lightning transients from top to bottom. All other variables being equal, if the tower height is doubled, the inductive component of voltage across each insulator will double. 2. Tower Surge Impedance - A transmission tower can be regarded as a network of metallic elements, each with a finite travel time for any transient current moving along it. In effect, the tower becomes a network of short transmission lines carrying current from the tower top 4-16
Transmission Line Lightning Performance Parameters
overhead groundwires to the earth below, where some of the current enters the earth resistance and some reflects back up the tower toward the top. Then the tower itself can be considered a short vertical transmission line. Like any transmission line its surge response (voltage per unit current) can be described by a scalar “surge impedance” and a travel time. The surge impedance and travel time are different for different tower geometries. A rough value for a conventional lattice tower might be 150 Ω, but this can vary substantially. Simple towers, including pole bonds, have travel times given by the tower height divided by the speed of light, while towers with crossarms have multiple paths that add complications. The contributions of tower surge impedance to lightning voltages across insulators are discussed in some detail in [I, Reds and 6] and can be explored with the L-5 applet provided with this guide. Chisholm, Chow, and Srivastava [6] developed useful equations for the surge impedance and travel time of typical transmission towers based on a narrow-waist steel lattice structure which was adopted by CIGRE [3]. A weighted average radius of the tower is first obtained by: Ravg =
(r1h2 + r2 (h1 + h2 ) + r3 h1 ) (h1 + h2 )
Equation 4-11: Average Tower Radius for Surge Impedance Calculations
Where: Ravg is the weighted average tower radius (m), r1 = tower top radius (m), r2 = tower midsection radius (m), r3 = tower base radius (m), h1 = height from base to midsection (m), h2 = height from midsection to top (m) Where, as an approximation, one can use half the average local width of the tower as an equivalent radius. The weighted average radius Ravg is then used in the following equation for equivalent tower surge impedance:
Z avg
⎛ ⎛ ⎞⎞ R ⎜ ⎜ tan −1 ⎛⎜ avg ⎞⎟ ⎟ ⎟ ⎜ h + h ⎟ ⎟⎟ ⎜ ⎜ ⎝ 1 2⎠ = 60 ln⎜ cot⎜ ⎟⎟ 2 ⎜ ⎜ ⎟⎟ ⎟⎟ ⎜ ⎜ ⎠⎠ ⎝ ⎝
Equation 4-12: Average Tower Impedance for Lightning Calculations
Specific surge impedance formulas for cylindrical and conical towers are also available in Chisholm et a1 [6] along with ways to add in the effects of crossarms. Insulator / Air Gap Geometries A lightning flashover at a tower (Figure XX) will usually occur from "live metal" (a line conductor, corona ring, yoke plate, etc.) to "ground metal" (a tower leg, crossarm, insulator 4-17
Transmission Line Lightning Performance Parameters
hanger, etc.). Much less frequently, it may occur phase-to-phase out on a span, or where phases are close together in a tower window. The geometry defining the likely flashover path or paths must be described before flashover calculations can be undertaken. If the shortest path is over an insulator, the first approximation of flashover strength will be governed simply by the string dryarc distance, not by the insulator material. The string dry-arc distance is the shortest possible distance from live metal to ground metal. This kind of approximation can be made because of the non-uniformity of both live metal and ground metal electrodes. If the distance Da, or DL from a line conductor to a tower crossarm or leg, respectively, is less than the dry-arclength, Dt of the supporting insulator string, then the shorter path, Da or DL, is assumed to define the flashover path. The creepage distance along an insulator surface is not used in specifying insulator flashover paths where lightning is concerned.
Figure 4-12 Flashover paths for a V-String Configuration
When analyzing the distances on V-strings and vertical suspension strings, the shortest distance between a high voltage conductor and a ground conductor defines the flashover path. When using a computer to analyze lightning flashover performance, the most practical way to define insulator/air gap geometries is to display a catalog of images of insulation arrangements and let the user select the image that best represents the line in question. The program can then make any adjustments to flashover strength that might be necessary. The fundamental problem in determining whether a gap or insulator will flash over under a lightning impulse is that flashover strength is sensitive to voltage waveshape. Flashover strengths of insulators and air gaps are determined with "standard" wave shapes, such as the 1.2/50µs standard lightning impulse, but rarely will this wave shape be created by lightning on an actual transmission line. A computer program can calculate the magnitude and wave shape of the voltage created across a line insulator or air gap, but it then has to decide whether that particular wave shape is sufficiently severe to cause flashover. If the wave shape departs substantially from a standard 1.2/50 impulse, the determination is not easy. Three different approaches are used for determining whether the insulation will flashover or withstand. 4-18
Transmission Line Lightning Performance Parameters
Volt-Time Curve Penetration Method This method is described in detail in [I]. It requires that a test volt-time curve (Figure 4-4) be known by experiment for a standard 1.2/50 impulse. If the time to flashover is greater than the 1.2 µs required to reach crest, the volt-time curve shows the crest voltage of the wave shape that caused failure and the time of breakdown. If failure is before the 1.2-µs crest time, the curve shows the instantaneous voltage and time at breakdown. An empirical fit of transmission line suspension insulator volt-time curves for strings of length L= 1 to 6 m in length is given by: 710 ⎤ ⎡ E = ⎢400 + 0.75 ⎥ L t ⎦ ⎣ Equation 4-13: Volt-Time Curve for Insulator Strings of Length L= 1 to 6 m
Where time t is in µs and flashover voltage E is in kV. If any voltage wave penetrates that volttime curve, breakdown is assumed to occur. This method is simple and computationally fast, but it loses accuracy rapidly as wave shapes depart from the 1.2/50 impulse. Nevertheless, it has been accepted as sufficiently accurate in several lightning flashover programs, including the IEEE FLASH program [4]. Disruptive Index Method Originally, this method was developed by Witzke and Bliss [7] to estimate the likelihood of failure of power transformers subjected to nonstandard lightning impulses, but it has been extended to air gaps and insulators. This method defines the term "disruptive effect" or DE as:
DE =
t =t s
∫ [V (t ) − E ] dt n
o
t =t 0
Equation 4-14: Disruptive Effect Integral for Insulation Strength with Nonstandard Waves
where: V(t) = instantaneous value of impulse voltage stress in kV/m, Eo is a threshold gradient in kV/m below which ionization will not occur, n is an exponent with a recommended value of n=2.5, Eo=300 kV/m and DE=1010 [8] to simulate Equation 4-13. The constant Eo in Equation 4.13 represents a minimum voltage gradient below which breakdown cannot occur for any wave shape. The exponent n is used to recognize that the breakdown process is not linear with the wave shape. When DE reaches a critical value, 4-19
Transmission Line Lightning Performance Parameters
breakdown is assumed to occur. There have been several variations of this approach. Darveniza [9] did extensive experiments on the method and concluded that it could be made to give good results for air gaps subjected to lightning impulses. Leader Progression Method This method was adopted by CIGRÉ [3] to evaluate flashover strengths of insulators and gaps to impulse waves of arbitrary shape. It attempts to simulate the dominant electronic processes that bridge a gap. It uses experimentally determined equations to solve for the streamer formation time ts and the leader propagation time tl as a function of electric stress in the air gap. Many researchers have examined these formation times, principally in Europe and Japan. In [Red 2006], the leader progression method was noted to be acceptable for air gaps, but not for insulator strings. It is also computationally slower than the other two methods, but it is certainly the method that best emulates the actual processes involved. Tower Ground Characteristics Tower grounding is a very important parameter in establishing the lightning performance of a shielded transmission line [1, Red 2006, 1002021]. Low tower ground resistance improves the effectiveness of lightning protection using overhead groundwires. Ground resistance for wire-frame approximations to solid shapes, such as a grid in a substation, consists of two terms: a geometric resistance of the overall shape to infinity and a contact resistance that corrects for the local resistance of buried wire to the nearby soil. Table 4-1 Geometric and Contact Resistance for Typical Surface Electrodes Electrode Shape Solid Disc, radius r
R geometric =
Ring, radius r, wire length L=2πr, wire radius a
R geometric =
Horizontal Square Plate, area A
R geometric =
Horizontal Square Ring, area A
R geometric =
Vertical Plate, length L, depth s,
g = L2 4 + s 2 , A = Ls Buried Wire, length L, depth s, wire radius a
4-20
Geometric Resistance
ρe ρe ρe ρe
0
2r
Rcontact =
2r 4 4
Contact Resistance
π π
A A
ρ1 ⎡ 1 ⎛ s ⎞ 1 ⎤ ln⎜ ⎟+ L ⎢⎣ 8π ⎝ 16a 4 ⎠ 4π ⎥⎦
0
Rcontact =
R geometric =
ρe ⎛ 11.8 g 2 ⎞ ⎟ ln⎜ 2πg ⎜⎝ A ⎟⎠
0
R geometric =
ρe ⎛ 11.8 g 2 ⎞ ⎟ ln⎜ 2πg ⎜⎝ A ⎟⎠
Rcontact =
ρ1 L
ρ1 ⎡ 1 ⎛ s ⎞ 1 ⎤ ln⎜ ⎟ − L ⎢⎣ 2π ⎝ a ⎠ 4 ⎥⎦
Transmission Line Lightning Performance Parameters
The total electrode resistance is the sum of Rgeometric and Rcontact in Table 4-1. In two-layer soil, the resistivity ρ1 of the top layer near the electrode affects Rcontact. If the electrode is large relative to the upper-layer soil depth (for a thin layer of soil on rock) then effective resistivity ρe is the bottom-layer resistivity ρ2. For typical transmission lines, ρe is somewhere between ρ1 and ρ2. Contact resistance varies with soil moisture content and also with the magnitudes and rise times of surge currents flowing into it [B]. Earth is fundamentally a very lossy dielectric, and, if current density in it gets high enough, dielectric failure will occur, causing a pronounced reduction in resistance. The non-linearity of contact resistance with current adds considerable complexity to the calculation of lightning surge voltages on lines. •
In cases when the contact resistance is a small fraction of the geometric resistance, it is sufficient to model the ionization effects by setting Rcontact = 0.
•
If contact resistance dominates geometric resistance, the EPRI Grounding Guide and Red Book provide details of suitable models, along with an applet (L1) to explore the effects on lightning performance.
There is strong evidence that the resistivity of many soils drops as frequency increases. The amount of the reduction varies, from 20% for sand to 60-90% for clay, and the reduction varies with soil moisture content as well. Where it is available, the resistivity at a frequency of 100 kHz or 1 MHz (or 1-µs pulse signals) is more suitable for lightning calculations than the value obtained at 100 Hz with standard test equipment. Transmission line grounds in contact with the soil are of several types: Buried Tower Grillage The lower portions of tower legs are buried in the earth. They sometimes rest on metal grillage, or are encased in poured concrete, to improve mechanical strength. A large fraction of lightning surge current will find its way into and out of the grillage or concrete. Thus, a pre-engineered entry and exit path, providing electrical continuity to any tower steel or reinforcing rebar, is recommended for long service life. Driven Ground Rods Ground rods comprise the most common type of transmission line ground in soil whose characteristics permit the driving of ground rods. References [1,2,3,8] contain formulas for lowfrequency resistance of single and multiple ground rods. Counterpoise Counterpoise may be either continuous or radial. A continuous counterpoise consists of a buried conducting cable connecting the base of each tower, whereas a radial counterpoise consists of one or more buried conducting cables extending radially outward from the base of a grounded tower and not connected to any adjacent tower. References [8,13] provide equations for the low4-21
Transmission Line Lightning Performance Parameters
frequency resistance of various counterpoise arrangements. Surge propagation velocities on buried counterpoise will usually range from c/2 to c/3 (where c is the velocity of light) because of the high dielectric constant of moisture-laden soils or rock. For this reason, the first 60 to 100 m of counterpoise length will most effective during any lightning transient lasting only a few microseconds. This limitation makes radial counterpoise much more effective than continuous counterpoise. Transmission Line Surge Arresters (TLSA) Prior to World War II, inexpensive expulsion tubes, or "protector tubes," were used as selfhealing gaps across insulators. These consisted of hollow fiber tubes configured to fail internally before the insulators could flashover. The arc inside a tube created high pressures, and the arc was blown out the grounded end of the tube and extinguished. However, as line fault current magnitudes increased, these devices became ineffective, and few are still in use. The application of TLSA across line insulators is now increasing in areas where the incidence of lightning and ground resistivities are both high. TLSA can be used in conjunction with or as an alternative to overhead groundwires and counterpoise, and they are becoming particularly attractive on lines where high reliability is required. Section 5 covers line arrester specification and selection, and Section X discusses TLSA placement for improved lightning performance. Nonlinear Corona Effects Nonlinear corona effects, particularly corona on overhead groundwires and phase wires, can change lightning surge voltages across insulators by as much as 15 percent. Surge corona on overhead groundwires effectively enlarges the electrical diameter of the wire, changing its surge impedance and the voltage it couples to nearby phases [1]. Surge corona on phase and overhead groundwires creates a pronounced distortion of traveling waves and also reduces crest voltages. In addition, phase-to-phase impulse flashovers, which are rare but more likely on compact lines, initiate hundreds of amperes of corona (streamer) current between wires prior to breakdown, and these coupled currents can have a pronounced effect on what happens at adjacent towers.
4-22
5 TLSA SELECTION AND SPECIFICATION
Introduction Transmission line surge arresters (TLSA) are designed to limit voltages between a particular phase conductor and the tower structure, thereby preventing flashovers of insulation on that structure. TLSA may also reduce flashovers on adjacent structures. The questions about TLSA design that utility engineers should consider are as follows: 1. Are the electrical characteristics of the TLSA adequate to prevent flashovers? 2. Is the energy withstand capability of the arrester adequate to withstand both lightning and switching duty? 3. Will the arrester withstand temporary overvoltages on the system? 4. If the arrester fails, will it prevent reclosure? Or, will it damage other line components? 5. Is the insulation system, including the water seals of the arrester and the housing material, able to withstand years of service without flashovers at power frequency voltages? 6. Is the strength of the arrester, including line disconnects, sufficient to withstand the mechanical requirements of handling and installation, wind and ice and Aeolian vibration? The selection of an arrester for a particular transmission line is the process of simultaneously satisfying these concerns with a single arrester type.
Transmission Line Arresters Spark gaps were one early form of lightning protection equipment used on overhead telegraph lines, starting in the 1850s. This form of protection was also used for lightning protection of power system components. Gap spacing is normally sized so that an arc would form, shunting dangerous current to ground, before the nearby insulation failed. The use of spark gaps provided control over the flashover location and the lightning current path to ground. Simple spark gaps have several shortcomings in power system applications. The sparkover voltage is a function of the length of the gap. Erosion of the electrodes from arcing and mechanical damage change the gap spacing, which, in turn, affects the sparkover voltage. The sparkover characteristics can also be affected by the moisture content and pressure of the air and by pollution. Sealed gas tubes were developed to solve some of these problems on telegraph and telephone systems. 5-1
TLSA Selection and Specification
The gas tube surge arrester is simply a spark gap, enclosed in a sealed insulated container filled with inert gas or air. The dielectric properties of the arrester are controlled by the selected gas and pressure (volts per mm) and the gap spacing (mm). Gas tubes filled with electronegative SF6 are in common use and offer excellent performance for repeated surges. The two primary design problems with application of spark gaps related to: •
Controlling the voltage at which the device operates (as a function of impulse duration)
•
Managing the power frequency current that flows after the gap operates (a function of system impedance)
In systems that are inspected regularly, a fuse in series with the spark gap can provide the interrupting capability for “power-follow” current. However, the fuse needs to be replaced after every operation of the protective gap. On power systems, the use of automatically reclosing circuit breakers provides the same function as a fuse, without the need for inspection and maintenance. The construction of an effective lightning surge arrester requires that the operating voltage be controlled for all sorts of surge durations, to ensure that the arrester operates at a voltage that is less than the insulation level of the protected equipment. To effectively block power-frequency current after a lightning discharge, arresters with highly non-linear voltage-current relations have proved by be advantageous. Research to develop lightning arresters that were better than rod gaps led in the 1930s to the development of the silicon-carbide valve arrester. These use a number of series spark gaps to control sparkover voltage. The series valve element limits power-follow current to about 100 A. With good (albeit complicated) design, using the magnetic fields from the power-follow current to snuff out the arcs, this level of current can be successfully interrupted at a current zero. Silicon carbide arresters were scaled down for application on distribution systems. At first, an external spark gap was used. However, it is difficult to extinguish 100-A arcs in air, so the spark gap was moved inside the arrester, where it was also possible to apply a number of methods to increase quenching capability. As a technical side note, it is relatively easy to quench a 100-mA arc in air, and this is one of the main reasons why externally gapped metal oxide arresters are now practical: the power-follow current on a modern arrester is usually well below 1 A, compared to 100 A for silicon carbide materials. The first distribution surge arrester with silicon carbide valve elements and a magnetic gap principle were marketed in the mid 1950s. These arresters continued to develop commercially into the early 1970s with emphasis on improved sparkover and discharge characteristics. Silicon carbide arresters traditionally used ceramic housings with metal rupture discs. The problems of moisture ingress into these metal-to-ceramic seals were never solved and this gave typical arrester life of ten to twenty years in service.
5-2
TLSA Selection and Specification
Figure 5-1 shows the internal complexity and cost in a silicon carbide lightning surge arrester with its multiple series gaps, magnetic coils, grading capacitors and resistors around the 60-mm silicon carbide blocks.
Figure 5-1 Internal Construction of Silicon Carbide Lightning Surge Arrester
The large number of components needed to obtain satisfactory power-follow interruption from silicon-carbide arresters made them costly, and also the high number of components affects reliability. Also, the arresters do not have fast sparkover characteristics, meaning that they provide limited protection against the steepest of lightning surge overvoltages. As soon as the metal oxide arrester, made from zinc oxide doped with several other metals, was developed by Matsuoka it became apparent that this material could be used to simplify the arrester design, improve reliability and eliminate the need for series gaps. The first gapless metal oxide varistor (MOV) arresters were constructed for television protection, but they were very quickly scaled up for substation protection of transformers. MOV arresters eliminated the series gap, since the MOV material does not conduct more than 0.3 mA at normal 5-3
TLSA Selection and Specification
line voltage (compared to the 100-A current in a silicon carbide arrester with the same protective levels). At the surge overvoltage knee point, the MOV material goes smoothly into conduction and returns to a nonconductive state when the voltage returns to normal. The volt-time characteristic of the MOV arrester is not much affected by the rate of rise of the impulse, addressing an important aspect in protection of insulation systems that are sensitive to fast transients. MOV arresters were also scaled for applications as ubiquitous as the power bars of multiple-strip outlets (now with LED indicators of power and protection functions) and for application on distribution systems. The first metal oxide arresters for distribution systems were introduced into the USA in 1977. With residual concerns related to thermal runaway after changes in the voltage-current characteristics with repeated stress, the first MOV arresters retained series gaps. However, these gaps were eliminated in later products as materials improved. As a point of historical interest, the first Metal Oxide Varistor TLSA [Koch 1985] used series gaps and they are still operating successfully in Virginia. Some details of these porcelain-housed units, mounted in cantilever, are shown in Figure 5-2.
Figure 5-2 General Electric 138-kV Gapped MOV TLSA in Virginia [Koch 1985, Zed 2004]
If the series gap in Figure 5-2 is not used, normal power frequency voltage appears across the arrester, usually producing a current of less than 1 mA. The power dissipation is minimal, although it does introduce high harmonics as most of the current is conducted at peak line-toground voltage. Higher currents from temporary system overvoltages (TOV) during faults or ferroresonance, can produce significant heating. If the TOV has sufficient magnitude and duration, the arrester block temperature can increase to the level where it will fail from thermal runaway. Like most semiconductor materials, MOVs have positive temperature coefficient.
5-4
TLSA Selection and Specification
Figure 5-3 Classification of Arrester Design Features [Richter et al 2004]
The main system for rating transmission line surge arresters (TLSA) uses the Maximum Continuous Operating Voltage (MCOV). The MCOV is the maximum line-to-ground, rms power frequency voltage that can be applied continuously to the arrester, and is set by the manufacturer. Voltages above the MCOV will eventually fail the arrester, depending on the overvoltage and the duration. Manufacturers specify curves of TLSA capability to withstand TOV. Typically, they can tolerate 150% of MCOV for 5 s and 110% of MCOV for 2000 h with negligible loss of service life.
Arrester Operating Characteristics Worldwide, TLSA are usually constructed with metal oxide varistor (MOV) blocks, usually cylinders of 30-60 mm diameter and 10-20 mm thickness. The blocks are stacked in series to give utilities modular increases in voltage rating. MOV arresters limit voltage by conducting current through the MOV blocks when voltage across the terminals rises above normal limits. The non-linear voltage - current characteristic that has a distinct and sharp turn-on voltage approximated by: ⎛ I ⎞ V =⎜ ⎟ ⎝K⎠
1/α
+ IR
Equation 5-1: Voltage-Current Relation for Typical MOV Elements
5-5
TLSA Selection and Specification
where: K is a function of the number of blocks, α is a non-linearity coefficient and R is an equivalent series resistance at high current. Typically, the nonlinearity coefficient α is a function of formulation, block diameter and application, with 10
5-6
TLSA Selection and Specification
Arrester Rating and MCOV Two rating systems are used to distinguish arrester capability: •
Maximum Continuous Operating Voltage (MCOV)
•
Duty cycle rating (sometimes referred to as "rating")
MCOV is the selection identifier of choice for most engineers. MCOV is defined as the maximum power frequency voltage that an arrester is designed to withstand continuously. It is published as an RMS line-to-ground voltage, and the published value should always be greater than the maximum line-to-line voltage/√3 for solidly grounded systems. For instance, if a 115 kV line is controlled such that the maximum continuous voltage is 1.06 per unit nominal voltage, then the MCOV of the arrester must be greater than (115 kV x 1.06)/43 = 70.4 kV. Arrester duty cycle rating has no link to system parameters. It is the voltage at which the arrester design will pass a duty cycle test as defined by ANSI/IEEE standard 62.11 [14]. Both MCOV and duty cycle rating are published in manufacturers' data, but duty cycle rating is less commonly used as an arrester descriptor [16].
Temporary Overvoltages All transmission systems are subject to fluctuations in power-frequency voltage. Any power frequency fluctuation that exceeds the normal system voltage is referred to as a temporary overvoltage (TOV). TOV may cause current to be conducted through the arrester, thereby subjecting the arrester to high levels of energy. If the arrester is to maintain its insulating properties when the system returns to normal voltages, the energy applied to the arrester by the current must be absorbed without damage to the arrester. Common sources of TOV are: •
Single line-to-ground faults
•
Load rejection
•
Ferroresonance
•
Circuit backfeeding from a higher voltage line
•
Ferranti voltage (lines with one end open)
•
Resonance
Factors that affect the magnitude and duration of TOVs on transmission lines include: •
System configuration
•
Operating practices
•
Relay and breaker settings
5-7
TLSA Selection and Specification
Arresters are designed to withstand some temporary power frequency overvoltages without damage. They can withstand TOV of lower magnitude longer than those of higher magnitude. However, the arrester TOV withstand times fall off rapidly with increasing voltage (because of the non-linear volt-ampere characteristic of the MOV valve elements). Manufacturers supply TOV time vs. magnitude data for all TLSA in the form of curves, with voltage usually given in per unit (p.u.) MCOV. These curves can be compared to system data to avoid problems associated with overheated and failed arresters. If TOV capability is exceeded for any predicted system event, then an arrester with a higher TOV must be selected. However, gapped silicon carbide station arresters may not protect MOV TLSA. In this case, direct comparison of all expected system overvoltages and TOV curves is necessary. In most cases the TOV capability of the TLSA will exceed that of MOV station arresters. For a quick assessment of the vulnerability of a TLSA to a TOV, it is necessary to compare the published TOV data for the TLSA with that of the MOV station arresters applied at the substation at the end of the line. If the TOV durations are longer for the TLSA, then the station arresters will protect the TLSA from damage from TOV. < an example of this graph here would be helpful >
Arrester Protective Levels and Insulation Coordination To apply TLSA properly, engineers must balance the protective level of arresters with the withstand capability of the line insulation. This means that engineers must select arresters with an MCOV that is: 1. High enough to prevent arrester failure on the system 2. Low enough to prevent external flashovers It is usually not difficult to satisfy these requirements in transmission applications. In fact, the task of deciding which TLSA to apply is more straightforward than determining whether or where to apply them. Because the principal insulation on transmission towers is external and self-restoring, engineers need only to consider the statistical flashover performance of a given transmission line when determining the appropriate arrester protective levels (PL). Arrester PL that are lower than necessary to prevent flashovers of the tower insulation offer no additional benefits such as increased equipment life. On the other hand, transmission line flashovers due to extreme, statistically rare events can be tolerated since permanent damage to the equipment is generally minimal. It is not necessary to choose the minimum TLSA MCOV that would be permitted by consideration of system voltage control and TOV application rules. However, it is important that the protective level of the TLSA be sufficiently below the CFO of the insulator in service at the maximum surge current expected through the arrester. For example, a 230 kV line might have 14 standard 5.75 x 10-inch insulators per phase, with an insulator impulse CFO of 1100 kV (derated 10% for poor atmospheric conditions) for a particular make of insulator. If the maximum continuous power frequency voltage to ground per phase is 140 kV, then one might select an arrester with the same 140 kV MCOV rating. If such an arrester has a maximum expected surge 5-8
TLSA Selection and Specification
current of 40 kA, its protective level might have a corresponding expected value of 740 kV (depending on the manufacturer). This is 67% of the flashover level and far below what would still provide good protection. When the MCOV of a gapless arrester is reduced, its cost will lower, but there will be a greater risk of failure. Hence, in order to improve reliability, and where tower-head space and funding permit, it may sometimes be advantageous to select arresters with MCOV ratings somewhat higher than the minimum value permitted, as long as the protective level at the maximum expected surge current is at least 15% below the insulator CFO. One philosophy for dealing with excessive energy dissipation in TLSA application on unshielded lines takes this one step further. The TLSA protective level for an unshielded line can be set very close to the insulator CFO, so that large-amplitude flashes on long spans will cause a few tripouts rather than arrester failures. It is also practical to reduce the MCOV and TOV requirements in TLSA applications by inserting a series gap. This practice is well established for older silicon carbide (SiC) nonlinear elements. The major difference in gapped SiC and MOV arrester design is that: •
Gaps for SiC arresters need to interrupt tens or hundreds of amps of power-frequency follow current at line voltage
•
Gaps for MOV arresters need to interrupt tens or hundreds of milliamps of power-frequency follow current at line voltage
In this respect, the MOV arrester functions nearly identically to a polluted insulator. The conditions for arc re-ignition on polluted insulators in the current range of 10 mA to 1 A are well understood [Rizk Elektra 1969]. The peak voltage necessary for re-ignition, Ûcx, is given by: 71.6 x Uˆ cx = 0.526 iˆm Equation 5-2: Peak Voltage for Reignition of AC Arc
Where Ûcx is in kV, x is the arc length in m and îm is the peak current in amps. For a 0.1-A peak power follow current in a hot MOV arrester under power-frequency voltage after a lightning flash, the peak reignition voltage of a 1-m gap will be 240 kV. After a 20% reduction to ensure withstand, this 1-m gap would be suitable for system voltage of 240 kV. Lightning Insulation Coordination Arrester manufacturers publish discharge voltage data for each arrester design as a function of lightning current magnitude. This is done typically for currents ranging from 1.5 kA to 40 kA, using an impulse current wave shape of 8/20 µs. The 8/20 µs current wave shape has a front time to peak of approximately 8 µs and a decay time to half the peak value of 20 µs, measured from the start of the wave.
5-9
TLSA Selection and Specification
Obviously, some TLSA can be subjected to currents higher than 40 kA in actual service, but 40 kA is somewhere near the upper limit for most factory-test capabilities. While speaking of testing limitations, the 8/20 µs test current has little relation to the currents that flow in MOV arresters, as it was originally standardized to simulate the current that gives a 1.2/50 µs voltage when applied to a gapped silicon carbide arrester. Modern computer analysis programs take the values of arrester voltage for the 8/20 µs wave and build internal models of the nonlinear response, similar to the approach used to obtain insulation strength models (DE) for nonstandard overvoltages from the standard insulator volt-time curves. One of the principal functions of the TFLASH lightning program is to compute maximum arrester currents and evaluate protective levels and risk of arrester failure for a broad range of currents. For most transmission line applications, the arrester PL (protective level - the arrester IR drop) at 40 kA can be used along with a 15% margin over insulator BIL (derated 10% for poor weather conditions) for estimating purposes. Figure 5-5 shows how surge currents might split at a pole with an arrester on the top phase only. This is assumed to be a 230 kV line with a derated insulator CFO of 1100 kV and a TLSA with a PL of 800 kV at 40 kA. A hit of 48 kA on the top phase causes 40 kA to flow through the arrester into the 20 ohm ground below and 4 kA fo flow out the top phase in each direction. A current of 40 kA through this particular arrester is equivalent to an arrester resistance of 20 ohms, making the total resistance to ground 40 ohms. This creates a total top phase voltage of 1600 kV and an 800 kV drop across the arrester (27% below the derated insulator CFO). The topmost insulator does not flash over. However, the 1600 kV on the top phase travels left and right to adjacent poles as a voltage wave, and this voltage is sufficient to flash over top phase insulators on both adjacent poles if they are not also protected by arresters. In addition, if the crossarm is not insulated, the 1600 kV on the pole may be enough to cause one or both of the unprotected insulators to flash over. If TLSA are not applied properly, there is always a risk that they may just transfer flashovers to nearby poles or phases that have no arrester protection. Conversely, if TLSA are properly applied, they can provide excellent protection. TFLASH can make these complex evaluations.
5-10
TLSA Selection and Specification
Figure 5-5 Typical Lightning Current Distribution on an Unshielded Transmission Line with a TopPhase Arrester having R=20Ω at 40 kA.
An inspection of data from arrester manufacturers shows that for most transmission lines there is more than one arrester model that meets MCOV requirements while providing adequate insulation margins. A more accurate statistical prediction of insulator flashover may be obtained by other methods that are outlined in Section 4 of this application guide and are used in TFLASH EPRI line performance software. These methods are better suited for estimates of future line performance, and sensitivity studies of the effects of different MCOV choices on flashover performance can be evaluated with this software. Switching Surge Insulation Coordination Under some conditions, TLSA can also suppress high magnitude switching surges that might otherwise flashover a line. The data from manufacturers includes a switching surge protective level for each arrester MCOV. Switching current levels are generally in the 500-to-2000 ampere range and are typically modeled or tested with 36/90 µs or 250/2500 µs waveshapes, although actual switching surges vary widely. However, on a long line the potential energy created by switching does not all appear at a single TLSA; it is distributed non-uniformly along the line and is stored between the phases as well as between phases and ground [17]. Tests have shown [17] that spacing arresters on each phase of a long line about six to ten kilometers apart will effectively hold the line switching surge levels to values near the arrester discharge voltages. If arresters are more than ten kilometers apart, the voltages between arresters tend to rise to values 5-11
TLSA Selection and Specification
considerably higher than the arrester discharge voltages. There is a hydraulic analogy in a bag of water: Press down on one part of the bag, and the water will rise in another part. Calculations of the effectiveness of TLSA in suppressing switching surges on long lines can best be simulated by EMTP calculations or Transient Network Analyzer studies. If TLSA are distributed along a line in some very non-uniform fashion (for example, arresters located only in high ground resistance areas 30 miles apart) the magnitudes of switching surges will also be non-uniformly distributed, and either worst case or statistical distributions of surge magnitudes of voltage magnitudes must be made. On a short line of less than ten km, switching surges are usually limited by station arresters, and the statistical distribution of surges will be limited at the high end by the arrester discharge voltages at about 500 amperes. Once worst case or statistical distributions of surge magnitudes have been determined, the basic switching surge insulation level (BSL) of the towers must be established, using insulator data, test data, or design curves as in reference [1, Red 2006]. Tower switching insulation strength is usually derated about 10% below its fair-weather values to allow for bad weather, and the greater the number of towers stressed by the same surge, the weaker will be the combined switching surge strength of all the towers taken together. Refer to references [1, Red 2006] for procedures to follow.
Energy Capability of TLSA The MOV arrester limits voltage by conducting current and absorbing energy. The energy absorbed by the TLSA is the voltage times the current integrated over the duration of the surge. Because the voltage changes very little during a surge due to the non-linear characteristic of the TLSA described above, arrester energies are not proportional to I2t, as in linear resistive circuit elements. Instead, the energy can be approximated by the following: t
t
t
0
0
0
1+1 / α ∫ (VI )dt ≈ V ∫ I dt ≈V ∫ Idt = VQ
The amount of energy that an arrester can absorb reliably in a lightning flash without suffering irreversible damage or overheating is specified by the manufacturer. This value is generally specified in kJ per kV of MCOV. A value of approximately 2 kJ/kV MCOV is common for TLSA manufactured in North America. Higher ratings of 5 or 10 kJ/kV MCOV are needed for unshielded distribution circuits. Crafty engineers can take advantage of this by locating lowcost, heavy-duty arrester-protected distribution phases above transmission circuits on shared rights-of-way. Lightning Energy Lightning is often approximated as a current source in arrester energy calculations as described in the previous chapter. The resistance of the arrester has little effect on the amount of current flowing through it because it is only a small part of the impedance from cloud to ground. 5-12
TLSA Selection and Specification
Therefore, TLSA from 42 kV through 765 kV can be assessed for energy capability with identical sets of statistical lightning current and charge distributions. It also follows that the amount of energy an arrester will absorb is proportional to its voltage, which is approximately proportional to its MCOV. Therefore, although the arrester energy capability increases with MCOV, the lightning energy that it must absorb for a given stroke is higher by approximately the same amount. Accordingly, increasing the MCOV of an arrester in a given installation does not improve the lightning energy withstand capability for catastrophic failure. As will be discussed in later sections, increasing the MCOV may reduce arrester failures due to other factors, including thermal stresses after a lightning energy event. TLSA sold today are designed to balance energy capability with economic considerations, including purchase price, size, and weight on the structure. Very low lightning failure rates were reported in an EPRI survey on TLSA usage which covered over 28,000 arresters installed at system voltages of 44 kV through 345 kV. This is an indication that for most applications the available designs are adequate. Anecdotal failure information and calculations both indicate that lightning energy is of greater concern on unshielded lines, where the phase conductors collect direct strikes in much greater numbers and resulting arrester energies are sometimes above the energy rating given by manufacturers. Yet failure rates even for these applications appear to be lower than calculations predict using energy limits supplied by the manufacturers, indicating that these energy limits may be conservative. Laboratory testing by [EPRI, Ringler et al] also indicate that the average MOV block can absorb nearly three times the energy claimed by the manufacturers. Switching Energy Unlike lightning energy calculations for arresters, switching energy is influenced by the arrester rating, and manufacturers have published information on switching energy versus line length for various system voltages. This information is usually published only for station-class arresters, however, because these arresters have been designed to absorb switching energy for entire lines. Indeed, station arresters often have an energy capability more than four times-greater than that of TLSA. Multiple TLSA that are mounted close together on the same phase conductor of a transmission line do not necessarily share the energy of a switching surge. The nonlinear characteristics of the MOV elements cause TLSA with even minimally lower resistance to absorb nearly all the energy, leaving parallel TLSA with relatively high resistance to absorb almost none at all (As shown in Figure 5-6, Effect of small variation in reference voltage on discharge current). Since statistical variations in the arrester population are only known for station-class blocks [Green Barrett], engineers should adopt a conservative approach and assume that among TLSA of the same design, 70-100% of the energy from a switching surge will be absorbed by just one arrester, if the arresters are within a few spans of one another. < This section is weak and should be supplemented with an applet, showing how α affects the energy sharing. Also, up-to-date recommendations from manufacturers should be provided, as the use of TLSA to control switching surges is classed as a miss-application.> 5-13
TLSA Selection and Specification
Figure 5-6 Effect of Small Variation in Reference Voltage on Discharge Current
For many transmission lines, particularly long lines, station arresters must absorb a substantial portion of the high energies of switching surges. While high frequency components of the surge may be clamped by the local TLSA near the switching event, the low-frequency components of the surge, which contain the bulk of the surge energy, should be absorbed at the end of the line by the station arrester. These arresters must be coordinated with the TLSA. The resistance of the station arrester must be sufficiently low to ensure that the energy is transferred to the station arrester and not to the TLSA. Coordinating the switching surge energy capability of the gapless, or shunt-gapped, station MOV arresters with the gapless MOV TLSA is straightforward. It involves comparing the published switching-surge PL data for the TLSA to the published switching surge PL for the station arrester in the substation at the end of the line. The station arrester switching-surge PL should be at least 10% lower than the switching surge PL of the TLSA. To coordinate the switching surge energy capability between the TLSA and gapped siliconcarbide (SiC) station arresters, engineers must ensure that the sparkover of the SiC arresters is not prevented by the TLSA. This requires that the station arrester sparkover level be at least 15% lower than the switching-surge PL of the TLSA. This should account for any upward drift of gap sparkover levels with age. If this margin cannot be achieved, assume that the switching energy will be absorbed by the TLSA and select a TLSA design that is capable of withstanding the switching energy produced on that particular transmission line. Alternately, the SiC arrester could be replaced with a suitable MOV unit. When an arrester becomes conductive due to internal failure, fault current is initially conducted through the arrester housing, causing internal pressure to build. If the current is sufficiently high and of long enough duration, the housing will burst or vent. This usually results in the formation of an external arc, thus relieving any further increase in pressure. TLSA are designed to remain intact on the structure when conducting fault current due to failure, thus minimizing danger due to flying or falling debris. The fault withstand capability (maximum magnitude and duration of fault current that a TLSA can safely withstand) is specified by the manufacturer and 5-14
TLSA Selection and Specification
demonstrated by design tests [14]. This capability is demonstrated for a single fault event. A second fault through the same arrester may cause a more violent event, but this scenario will normally be prevented by the operation of the arrester disconnect. TLSA fault current withstand capability should be higher than available fault current and maximum breaker clearing times at all structures where TLSA are applied.
Arrester Failures Careful selection of TLSA for application on transmission lines can prevent many arrester failures. However, no transmission component will last indefinitely. Even the most reliable components such as polymer insulators have annual failure rates of about 0.5-1% per 10,000 units [EPRI APhillips Guide]. As discussed earlier, TLSA are exposed to a number of stresses that could lead to failure, including: •
Handling load damage
•
Installations that restrain conductor movement, causing broken leads
•
Housing damage, weathering, and possible water ingress
•
Vibration fatigue damage to flexible components
•
Excessive energy input from lightning
•
Excessive energy from system events, such as switching or TOV
•
Manufacturing defects
Arrester failures, though expected at the same rate as other line components, must be kept to a minimum if TLSA are to improve the reliability and performance of a transmission line. Since the most frequent TLSA failures are those associated with flexible components, one approach favored by at least three manufacturers is to eliminate these components through the alternative, using series air gaps to the isolate faulted arresters. Engineers must realize that even if all arrester failures, other than energy failures due to lightning, were eliminated, they would still need to analyze statistically the likelihood of failures caused by lightning strikes, which can range in magnitude and duration from the benign to extremely high values exceeding 200 kA. It can be economically impractical to design TLSA to withstand every stroke. In these cases, as mentioned above, coordination of insulator flashover and TLSA protective levels can lead to an acceptable, non-zero lightning tripout rate along with satisfactory energy duty. A similar philosophy is recommended [IEEE 1243] for overhead groundwire placement, with a recommended design goal for a shielding failure outage rate of 0.05 tripouts per 100 km per year. Electrical Failure Modes When TLSA are exposed to energies beyond their capability, shorted by water ingress, or have damaged or defective valve elements, they will fail in a low-resistance mode. This occurs because the TLSA become conductive at power frequency voltages and, therefore, cause a line5-15
TLSA Selection and Specification
to-ground fault, which requires a breaker operation to clear. The TLSA failure is non-selfrestoring and will cause a line lockout if the arrester is left connected to the line. Arrester Disconnects To prevent the problem of line lockout, TLSA currently manufactured in North America are equipped with a frangible link, called a "disconnect," which separates the arrester from the line if the arrester is internally faulted. This disconnect does not clear the fault current but does allow successful reclosure after the breaker operation. European and Japanese practice relies increasingly on series air gaps rather than flexible disconnects to provide this isolation. Manufacturers can supply curves of fault current magnitude and duration versus disconnect operation. Unless the available fault current on a transmission line and the corresponding breaker setting is very low, disconnect operations on transmission lines should be as reliable as they are on distribution lines. Disconnects for distribution-class line arresters normally operate at a charge level of about 10 coulomb [Lat CEA DistnArresterGuide]. The prototype MOV TLSA in Figure 5-2, and TLSA sold in Japan and Europe, have no flexible disconnects. Instead, they have large external gaps that sparkover to the arrester elements under lightning, but not switching, overvoltage conditions. If an externally gapped arrester fails, each surge induced sparkover causes a breaker operation, but line lockouts are prevented because the external gap is sufficient to withstand normal power frequency voltage. When an arrester fails and power frequency current passes through the disconnect, an explosive charge is ignited that shatters the disconnect housing. The disconnect may be supporting either the arrester or a connecting lead, which then swings or falls clear. An air gap, sufficient to hold off power frequency voltage, is thus produced between the failed arrester and the line conductor. If examination of a failed disconnect indicates that the explosive charge has not operated, then it is likely that the arrester is sound and can be put back in service with a new disconnect. An arrester with a disconnect that has operated should not be reenergized because of the danger of an internal short, which could cause a violent arrester failure. Mechanical failures of the disconnects when there is no fault in the arrester produce the most common nuisance situation in which the arrester is unnecessarily taken out of service. Utility engineers also should be aware of the mechanical capabilities of TLSA disconnects and should respect these ratings when restraining conductor motion. Forces on transmission spans are considerably large than those on distribution spans, where the disconnect technology was first applied. Another nuisance situation occurs when surge currents set off the explosive charge and cause a disconnect operation when the arrester has successfully withstood the surge event. Energy tests on arresters were performed at the EPRI Power Delivery Center and by [Darveniza et al multipulse]. In these tests, there were many cases where disconnects operated as a result of the surge current even though there was no power-frequency source was connected to the arrester. In all of these cases the energy level absorbed by the arrester was in excess of the manufacturer's 5-16
TLSA Selection and Specification
rating, but in most cases the arrester went on to withstand energy surges of much higher magnitude. Figure 5-7 shows that typical 63-mm diameter MOV blocks can absorb up to 20 coulomb, well above the 10-coulomb charge needed to fire a distribution-class disconnect. Manufacturers should be consulted on the high-current and charge capabilities of disconnects, particularly for TLSA application on unshielded lines.
Figure 5-7 Comparison of 63-mm MOV Block Charge Absorption at Destruction with Firing Level for DLSA disconnect [Lat CEA Guide, CEATI 3312A, permission or substitute needed]
In normal operation the TLSA disconnects are in a load path that may be required to support the entire or partial weight of the arrester, or they may be required only to support the connecting lead, depending on the mounting configuration. The mechanical stresses to which disconnects are exposed is highly dependent on such factors as the arrester weight, the mounting geometry, wind exposure, and arrester handling. Appendix D can be used to assist in load calculations for some common installation configurations. There have been reports of mechanical failures of disconnects by surveyed EPRI member utilities. Most of these occurred on earlier TLSA designs. To investigate possible causes of disconnect failures, tests were conducted at the EPRI Power Delivery Center in Haslet, Texas (PDC-H). Disconnects were tested for ultimate strength in tension and in shear. Detailed information on test setups and results are given in Appendix A. As expected, disconnects are the weak link in the load path. The ultimate strength in tension and shear was between 1000 lbs. and 2000 lbs. for all samples, compared to housing strengths on the order of 5000 lbs. These figures should be adequate as long as the strength of TLSA assemblies is not compromised by excessive handling, maintenance or installation loads and the arresters are never configured in ways where they will restrain conductor motion. However, Chisholm suggests that other links in the gapless arrester integrity are not as robust. T he ring tongue pull-out with 4/0 conductor is rated at 140 pounds, or 400 pounds with crimp connectors that meet MIL-T-7928. Transfer of bending moment to the arrester mounting studs, breaking the stud at the top of the arrester, is another common problem. 5-17
TLSA Selection and Specification
At Haslet, fatigue tests were also performed in order to determine susceptibility to failure of disconnects due to Aeolian vibrations of line conductors. Again, results are presented in detail in Appendix A. The results of tests on these two designs indicate that vibration should not cause tension fatigue failure on sound disconnects with these designs within a reasonable lifetime. Shear fatigue tests were not performed. Since data indicate that disconnects are adequately designed for installed mechanical loads, the greatest opportunity for mechanical compromise may be during storage, transport, installation, and maintenance. In order to prevent nuisance mechanical failures of disconnects, it is recommended that they be given special treatment during these vulnerable periods. Any arrester with a disconnect mounted to it should not be stood on the disconnect end in storage or transport. Disconnects that show cracks, chips or other signs of mechanical shock or damage should be replaced before the arrester is installed. The arrester should never be suspended from, or swung by its disconnect lead. When installing arresters, do not hang them unrestrained from the disconnect end before securing the arrester, as this may subject them to excessive bending and torsional loads. Do not hang tools or equipment or support body weight on either the arrester or a lead connected to the disconnect. Use the packaging supplied by the manufacturer for shipping and transport.
TLSA Housings TLSA construction is similar to that of non-ceramic insulators (NCI). They are typically constructed with line and ground end fittings connected by a fiberglass reinforced core that provides the mechanical strength. Unlike insulators that use solid, pultruded fiberglass rods, the fiberglass core is hollow and surrounds the MOV blocks and internal hardware. Generally, the core is not continuous but consists of multiple sections attached with metal hardware. Surrounding the core is a weathershed that is essential in maintaining the insulating properties of the arrester at normal voltages. Just as is the case with NCI, any compromise of the housing will allow moisture to penetrate the housing, possibly causing failure. Weathershed materials are typically EPDM rubber / silicone alloy or silicone rubber with alumina trihydrate filler. Various external attachment hardware complete the tower installation, including the arrester disconnect, flexible lead and duckbill clamp for conductor mounting. TLSA for line voltages above 230 kV require grading rings to avoid corona, to maintain satisfactory voltage distribution on gapless MOV disks or to provide series gaps for the preferred gapped installations. These rings are typically designed and provided by the TLSA manufacturer for a specific tower installation configuration. In icing conditions, galloping causes torsional conductor oscillations at frequencies that can excite the natural pendulum frequency of the arrester. The arrester makes a poor detuning pendulum [Havard EPRI ref] because it cannot withstand the significant bending moment at the attachment stud. Arresters should not be hung from the conductor in areas where galloping may occur. [Chisholm 3312A, INMR 2005]. 5-18
TLSA Selection and Specification
TLSA Installation and Handling Arrester installations vary considerably for different tower configurations. The arrester manufacturer can be consulted to suggest a TLSA installation configuration for a given tower design. When installing TLSA on an existing structure in ideal situations, electrical clearances will be maintained. Some clearance compromise may be acceptable to the design engineer because surge voltages are limited by the presence of the TLSA, but line workers have been slow to accept this compromise if there is no way to verification that the arrester is functioning properly. The designer should also consider the impact of the TLSA installation on live working practices, if any. Mechanical loading on TLSA and other tower components also should be considered for a TLSA installation. Appendix D can be used to assist in mechanical load calculations for some common installation configurations. These configurations include the following: 1. Arrester parallel to line insulator, vertical installation 2. Arrester parallel to line insulator, horizontal installation 3. Arrester mounted to structure - pole or tower mast 4. Arrester hung from the conductor 5. Arrester hung from the crossarm Two load cases were considered for each configuration: 1. Service (or everyday) loads 2. Ice loads A geometric analysis for the static load cases was performed for each configuration. In general, TLSA mechanical loads are usually modest compared to those carried by insulators of similar dimensions since arresters are not required to carry conductor loads. Because the arresters are relatively heavier than comparable non-ceramic insulators, loads do increase for higher voltage designs. In order to determine TLSA mechanical capabilities, tests were conducted at PDC-H. Arrester assemblies were subjected to breaking loads in tension and bending, and the results of these tests are presented in Appendix A. The data gathered in these tests indicate that the arrester housings tested have adequate strength for in-service loads. The assemblies were considerably weaker in bending than in tension, so cantilever installations should be avoided with these designs. The loads to which TLSA are subjected in transport and handling are much harder to predict than in service loads. Similar products, such as non-ceramic insulators, have different construction and may withstand rougher handling than TLSA. Practices of line crews and storage facilities should be evaluated to determine that manufacturer recommended practices, if any, are being followed.
5-19
TLSA Selection and Specification
Handling and Installation Recommendations Based on the results of the mechanical testing program and discussion with manufacturer and utility personnel, the following recommendations are made for handling and installing TLSA: Handling 1. Follow manufacturer's instructions when available. 2. The ceramic internal MOV blocks used in TLSA are brittle. Do not drop, throw, or otherwise shock the housing. 3. Do not modify the manufacturer-supplied shipping crates for transport. 4. Before unloading, carefully inspect shipping containers for damage that may have occurred during shipping. 5. Unpack TLSA from packing materials in such a manner that the outer weathershed housing of the TLSA is not cut or torn. 6. When out of container, TLSA should rest flat on the ground void of rocks, abrasive or chemical products. Ground cover should be used-when possible or in cases of difficult ground. 7. Do not wash TLSA using solvent based materials. 8. Do not subject TLSA to unnecessary compression, torsion, vibration, or bending loads (other than natural bending due to its own weight before installation). 9. TLSA are not designed for cantilever loading. Be gentle in handling them. Hoist TLSA into position by attaching straps and slings to one metal end fitting only (do not make attachments to the rubber-coated housing or weathersheds). Exercise extra care when long units are involved. Installation and Maintenance 1. Follow manufacturers' instructions when available. 2. Inspect TLSA for cuts, abrasions, or core exposure before installation. Do not install any unit showing damage to the sheath, cut sheds, or exposed core. 3. Exercise care not to unduly bend or twist the arrester during installation. 4. TLSA should never be used as anchoring points for safety belts, or tools. 5. Never climb TLSA or hang onto flexible leads. Arrester Markings TLSA installation should be performed only after inspecting the arrester for obvious defects and checking nameplate information. Arresters that are misapplied at voltages higher than the MCOV on the nameplate may fail when energized. Extra care should be taken when different arrester models are being installed simultaneously or when different arrester models are stored or 5-20
TLSA Selection and Specification
transported together. Arrester failures are nuisances at best and possibly dangerous to anyone in the immediate vicinity. When specifying arresters, engineers should require that the following markings be placed on the arresters, and this information should be checked before the arresters are installed: •
Manufacturer
•
Model Number
•
MCOV
•
Date of manufacture
Before arrester installation, engineers should check to see whether the TLSA MCOV is above the maximum line-to-ground voltage of the system. If it is not, the arrester should not be installed. The maximum line-to-ground voltage is determined by the voltage control on the system and is generally less than 1.1 p.u. nominal for solidly grounded systems. Table 5-1 External Gap or MCOV Requirements for TLSA Nominal LineGround Voltage (rms)
Nominal Air Gap to Interrupt 100 mA peak in Half Cycle
42 kV
24.2 kV
135 mm
25.5 kV
27 kV
44 kV
25.4 kV
140 mm
27 kV
28 kV
69 kV
39.6 kV
220 mm
42 kV
44 kV
100 kV
57.7 kV
320 mm
61 kV
63.5 kV
115 kV
66.4 kV
370 mm
70 kV
73 kV
138 kV
79.7 kV
445 mm
84 kV
88 kV
161 kV
93 kV
520 mm
98 kV
102 kV
230 kV
133 kV
740 mm
139 kV
146 kV
275 kV
159 kV
890 mm
167 kV
175 kV
345 kV
199 kV
1110 mm
209 kV
219 kV
500 kV
289 kV
1610 mm
303 kV
318 kV
765 kV
442 kV
2470 mm
464 kV
486 kV
Nominal LineLine Voltage (rms)
Minimum MCOV Required for Voltage Control Within 1.05 p.u. L-G
1.1 p.u. L-G
5-21
6 PLACEMENT OF ARRESTERS FOR IMPROVED LIGHTNING PERFORMANCE
Economics The optimum lightning performance of a transmission line could very likely be attained if an arrester were installed on every tower and every phase. This trivial solution would, in most cases, involve a prohibitively large capital investment. However, in many cases, it is possible to prevent most flashovers by placing fewer arresters at optimum locations. The efficient application of TLSA to improve line performance requires the investigation of all available mitigation options and the weighing of the performance benefits of each against real cost. Estimating the effects of changes in structure design, shielding, grounding, and arresters on the lightning performance of transmission lines is a necessary step in this process. Before making these calculations, however, a basic knowledge of options to investigate in various situations is essential, for it can reduce the number of iterations and, therefore, the total design effort.
Backflashover Protection Backflashover is an insulator flashover that results from elevated tower voltage rather than elevated phase voltage. As described in Chapter 3, the tower voltage is a function of three parameters: the magnitude and wave shape of the lightning current traveling down the tower, the tower surge impedance, and the tower ground impedance. However, because an insulator will flash over if there is a sufficiently high voltage across it for a sufficiently long time, the operating and induced voltage of the individual phase conductors is also of interest. Phase Location of TLSA Phase insulators are more or less susceptible to flashover from lightning surges, depending on the location of the conductor. TLSA located on some phases can help protect other phases from flashover. The following subsections describe how conductor location can be used to locate the best position(s) for TLSA on a tower. Coupling to Overhead Groundwires The voltage of the phase conductor does not remain constant when either the overhead groundwire or the tower top receives a lightning stroke. The overhead groundwire voltage is 6-1
Placement of Arresters for Improved Lightning Performance
coupled to the phase conductors according to the coupling factor as described in [1, Red 2006]. The voltage of the phase conductor is affected by the portion of the current flowing outward on the shield wires according to the coupling coefficient Kn. Neglecting other voltages imposed on conductor n, such as power frequency voltages and reflections from adjacent towers or the tower base, we have: Vn = K n ⋅ Vt =
Z n1 + Z n 2 ⋅ Vt Z 11 + Z 12
Where: Vn is the voltage of the nth phase conductor Vt is the voltage of the tower top, Kn is the coupling coefficient, Zn1 and Zn2 are the impedances from the phase conductor to the images of the overhead groundwires 1 and 2, Z11 is the self surge impedance of the groundwires and Z12 is the mutual impedance between groundwires, where Zab = 60 ln [ (distance from conductor b to image of conductor a )/ (distance from conductor b to conductor a) ] As Kn increases toward unity, the voltage across the insulation for conductor n decreases because the conductor voltage more closely follows the tower voltage. The coupling coefficient Kn can be calculated for all phase conductors on a tower. Again, from [1,Red 2006], for a tower with two shield wires at equal heights above the ground: The equation for a single overhead groundwire is obtained by setting Zn2 and Z12 to zero. When there are three or more shield wires, as in the case when one of the phases is protected by an arrester, then the coupling coefficient is obtained by: •
Inverting the square matrix of self (Zii) and mutual Zij surge impedances, a 4x4 matrix when there are three shield wires
•
Summing the admittance values Yn1…Ynn for the undriven conductor
•
Dividing this sum by –Ynn to obtain the coupling coefficient Kn.
Appendix 12.3 of [1] and < presumably > Applet TLSA-1 carry out these computations and the help file for TLSA-1 gives further details. In all cases, the magnitude of Kn for each unprotected phase conductor differs by the ratios of their mutual impedance to the shield wires. This mutual surge impedance is Zab = 60 ln [ (distance from conductor b to image of conductor a )/ (distance from conductor b to conductor a) ] While not rigorously proportional because the image distance, a,,, changes slightly with phase location, the mutual surge impedance, and therefore the coupling coefficient, is nearly inversely proportional to the distance between the phase conductors and the shield wires. Therefore, on a shielded line that is to be protected from backflashover: •
6-2
The phase conductors farthest from the overhead groundwires will have the highest-stressed insulation and will be statistically more prone to flashover.
Placement of Arresters for Improved Lightning Performance
•
Adding arresters to the phase conductors farthest from the overhead groundwires will make the largest improvement in coupling coefficient to the unprotected phases, boxing them in a sort of Faraday cage.
This usually means that bottom-phase arresters of double-circuit lines are the best candidates for TLSA application, provided that the overhead groundwires are to be retained. Crossarm Voltage On towers with multiple crossarms, the voltage of a particular crossarm is a function of its height. The voltage is often calculated as the linear interpolation of the voltage between the tower top and the tower base. Figure 6-1 is a plot of the voltage as a function of time at various points on a vertical double-circuit tower [Red 2006]. In this calculation, the lower crossarm has the lowest voltage.
Figure 6-1 Plot of Voltage versus Time at Various Points on a Double-Circuit Tower using L-5 Applet and Step Waveshape (final to use CIGRE concave)
Figure 6-2 is a graph of insulator voltages for the same tower shown in Figure 6-1, but with the relative coupling from the overhead groundwires taken into account. Even though the lower crossarm has the lowest absolute voltage, the bottom phase has the greatest insulator stress because of the reduced coupling coefficient. Therefore, on this vertical double-circuit tower, an arrester should be installed on the lowest phase(s) first. Then, if line-performance calculations indicate that further performance improvement is desired, the engineer could model an arrester on the middle phase. < Figure to be prepared from Applet > Figure 6-2 Plot of Voltage versus Time at Various Points on Double-Circuit Tower, Taking Into Account Relative Coupling from Shield Wires (final to use CIGRE concave)
Applying arresters to the bottom phase of a tower has an additional effect on performance: A phase conductor with an arrester approximates an additional shield wire on the structure. The 6-3
Placement of Arresters for Improved Lightning Performance
lower phase-conductor voltage is constrained to be no greater than the crossarm voltage minus the PL of the arrester. The PL voltage can be relatively low in magnitude compared to the towertop voltage. The coupling coefficient to the middle and top phases is, therefore, increased because of the proximity of the lower phase wire, which acts as an additional shield wire. As a result, the two remaining phase conductors are less likely to flash over. Similarly, applying arresters to the outer phases of a horizontal circuit increases the coupling to the middle phase, thus reducing the incidence of back flashovers on that phase. TLSA Location for High Tower Footing Resistance In areas of high soil resistivity it is often expensive to attain tower ground resistances low enough for acceptable lightning performance. In these areas, a high percentage of strikes to the shield wires will cause back flashovers on the line. Where the soil is rocky, or where there are exposed ridge crossings with towers on bare rock, counterpoise installation may be impractical and may not provide satisfactory results. In such situations, TLSA may provide an economical solution, for they can eliminate a relatively high percentage of outages with a relatively small investment. Consider a section of line consisting of 10 steel lattice towers with one horizontal 138-kV circuit. The tower footing resistances, as measured with a low frequency technique, are indicated in Table 6-1. Table 6-1 Footing Resistance at Steel Lattice Towers along Hypothetical 138-kV Transmission Line Tower
1
2
3
4
5
6
7
8
9
10
Ohms
25
35
20
205
250
100
155
30
35
25
Fault location relays indicate that while the entire 100 mile line experiences approximately 12 flashovers/year that are associated with lightning, eight of those flashovers involve these ten towers. An inspection reveals that the tower grounds are intact, but the resistances shown in Table 6-1 are much higher than the levels reported when the line was built. Adding counterpoise at these towers would lower the resistances, but it would involve transporting the necessary digging equipment four miles over difficult terrain. Adding arresters to all three phases on towers 4,5,6, and 7, where resistance is highest, seems a good, economical alternative, but it would not reduce the lightning flashovers as much as desired. To achieve acceptable performance, TLSA should also be placed on towers 3 and 8, which are the first structures on each side of the ridge that have low footing resistance. The following discussion shows why placing TLSA on these structures is important. Figure 6-3 is a qualitative depiction of traveling waves on the left span when a 50 kA stroke hits Tower 5. This simplified schematic ignores some voltage reflections and tower surge impedances and assumes long spans. 6-4
Placement of Arresters for Improved Lightning Performance
Figure 6-3 Schematic of Traveling Waves Propagating Towards a Structure with Low Footing Resistance: If Tower 3 has no arrester, it may flashover.
The stroke current of 50 kA hitting Tower 5 divides, part going out the shield wires in both directions, part going down the tower into the ground resistance, and part flowing through the conducting arrester into the phase wire where it again splits and goes in both directions. Neglecting the influence of the tower surge impedance (which is small at the crest of a double exponential wave, anyway) the current divisions are roughly those shown. To prevent the phase insulator at Tower 5 from flashing over, the arrester brings the voltage on the phase up to within 400 kV of the crossarm potential, thereby putting a 3.1 MV surge on the phase wire. This wave and the 3.5 MV wave on the shield wire travel together to Tower 4. At Tower 4 the voltage wave on the shield wire sees a 170 ohm resistance to ground, which is much less than the shield wire 400 ohm surge impedance, so the tower top voltage at Tower 4 tries to fall to a value much lower than the incoming 3.5 MV and lower than the 3.1 MV coming in on the phase wire. This causes the arrester on Tower 4 to conduct backwards, bringing the voltages headed toward Tower 3 to nearly the same potential. At Tower 3 the voltage on the tower is pulled down nearly to zero (157,kV), whereas the voltage on the phase remains at 2.1 MV. If the BIL of the insulator is lower than 2100-157 = 1943 kV, the insulator may flash over, unless arresters are installed on this tower also. Beyond Tower 3 arresters are not needed because a Tower 3 arrester will have pulled the phase voltage down below the insulator flashover value. When installing arresters on towers with high footing resistances, engineers should keep in mind that arresters should also be installed on the first structure in each direction that has low footing resistance (in this case Tower 3 in Figure 6-3). Line performance software can be used to analyze situations where footing resistance reduces gradually.
Unshielded Applications The lightning performance of unshielded transmission lines can be improved dramatically by the use of TLSA. There have been numerous instances in which TLSA have replaced the overhead shield wire on the top phase of delta and vertical phase arrangements. These applications generally have been on transmission lines of 69 kV and below, but there is considerable interest 6-5
Placement of Arresters for Improved Lightning Performance
in retrofitting higher-voltage lines that were constructed without shield wires because of relatively low lightning activity. Arrester Energy Lightning-caused energy failures of TLSA are of highest concern on unshielded lines. This is because TLSA mounted on phase wires on unshielded lines are required to absorb much more lightning energy than arresters mounted on shielded lines. The reason is that phase wires on unshielded lines are subject to many more direct strikes than those on shielded lines thus subjecting TLSA to more lightning energy. In 1994, Williams [15] conducted an Electromagnetic Transients Program (EMTP) computer study to find out how much more lightning energy is absorbed by TLSA on unshielded lines. He found that arresters on unshielded lines absorb energies that are approximately an order of magnitude higher than those on shielded lines. In his study, Williams tested identical TLSA, each with an MCOV of 42 kV and a switching surge energy rating of 92 kJ. The study modeled TLSA on the phase wire at a tower with a 50-Ω footing impedance and subjected both shielded and unshielded cases to an identical, simulated stroke of lightning current. With a single 50 kA stroke, Williams reported, the arrester on the shielded line absorbed approximately 9 kJ, while the arrester subjected to a stroke directly on the phase wire absorbed approximately 150 kJ. Although the difference is dramatic, it may not be a cause for alarm. When an arrester absorbs energy above its rating, it does not necessarily fail. Although calculations indicate that a large number of arresters on unshielded lines are subjected to high energies with some frequency, the reported failure rates are low. This anecdotal evidence suggests that installing arresters on unshielded lines is a sensible approach, and many such lines are now in service. However, in order to incorporate arrester failures on unshielded lines into line-performance estimates, the issue of failure probability needs to be addressed. For that reason, data from tests performed at the EPRI Power Delivery Center - Lenox (PDC-L) in late 1996 are incorporated into Appendix B. These data were be used to generate arrester-failure probability curves that are now used the TFLSAH software. Vertical Circuits Phase arrangements in which the top phase conductor provides a good shielding angle for phase conductors below it are solid candidates for top phase arrester applications. Applying arresters to the top phase of every tower effectively prevents flashovers of the top phase, and if the shielding angle is adequate, the lower phases are protected from direct strikes. Shielding angles of 20° or less are ideal, but greater shielding angles may still yield a substantial improvement in lightning performance (See Figure 6-4). If repositioning the phase wires is possible, for instance when designing a new transmission line, and top phase arresters are to be applied, then an electrogeometric model should be used to position the conductors. This process is similar to that suggested for a shielded transmission design.
6-6
Placement of Arresters for Improved Lightning Performance
Figure 6-4 A Schematic showing the Shielding Angle on an Unshielded Transmission Line with the Top Phase protected by TLSA
Performance benefits derived from the application of arresters to the top phase of a transmission line are highly dependent on the footing resistance of the towers. The arrester conducts lightning current from the phase wire to the tower in order to maintain low voltage between the top phase and tower top. This current must then flow through the tower ground, and if resistance is high, the tower potential will rise. The other two phases will then be subjected to back flashovers just as on any shielded transmission line. The lightning performance of a line with an arrester on the top phase conductor at each tower, and with an adequate shielding position, should be comparable to or even better than a similar line with a shield wire and no arresters. There are three reasons for improved performance: 1. Lightning flashovers are virtually eliminated on the top phase conductor, thus leaving only two phases subject to back flashovers instead of three. 2. A struck top phase conductor will have a higher potential than an equivalent shield wire due to a voltage equal to the PL of the arrester. A struck top phase conductor is also generally closer to the lowest phase wire than a shield wire would be. These two factors increase the coupling voltage to the other phase conductors and reduce the chance of back flashovers on these phases. 3. The elimination of the overhead groundwires allows the tower height to be reduced, thus reducing the line shadow width slightly and, therefore, the number of strikes to the line. However, there are two factors that may reduce line performance in comparison to a similar line with a shield wire and no arresters: 1. The number of induced flashovers due to near misses may be higher. 2. The energy failure rate for arresters is statistically higher on unshielded lines, although the actual number of failures still may be low.
6-7
Placement of Arresters for Improved Lightning Performance
Induced flashovers are insulation failures resulting from nearby lightning strikes that terminate on trees, light poles, buildings, or the ground. Shield wires protect transmission lines from induced flashovers by conducting induced current through the tower to the tower grounds. These currents are beneficial because they couple the induced voltage to the phase wires, thereby reducing the voltage across the tower insulation. The resistance of the TLSA installed between the top phase wire and the tower top will reduce the magnitude of the shielding currents. Therefore, the number of induced flashovers caused by near-miss lightning strikes may be higher. Induced flashovers become statistically rare when the CFO of the tower insulation system is above 500 kV. As such, induced flashovers are usually statistically rare on transmission lines of 115 kV and above. Another advantage resulting from the replacement of shield wires with top phase arresters is the elimination of shield wire losses. These losses are negligible if the shield wires are isolated from the tower by insulators, but they can be substantial at full load if they are not isolated, as described in (Red 2006). Shield wire losses should be compared to arrester losses, which are continuous but small. In many cases, depending on load variations, arrester losses are less than non-isolated shield wire losses. Deciding whether to install top phase arresters or shield wires is a question of balancing the economic considerations of the two options against the resulting like performance. Engineers must compare the cost of installing and maintaining an arrester on each pole to the cost of stringing and maintaining shield wires, along with the cost of losses for both options. In addition, they must confront the problem of predicting both maintenance costs and line performance for a line with arresters, a problem made more difficult in view of the question about failure rates noted earlier. In any case, line performance software is a valuable tool for comparing the various design options and arriving at a decision. Horizontal Circuits Unshielded horizontal circuits can also benefit from the application of TLSA. These differ from vertical circuits in that both outside phase wires, while exposed to direct lightning strikes, often provide the center phase with a high degree of shielding from direct strikes. Only descending leaders that are directly over the line are likely to hit the center phase. Table 6-2 shows the strike frequency calculated for a 60-foot-high transmission line at two different phase spacings. The table was generated with the electrogeometric model in the EPRI Lightning Protection Design Workstation, Ver. 3. Table 6-2 Flash Incidence for 161 km (100 miles) of a Horizontal Circuit, 18 m (60 feet) above Flat Terrain with Ground Flash Density of 3.9 per km2 per year
6-8
Phase Spacing (ft.)
Total Flashes to Line
Flashes to Left and Right Phases
Flashes to Center Phase
% of Flashes to Center Phase
20
107
103
3.8
3.6%
10
103
101
1.9
1.9%
Placement of Arresters for Improved Lightning Performance
Table 6-2 data indicate that protecting the outside phases of a flat horizontal circuit with TLSA could be an effective means of improving overall lightning performance. As with all transmission line designs, detailed studies using line performance software should be conducted and other mitigation options should be evaluated so that the most economical measures can be employed.
Transmission Lines over Unchanging Terrain When long stretches of transmission lines are located over uniform terrain with little or no differences in GFD, natural shielding, soil resistivity, or tower geometry, designers have the challenge of optimizing a few tower designs for optimum performance at minimal cost and then applying them along the length of the line. Design options include applying TLSA periodically to every second or third tower or protecting a vulnerable phase on every structure. When applying the first option, it is assumed that an arrester installed on a structure will afford some protection to adjacent structures. Arresters do provide adjacent structures some protection from flashovers caused by low frequency overvoltages such as switching surges and line drops. However, when a lightning stroke hits a line, TLSA provide very little protection for adjacent structures. This is because the impedance to ground through that remote arrester includes the surge impedance of the transmission line between the structures. This surge impedance is on the order of 400 Ω in each direction. If a typical stroke hits a phase wire near a tower that is a span away from the nearest arrester on that phase, the prospective voltage V will be in addition to the arrester PL:
V = IZ ≈ 30kA ⋅ 200Ω ≈ 6MV This voltage is beyond the withstand capability of most transmission insulation. Similarly, if the shield wire is struck, the 400-Ω surge impedance of the shield wire and the return path along the phase wire predominates over the few ohms of resistance from the TLSA at the next tower. There is minimal protection against backflashover at the stricken tower. As a first approximation, applying TLSA on half the structures on a uniform line will prevent no more than half the flashovers that the same TLSA on every structure would prevent. It is often more effective to apply TLSA to every tower for the most vulnerable phase for a portion of the line than to spread the arresters uniformly over the entire line. Case studies for both shielded and unshielded lines over uniform open terrain are included in Appendix C. A summary of these cases is included in
6-9
Placement of Arresters for Improved Lightning Performance
Table 6-3 Flashover Data for 161 km (100 miles) of Unshielded Transmission Line for Various TLSA Installations
No Arresters Nominal Voltage (kV rms)
69 kV
Tower material
Steel
Tower footing resistance (ohms) for all towers
25 Ω
Crossarm material (unbonded without arresters)
Wood
Ground Flash Density, Flashes per km2 per year
3.9 / km2 / yr (10 / mi2 / yr)
Line Length
Top-Phase Arrester at Every Tower
161 km (100 mi)
Arrester Locations
None
ABC every 2nd pole
A every pole
Arresters per mile
0
26.4
17.6
Direct Flashes / yr Direct-Flash Flashovers / yr
6-10
Arresters All Phases, Every Second Tower
101 100.75
49.8
18.5
Induced Flashovers / yr
1.08
0.01
0.08
Total Flashovers / yr
101.8
49.8
18.6
Placement of Arresters for Improved Lightning Performance
Table 6-4 Flashover Data for 161 km (100 miles) of Vertical Circuit, Steel Pole, Shielded Transmission Line for Various TLSA Installations
No Arresters
Arresters All Phases Every 2nd Tower
1 Bottom Phase Arrester Every Tower
2 Bottom Phase Arresters Every Tower
Nominal Voltage (kV rms)
69 kV
Tower material
Steel
Tower footing resistance (ohms) for all towers
25 Ω
Ground Flash Density, Flashes per km2 per year
3.9 / km2 / yr (10 / mi2 / yr)
Line Length
2 Bottom Phase Arresters Every 2nd Tower
161 km (100 mi)
Arrester Locations
None
ABC every 2nd pole
B every pole
BC every pole
BC every 2nd pole
Arresters per mile
0
26.4
17.6
35.2
17.6
Direct Flashes / yr
108
108
108
108
108
Direct-Flash Flashovers / yr
34.1
19.4
18.5
9.9
22.4
Induced Flashovers / yr
0.08
0
0.08
0.08
0.08
Total Flashovers / yr
34.2
19.4
18.6
10.0
22.5
Compact Transmission Lines A compact line is defined as a line that takes advantage reduced phase-to-phase spacing, with reliance on control of overvoltages through other means such as surge arresters or closing resistors in circuit breakers. It can be argued that any extra-high voltage line is “compact”, since most designs at 345 kV and above have higher phase-to-phase voltage stress per meter of phase separation than lines built for system voltages below 300 kV. Compact Transmission Lines with Overhead Groundwires and TLSA One direct consequence of reduced phase spacing is that the insulator dry-arc distance in compact lines is also reduced. In contaminated areas, additional sheds or deeper skirts can add the necessary leakage distance, but large-diameter sheds do not increase the metal-to-metal dry arc distance very much. This means that compact lines built with 115-kV dry arc distances and operated at 230 kV will generally had the lightning performance of 115-kV lines. 6-11
Placement of Arresters for Improved Lightning Performance
< Insert table of lightning performance of compact lines extracted from survey > While the reduced insulation level degrades lightning performance, closer phase-to-overhead groundwire spacing also improves the electromagnetic coupling, indicated by Csp in Figure 6-3. This means that a higher fraction of the tower-top impulse voltage appears on the floating phase conductors. Since the insulator impulse strength must withstand the difference in voltage between the tower voltage and the phase conductor, any increase in phase voltage reduces stress. An example of the increase in coupled voltage with compact geometry is given by Table 6-5. Table 6-5 Effects of Compact Line Insulation and Phase Spacing on Lightning Performance (Outages per 100 km per year) OHGW and shielding Insulation Phase Spacing Rfooting=20 Ω Rfooting=30 Ω Rfooting=50 Ω Rfooting=100 Ω
±2 m at 14-m height 11-m phase conductor height at tower 2 m (14 discs) dry-arc distance 1 m (7 discs) dry-arc distance -4m 0m 4m -2m 0m 2m -4m 0m 4m -2m 0m 2m 1.1 0.9 10 9 2.1 1.8 17 15 4.0 3.3 23 20 9.6 8.2 35 33
Ng=6 flashes / km2-year Both phase configurations have good shielding from direct flashes. The improved coupling with close phase-to-phase spacing offers a significant benefit of between 10 and 20% better lightning backflashover performance. However, this alone cannot make up the deficit in backflashover rate related to the reduced insulation of a typical compact line. A number of options can be considered to improve the dismal lightning performance of the compact transmission line (-2, 0, 2 m phase spacing) with 1-m dry arc distance. Some possible options would be: •
Improve the grounding resistance by a factor of two
•
Improve the coupling by using bundled overhead groundwires
•
Increase the insulation strength
•
Add line surge arresters
The relative merits of each option are ranked in Table 6-6. Improving coupling by adding more OHGW gives a modest improvement in lightning performance compared to the reference compact line. However, it seems to be more practical to convert the horizontal circuit to a vertical arrangement using braced posts, and to then convert the bottom phase to a shield wire by fitting it with a suitable transmission line surge arrester.
6-12
Placement of Arresters for Improved Lightning Performance Table 6-6 Options for Improving Compact Line Lightning Performance (Outages per 100 km per year) OGHW -2 and 2 m at 14-m height 11-m phase conductor height at tower 1 m (7 disc) dry-arc distance -2m 0m 2m phase spacing Basic Performance Twin Bundle of both OHGW Cut Rfooting by 50% Increase to 1.5-m insulation Single OHGW on Pole, 1.5-m Braced Posts, Bottom Phase TLSA
Double OHGW Compact Line with 1-m Dry Arc Distance
Rfooting 20 Ω
Rfooting 30 Ω
Rfooting 50 Ω
Rfooting 100 Ω
9 6.5 2.6 2.4 1.7
15 11 5.7 4.4 3.2
20 16 12 7.5 5.8
33 28 20 17 13
Single OHGW Compact Line with 1.5-m Dry Arc Distance, Bottom Phase TLSA
14 m 1.5 m 2m
11 m
Figure 6-5 Options for Improving Compact Line Lightning Performance
Compact Unshielded Transmission Lines with TLSA A number of developments have completely shifted the paradigm for lightning protection of compact lines from achieving expensive improvements in footing resistance to the alternative of improving coupling and limiting overvoltage levels using transmission line surge arresters. With energy absorption capability at destruction in excess of 1500 Joule per cm3, and corresponding 6-13
Placement of Arresters for Improved Lightning Performance
charge levels of 10-20 C for IEC Class C (62-63 mm diameter) zinc oxide blocks, there is now a possibility that overhead groundwires can be completely eliminated. This reduces the height and visual impact of lines, giving a new axis of line compaction. •
A partial list of the improvements in TLSA includes:
•
Adoption of metal oxide varistors as the nonlinear elements
•
Production experience with a wide range of MOV formulations
•
Control of the V-I characteristics if TLSA that affect energy sharing
•
Adoption of sealed polymer housings to improve reliability
•
Introduction of external series gaps as an alternative to explosive disconnects and hinged installations to reduce cost and improve reliability
•
Proliferation of suppliers, leading to a competitive market
Transmission line surge arresters have proved to be quite reliable in both shielded and unshielded applications. Calculations based on the V-I characteristics of line surge arresters suggest that the sharing of energy among parallel arresters on protected circuits could be a problem. However, to date there have been few failures from excess energy, and most problems have been related to problems that crop up when an arrester is used to restrain conductor motion. In the modern compact line, restraint of conductor motion is important for maintaining minimum phase-to-phase clearances under high-wind and galloping conditions. These clearances affect the choice of tower head geometry and insulator configurations. The fact that conductor motion is restrained at the tower in typical compact-line design practice also makes application of line surge arresters more practical. Arresters can be installed in parallel with one of the rigid elements, and the use of braced post designs with two-section posts can automatically provide the necessary series gap for consistent operation of gapped TLSA. The most promising compact phase arrangement uses a three-phase delta on a single pole as shown in Table 6-6. This line was designed to be a 115-kV upgrade of a 44-kV subtransmission line, with reliability achieved by meshing of multiple supply lines rather than through good lightning and contamination performance of any individual line. The configuration remains attractive today, although as illustrated live-line work methods require a number of additional steps to establish safe clearances. An unshielded compact transmission line designed today could treat the center phase, supported on a polymer post, with a line surge arrester. This would make it function as an overhead groundwire under lightning conditions. The arrester voltage will add to the tower-top voltage, and this will actually improve the lightning performance of the unprotected phases. It is also possible to stack a pair of standard polymer post insulators in series, providing the recommended air gap from central flange to phase conductor, given by those manufacturers who supply externally-gapped TLSA rather than, or as well as, TLSA with explosive disconnects.
6-14
Placement of Arresters for Improved Lightning Performance
Figure 6-6 Typical 115-kV Compact Line Geometry from 1980, using Polymer Post and Semiconductive Glaze Bell Insulators [Ontario Hydro 1980]
6-15
7 APPLICATION SOFTWARE
Introduction Methods for estimating transmission line lightning performance are outlined in Chapter 6 of the EPRI Transmission Line Reference Book: 200 kV and Above [Red 2006]. These calculations are time-consuming to perform by hand, even with the applets provided, when there are several mitigation options to investigate. In general, it is impractical to undertake any such calculations without a computer and the proper software. The same is true for evaluating possible strategies for improving lightning performance. The placement of arresters, the improvement of tower grounds, the placement or relocation of shield wire, and the replacement of shield wires with arresters should be evaluated with the proper computer modeling in order to compare the effectiveness of each and, ultimately, to select the most cost-effective lightning performance design.
Lightning Performance Design Workstation (LPDW) Several software packages can be used to simulate the effects of lightning on transmission lines. Programs giving a highly detailed model of power system transients, with user-defined source current and attachment point, include EMTP, TLP, ATP, Matlab Power System Blockset and PSCAD. Programs giving a simplified model of the power system transient, but incorporating models for the lightning source current statistics, include the IEEE FLASH program provided with IEEE Standard 1243. Two programs, SIGMA slp and TFLSAH, offer the specific features that allow tower-by-tower parameter variation along the statistical variations in lightning surge currents. In particular, the distribution of soil resistivity or footing resistance along the line can be entered if the tower-totower variation has been established. The balance of this section will discuss the EPRI TFLASH routine, Version (?4.1).
TFLASH Overview This section is not intended to be a step by step guide through the TFLASH program or a detailed description of the algorithms used. These are covered in the online tutorials and help features of the program. The on-line information will be kept updated as new research data is incorporated into the program and algorithms are changed or updated. This section is merely a summary of TFLASH capabilities and features. 7-1
Application Software
TFLASH is a Windows TM based program that allows the user to model an entire transmission line or sections of a line quickly and easily on a PC. The line can have an arbitrary number of towers, and there are no restrictions on the number or order of unique towers that can be entered in the model other than the memory and computation capability of the computer used. Several other programs allow for a limited number of unique towers that can be repeated periodically. TFLASH provides libraries of towers and equipment to assist the user in building a model. Once the model is completed, the user can perform a number of different analyses.
Building a TFLASH Model TFLASH Capabilities Before describing the details of constructing a TFLASH model, the user should be aware of general capabilities and operating characteristics of the software, which are outlined below: •
TFLASH can simulate a maximum of three shield wires and 12 phase wires, for a total of 15 conductors.
•
TFLASH can simulate up to four circuits.
•
The number of conductors must remain constant throughout the model. For instance, a single-circuit tower with a shield wire cannot be connected to a single circuit tower without a shield wire. The first tower has four conductors and the second has only three. (This scenario can be analyzed with two separate models, however .)
•
TFLASH cannot perform analyses for corridors with circuits on different towers.
•
TFLASH will calculate circuit response to a discrete, specified lightning event at a tower or series of towers. Output graphs will plot specified voltages and currents against time for specified locations on the span or tower and will also report arrester energy absorption.
•
TFLASH will estimate the average statistical performance of a transmission line or line section for the full spectrum of lightning events that would be expected for a given location. This spectrum can be selected from IEEE or CIGRE Curves or from local data when the area is covered by the North American Lightning Detection Network (NALDN).
•
TFLASH will analyze simultaneous multi-phase or multi-circuit flashover performance.
General Procedure for Constructing a Line Model To construct a tower model, the user should be aware of the information that TFLASH requires and the procedure for supplying that information. First, the user must select a tower from a library of tower structures, as shown in . Once a tower is selected, the user must enter information about the tower: •
General properties
•
Ground system
7-2
Application Software
•
Equipment on the tower
Figure 7-1 Tower Modeling Screen from EPRI TFLASH (dummy)
The tower general properties are as follows: •
The length of the span that connects the tower to the next tower.
•
The ground flash density (GFD), which is given in terms of the number of lightning strokes to ground per square kilometer or per square mile per year. This can be determined from the NLDN program data base for the U.S., or it can be specified by the user.
•
The location of natural shielding, or lack thereof, from hilltops, valleys, trees, buildings, and other structures
The user has a number of options for the tower grounding. First, the user can select from three different ground types: •
Driven rods
•
Continuous counterpoise
•
Radial counterpoise
The user can then enter the ground resistance directly. If the ground resistance is not known, or the user selects driven ground rods, TFLASH will calculate the resistance using the following data: •
Soil resistivity
•
Ground rod diameter
•
Ground rod length
7-3
Application Software
•
Number of ground rods
If the user selected a continuous counterpoise, the following must be specified: •
Soil resistivity
•
Wire diameter
•
Depth to which the wire is buried
If the user selected a radial counterpoise, all of the data that would have to be entered for a continuous counterpoise must be entered, PLUS: •
The number of branches that make up the counterpoise
•
The length of each branch
The tower equipment includes conductors, insulators and arresters. As Figure 7-2 illustrates, for each conductor in the tower, the user must enter: •
The type and name of the conductor (i.e., ACSR, Pheasant). These values can be selected from a database of conductor types and names.
•
The conductor use (i.e. Circuit 1, Phase A)
•
The height of the conductor above the ground
•
The horizontal location of the conductor from the center of the tower
•
The sag of the conductor
7-4
Application Software Figure 7-2 Conductor Information Screen from TFLASH (dummy)
For each insulator, the user must enter: •
The type and name of the insulator (Again, these are also in a database to assist the user.) The name field also contains the number of units in the string for ceramic suspension insulators.
•
The geometry of the insulator (i.e., a V-string, an I-string, or a dead-end)
For each arrester, the user must enter: •
The arrester name and rating (These are also in the database.)
•
The series gap length, if gapped arresters are used.
There are many more options within TFLASH. This section is intended to give the reader an idea of the minimum data required to get TFLASH results. When all of this information has been provided, the tower is complete. Then the user can copy the tower, create new towers, or copy repeating sequences of towers until the line is modeled.
Analyzing a TFLASH Model Once a TFLASH model has been constructed, two different types of analyses can be done. The first calculates the statistical performance of the line, and the results it produces represent the "average" yearly performance of the line if the line were observed for a long time. The second type of analysis simulates the performance of the line during a single, user-specified lightning event. Both types of analyses are useful lightning performance calculations. The Classical Solution - The Average Performance of the Line The process of determining the average performance of the line is called the classical solution. The classical solution works by first using the electrogeometric model (EGM) to determine the distribution of strokes along the line. This is accomplished by applying the EGM at each unique tower at back midspan, quarter-span at the tower and the forward quarter span for strokes of incremental magnitude and perpendicular distance. A traveling wave model is then initiated that computes the propagation of currents through the towers, grounds, arresters and spans. Voltages are then computed to determine whether a stroke caused a flashover. The process is repeated for strokes of various peak currents until the critical current is found, below which there are no more flashovers. Menu options include performing calculations only to the first insulator flashover which is a relatively quick task for the computer, or continuing calculations to also determine if multiple phase or multiple circuit flashovers occur. Other options include limiting calculations to any continuous sequence of towers or performing calculations on the entire line model. The effects of power frequency voltage can also be included if desired. 7-5
Application Software
The statistical results of the solution are presented in terms of the average number of flashovers per year that the line will experience if operated for a very long time. This data is arranged into a number of different reports: •
Phase Flashover Report
•
Tower Flashover Report
•
Multiple-Phase, Single-Circuit Flashover Report
•
Multiple-Circuit Flashover Report
•
Arrester-Failure Report
The Phase Flashover Report lists the average number of times a year a given phase would be expected to have a breaker operate. For example, phase A of circuit 1 may be expected to flashover 11.52 times per year. This is a convenient overview of the total number of breaker operations that could be expected in a year. The Tower Flashover Report lists the average number of times a year that an insulator flashes over on a given tower. For example, tower #34 may have an insulator flashover 1.66 times per year. While the user will find this information particularly helpful in identifying problem sections on the transmission line, caution must be exercised in using this information for arrester placement. Chapter 5 provides information on arrester placement strategies. The Multiple-Circuit Flashover Report lists the average number of times a year that more than one circuit trips out. For example, the combination of circuit #1 and circuit #2 may flashover simultaneously 0.16 times per year. This is an important number in terms of power quality because multiple-circuit outages can be far more serious than single-circuit outages. The Multiple-Phase, Single-Circuit Flashover Report lists the number of times two or more phases of the same circuit flash over. For example, an A to B phase-to-phase flashover of circuit #1 may occur 0.34 times per year. This is not as serious an occurrence as a multiple-circuit flashover, but it is of greater consequence than a single phase flashover. The Arrester Failure Report gives two important pieces of information: the total number of arresters that fail (and therefore must be replaced), and the total number of additional breaker operations that arrester failures cause. For example, in one year 3.22 arresters on a given line may be expected to fail and be replaced. Of those failures, 2.80 per year would cause a breaker operation even though an insulator did not flashover. This is useful in terms of making arrester lifetime assessments and in determining the relative improvement in lightning performance from the application of arresters. Oscillographs - Line Behavior for a User-Specified Stroke The second type of analysis that the user can perform is to simulate the response of the line to a single lightning event. The user defines the stroke characteristics, and location on the line and Tflash computes the voltages and currents at various points on nearby towers. This type of 7-6
Application Software
analysis is helpful in determining how arresters improve performance. For example, if NLDN data indicate that a particular set of towers was hit with a 65 kA stroke (which subsequently caused a flashover), the user could simulate the single lightning event using the oscillograph. The user could then try different mitigation techniques to preventing the event from happening again. This feature can also be used when the classical solution Tower Flashover Report lists a section of line as a problem area. The user could focus on this section of line using oscillographs to evaluate various mitigation options. General Procedure / Sample Application As an example, assume that the user has a transmission line ridge crossing with a high exposure factor and high footing resistance. A first step could be to build a line model without arresters and to run the classical solution. Using the Phase Flashover Report, the user could obtain an estimate of the total number of flashovers expected each year and compare it with past performance. Next, the user could use the Tower Flashover Report to find the problem locations. If the user could also run an oscillograph solution for a sample stroke for an indication of which conductors would benefit the most from the application of TLSA. The user could then edit the model, adding arresters at various points in the section of line. Finally, the user could run the classical solution again, checking the Phase and Tower Flashover Reports so see if there has been an acceptable improvement in the line. These techniques also could be applied to test the performance of different tower designs and different line routing tower placements, all with the goal of optimizing the design of a transmission line. Other mitigation techniques, such as the addition of counterpoise, can be applied. Then an economic analysis can be used to determine the most cost-efficient mitigation strategy. Similar analyses can be performed on long transmission line sections through open ground, trees, valleys, and ridges to determine mitigation strategies.
7-7
8 CASE STUDIES
Applications of TLSA on system voltages from 42 to 765 kV have already been reported around the world. This chapter organizes a number of important examples according to system voltage. The case studies are generally organized as follows: •
Estimates of the OHGW protection, expressed as a line tripout rate, as a function of soil resistivity, for typical single circuit horizontal and double circuit vertical configurations
•
Estimates of the improvement (reduction) in line tripout rate, when additional impulse strength of wood insulation in series with polymer or porcelain is used
•
Estimates of the line tripout rate when TLSA are added to the two phases farthest away from the OHGW, making underbuilt shield wires that reduce the backflashover rate
•
Estimates of the line tripout rate when TLSA are used instead of OHGW, mounting the TLSA on the highest outboard phases. The arrester failure rate from excess energy dissipation is calculated, using typical span lengths for this system voltage and tower type.
In many cases, the line geometries have been contributed by EPRI members. These designs represent good engineering practice for a particular region, with regard especially to wind and ice loads, but may not be suitable in other regions with different climates. Where available, lines that have actually been treated with TLSA are used, so that the “before and after” predictions can be compared with observations.
44 kV Case Studies Comparison of OHGW versus TLSA using Customer Momentary Disturbance Benchmark In an area with a ground flash density that ranges from 0.5 to 3 per km2 per year, one utility recently re-evaluated an engineering instruction for lightning protection of subtransmission lines, originally written in 1973. At this utility, subtransmission lines often have no lightning protection at all. The insulation of the lines, including wood and porcelain in series, usually prevents flashovers from induced strokes which have a typical maximum of 300-420 kV. At this utility, it has been accepted that direct strokes to subtransmission lines always cause a flashover and a breaker re-close operation. Since there are thousands of customers on each subtransmission circuit and the circuit topology is radial, each lightning strokes causes thousands of momentary customer-interruptions. 8-1
Case Studies
The utility considered two techniques to reduce these interruptions at the source: •
Installing a grounded shield wire above the phase conductors to intercept the lightning stroke
•
Installing lightning arresters that shunt the lightning current safely to ground without allowing a power follow current that would trip the breaker
Studies carried out for the utility found that the effectiveness of each approach techniques depends on a number of factors including the resistivity of the earth, the resistance of each ground, the characteristics of the lightning surge (rate-of-rise etc.) and the physical arrangement of the circuit conductors. Analysis identified the geographic locations where overhead groundwires and arresters could be effective for reducing momentary interruptions. Further study evaluated the cost, effectiveness and other issues that affect the practical implementation. Figure 8-1 shows the typical overhead groundwire effectiveness, calculated using the SIGMA slp software described by [Sadovic 2000]. Delta Configuration, Ground Spacing 120 m, Resistivity 250 Ωm
Probability of Flashover
1 180 kV BIL 300 kV BIL 450 kV BIL
0.8
0.6
0.4
0.2
0 0
20
40
60
80
100
Footing Resistance (Ω)
Figure 8-1 Probability of Flashover on 44-kV Line in Delta Configuration with Overhead Groundwire
The project concluded that there were locations of low soil resistivity where overhead wires would be effective for reducing the momentary outages caused by direct lighting stokes. The overhead groundwire was not recommended in rocky areas, constituting more than two-thirds of the service territory but less than half of the line length. The effectiveness of both OHGW and top-phase TLSA treatments was found to vary with ground resistance of each ground point. A ground resistance of less than 25 Ω was recommended for either approach. 8-2
Case Studies
Overhead groundwires on 44-kV lines have low installation cost if fitted when lines are built or reconstructed. OHGW retrofit onto existing lines is expensive and, more importantly, the upgrade requires an extended outage. For existing 44-kV lines, TLSA can be effective at reducing momentary outages from direct and induced lightning strokes, especially on vertical and delta configuration lines where only the top phase needs to be protected. Arresters are particularly suited to retrofit situations. Financial analysis was carried out using the cost per avoided momentary customer outage, described in the EPRI Red Book [EPRI 2005 Chapter 6] for transmission systems. Analysis for this utility gave a value of $US 1.20 per avoided customer momentary disturbance (CMD). As an example, a 15m high, delta configuration line with CFO of 300 kV was considered in an area with GFD=1 flash per km2 . Every pole, on 50 m spacing, had a 25-Ω ground resistance. The cost assumed that OHGW would installed when the line is constructed or rebuilt, since as a retrofit it would cost over $45 per meter. Table 8-1 shows some additional details, including the additional cost of grounding to make sure that footing resistance is suitably low in highresistivity soil. Table 8-1 Comparison of Costs for 44-kV Lightning Protection Options Untreated Earth Resistivity Ω-m 1000
Flashover /100km/y
Cost $US/m
Overhead Groundwire Flashover Cost /100km/y $US/m
15 15 19
0 0 0
1.2 3 6
6.4 7.6 14.3
Top-Phase Arresters at Every Pole Flashover Cost /100km/y $US/m 8 9 10
8.0 9.3 16.0
TLSA were found to be 8% cheaper than OHGW in areas with soil resistivity of 1000 Ωm but three times more expensive in areas of 75 or 250 Ωm soil. The cost per avoided CMD on the 44-kV system for this utility was found to be much higher than the costs of applying similar solutions at the transmission level. Values ranged from $US 5.5 to $US 200 per avoided CMD when making improvements on 27.6-kV or 44-kV lines. This means that affected customers, rather than the utility, would need to support the costs of subtransmission line treatment. Application Experience with 44-kV Arrester Application At Hydro-One, subtransmission line supplies to sensitive auto manufacturing plants were upgraded for improved reliability using TLSA [Debu Kundu Case Studies]: •
Case I 'Line Arresters' application on an 81 km, 44-kV circuit in 1992
•
Case II 'Line Arresters' application on 44 kV circuits in 1997, 1998 and 1999
•
Case III 'Line Arresters' application on an 27.6 KV circuit in 1995
8-3
Case Studies
46 kV Case Studies Use 1002019 46-kV single circuit unshielded / ungrounded from pp 5-11 to 5-13
66 kV Case Study The use of gapped transmission line surge arresters in Japan at this voltage level has had remarkable success in improving power quality. [refs]
69 kV Case Study TU Electric identified 720 km (450 mi) of unshielded 69-kV transmission lines with unacceptable lightning performance in 1989. They considered a number of options and selected two for trials, shield wires and arresters on all three phases.
Figure 8-2 69-kV Line Configurations considered by TU Electric for Improved Lightning Performance
48 km of H-frame construction were treated with shield wires and bayonets as shown in Figure 3-1. Another 200 km were treated with line surge arresters by 1992, with details shown in Figure 8-3. TXU Electric may be able to update the number of lines treated.
8-4
Case Studies
Figure 8-3 Application of TLSA on TU Electric 69-kV Lines [Sanders and Newman 1992]
The long-term performance of these circuits has been .
115 kV Vertical Line Case Study Use 1002019 starting at page 5-16 and going to 5-27. These look to be real applications, so try to follow up with how they performed.
Figure 8-4 Voltage across 115-kV or 138-kV class TLSA compared to Insulator Flashover Levels
8-5
Case Studies
Figure 8-5 Strategy for Mounting TLSA with Suitable Mechanical Rating to Restrain Conductor
8-6
Case Studies
115 kV Horizontal H-Frame Line Case Study
Figure 8-6 Mounting of TLSA with Insufficient Horizontal Clearance
This would also be a good place to introduce the need for revised live-work methods after TLSA are installed. 8-7
Case Studies
115 kV Horizontal Steel Lattice Line Case Study [Tarasiewicz et al 2000] report on the process for optimizing placement of TLSA on a 115-kV line, constructed with 1.02 m (seven discs) and a single overhead groundwire, offset to one side as shown in Figure 8-7.
Figure 8-7 Single Circuit 115-kV Structure with Single OHGW Lightning Protection [Tarasiewicz 2000]
The phase with reduced protection from shielding failures was fitted with TLSA. Some towers were selected for treatment with TLSA on all three phases, based mainly on the local tower footing resistance from construction records. 115-kV Horizontal Line: Predicted Outage Rate Distribution of resistivity varies widely from tower to tower, located in “Canadian Shield” with variable overburden depth over granite. The overall outage rate of 2.2 per 100 km per year was predicted for the line with its existing configuration. This rate was composed of 52% backflashovers and 48% shielding failures. It was not possible to consider the use of additional wood insulation on this steel lattice structure, or to fit a longer insulator, as this would compromise the conductor-to-tower and ground clearances. 115 kV Horizontal Line: Outage Rates with Partial TLSA Treatments With the high variation in footing resistance from tower to tower, [Tarasiewicz et al 2000] found that there was a wide variation in the cost and effectiveness of partial arrester treatments. Their analysis is summarized in Table 8-2.
8-8
Case Studies Table 8-2 Reduction in Total Lightning Outages for Nine Treatment Options
Option
Description
Cost
Predicted Lightning Outage Rate per 100 km-yr
Reduction compared to “Do Nothing”
0
Do Nothing
0
2.2
0%
1
TLSA, 3-φ, on 10% of Towers
0.1
1.14
48%
2
TLSA, 3-φ, on 22% of Towers
0.22
0.94
57%
3
TLSA on Unshielded (Blue) Phases
0.29
0.72
67%
4
Second OHGW
0.78
0.59
73%
5
TLSA on Blue Phases + 10% 3-φ
0.36
0.26
88%
6
Second OHGW + TLSA, 3-φ on 10%
0.88
0.22
90%
7
Second OHGW + TLSA, 3-φ on 10%
0.99
0.13
94%
8
TLSA on Blue Phases + 22% 3-φ
0.44
0.11
95%
9
TLSA, 3-φ, on all 398 Towers
1.00
0
100%
With its excellent predicted performance at only 44% of the cost of a full treatment on all phases, option 8 was selected. 115 kV Horizontal Steel Lattice Line: Lessons Learned The installations were completed in 1997. The ongoing improvement in performance demonstrates that it is possible to afford adequate lightning protection levels by selectively applying surge arresters only to the towers most sensitive to the backflashover and shielding failures. This means that arrester protection can be limited to the structures with: •
High exposure to direct lightning strokes
•
High grounding resistance
This installation was actually funded by one customer, whose relatively small 25-MW load to a continuous chemical process was adversely affected by voltage dips and momentary interruptions. The performance of the 115-kV circuit, at 2.2 outages per 100 km per year, met the “Class C” security requirements applied for this utility to its 115-kV system, and there was no statutory requirement to do better. Client payment was based on satisfactory performance of the TLSA treatment, using lightning location network data to quantify the number of annual challenges to the line. This co-operative arrangement could be a good model in other jurisdictions where customers can make investments to improve their electric supply quality.
8-9
Case Studies
Line disconnect leads on the TLSA failed at a high rate, and were replaced with an upgraded design that threaded the flexible lead through a strain relief chain. Lead failures were not noted by helicopter patrols, as inspectors were unfamiliar with the new TLSA technology. Pictures of good and bad arrester installations proved to be helpful.
138 kV Case Studies The first application of metal oxide TLSA was made in the AEP system in Virginia. This was a science project to establish whether the energy duty in an area of difficult grounding could be managed with the relatively new technology. [Koch et al 1985] give a number of important details. All six phases were treated with gapped arresters, mounted in cantilever to restrain conductor motion using ceramic housings.
154 kV Case Study The use of gapped transmission line surge arresters in Japan at this voltage level has had good success in improving power quality. However, the use of TLSA at this system voltage in Japan is not widespread because similar improvements can also be obtained with better grounding.
161 kV Case Studies These line geometries were contributed by TVA, who make extensive use of this system voltage. The TVA service area has a wide range of soil resistivity as shown in [TTHOR overall figures].
230 kV Case Studies 230 kV Horizontal Line: Application Experience A pair of unshielded 230-kV lines, with horizontal configuration, sharing the same right-of-way and grounding system, bring power to the Avalon Peninsula in Newfoundland. Rather than OHGW, these lines use single pole reclosing to mitigate single-phase to ground lightning faults. This is an area of heavy ice and wind loads where the extra ice load on OHGW increases line cost, and also an area of high resistivity where OHGW are less efficient. Faced with unsatisfactory performance of their single pole reclosing protection from a high fraction of multiple-phase and multi-circuit tripouts, N&L Hydro initiated studies to rank suitable options. Improved grounding was considered. However, there was also strong evidence that the grounding improvements completed ten years before had been attacked by corrosion and had reached their end of life. These improvements used single radial counterpoise between towers, with some cross-bonding between circuits to supplement the natural transfer of potential enhanced by the two-layer soil.
8-10
Case Studies
This project benefited from the commercial introduction of 230-kV TLSA about six months after the grounding study reached its conclusion. TLSA with IEC Class “C” energy rating (roughly 62-mm block diameter) were fitted to every insulation point on one of the lines. This treatment has been fully effective. Lightning outages continue to occur on the untreated line, but none have occurred on the unshielded line with TLSA on every phase. [H. Richards, CEATI seminar 2006, discussion] 230 kV Circuit with 35-kV Underbuild Take case from 1002019 starting page 5-7. This is a poor-performing design with single OHGW and 3.6 outages per 100 km. Convert it to a good-performing design by discarding the OHGW, going to 90-ft pole with A phase having zero offset and TLSA – then show the drastic improvement when the bottom three conductors (35 kV) are also fitted with TLSA.
275 kV Case Studies The lightning ground flash density in South Africa increases from west to east, reaching levels of 12 or 14 flashes per km2 per year, similar to Florida. Grounding of transmission lines in South Africa is likely to be difficult in the areas highlighted in < figure from world atlas of conductivity. With a high level of industrialization along with difficult conditions, South African researchers have offered important leadership and insight in lightning protection, including studies of lightning parameters on instrumented masts, development of a more suitable lightning flash counter with calibrated range and a high rejection of cloud-flash interference, and comparison studies of gapped versus gapless TLSA on 275 kV lines. In Japan, at 275 kV, issues with winter lightning and the effective use of partial treatment have been recently discussed by (Tsuge and Yamada 2006) for Chubu Electric and (Araya et al 2006) for Hokuriku Electric Power Company.
400 kV Case Study Horizontal, unshielded line in Norway described in [Loudon 1998]
8-11
Case Studies
Figure 8-8 Typical 400-kV Line Geometries at Statnett (Norway)
Figure 8-9 Compact 400-kV Unshielded Design with Top-Phase TLSA
500 kV Case Studies Pilot project, gapped 500 kV TLSA in Japan. BCTC are looking at TLSA for their unshielded 500 kV.
8-12
Case Studies
765 kV Case Study With its high level of insulation, transmission lines operated at 765 kV have provided excellent lightning performance. Some of this good performance is related to the large footprint at the tower base, typically four poured foundations on centers of 12 m (40 feet). This acts like four, large, independent ground rods and tower resistance of less than 20 Ω is achieved in poor soil of 1000 Ωm resistivity.
Figure 8-10 Traveling Waves near Open Terminal
AEP applied polymer arresters to attenuate the reflected voltage at open terminals as shown in Figure 8-11. These arresters were mounted on the tower, making them TLSA. They also eliminate the possibility of a close-in lightning fault on the protected tower.
8-13
Case Studies
Figure 8-11 TLSA on AEP 765-kV Line for Switching Surge Control
8-14
9 REFERENCES ANSI/ IEEE Std. 4-1992 Standard Techniques for High-Voltage Testing ANSI/ IEEE-62.11-1993 "Standard for Metal Oxide Surge Arresters for Alternating Current Power Circuits" CEI/IEC 99-4 1991-11 "International Standard: Surge Arresters Part 4 Metal Oxide Surge Arresters without Gaps for AC Systems" ANSI/ IEEE C62.22-1991 "IEEE Guide for the Application of Metal-Oxide Surge Arresters for Alternating-Current Systems" ANSI/ IEEE C62.1-1989 "Standard for Gapped Silicon-Carbide Surge Arresters for AC Power Circuits" ANSI/IEEE C62.2-1987 "Guide for the Application of Gapped Silicon-Carbide Surge Arresters for AC Systems" IEEE Standard 1243/1997, "Guide for Improving the Lightning Performance of Transmission Lines" CEI/IEC 99-1 1991 "Part 1: Non-Linear Resistor Type Gapped Surge Arresters for AC Systems" V. Mazur and L. Ruhnke, “Evaluation of Lightning Protection System at the WSR-88D Radar Sites”, NOAA Final Report, May 2001 Combined Satellite- and Surface-Based Estimation of the Intracloud–Cloud-to-Ground Lightning Ratio over the Continental United States DENNIS J. BOCCIPPIO, KENNETH L. CUMMINS, HUGH J. CHRISTIAN AND STEVEN J. GOODMAN, Monthly Weather Review Vol. 129, January 2001 R. Zemeckis and R. Gale, “Back to the Future”, 1985 The Cost of Power Disturbances to Industrial and Digital Economy Companies, EPRI, Palo Alto, CA, June 2001. TR-findout 1. J.G. Anderson, "Lightning Performance of Transmission Lines," Chapter 12, Transmission Line Reference Book - 345 kV and Above, Second Edition, Electric Power Research Institute, Palo Alto, CA, 1987. 9-1
References
2. Guide for Improving the Lightning Performance of Transmission Lines, IEEE Standard 1243/1997. Dept., 445 Hoes Lane, Piscataway, NJ 08855-1331. 3. CIGRE Working Group 01 (Lightning) of Study Committee 33 (Overvoltages and Insulation Coordination), Guide to Procedures for Estimating the Lightning Performance of Transmission Lines, CIGRE Brochure #63, October 1991, Paris. 4. Appendix of Reference 2. 5. K. Berger, "The Earth Flash," Chapter 5, Lightning, Vol. 1, Edited by R. H. Golde, Academic Press, New York, 1977. 6. W. Chisholm, Y. L. Chow, K. D. Srivastava, "Travel Time of Transmission Towers," IEEE Trans. PWRD, Vol. PAS-104, No. 10, October 1985, pp. 2922-2928. 7. R.L. Witzke, T. J. Bliss, "Coordination of Lightning Arrester Location with Transformer Insulation Level," AlEE Trans., Vol. 69, Part 1,1950, pp. 964-975. 8. P. Chowdhuri, Electromagnetic Transients in Power Systems, John Wiley & Sons, New York, NY USA. 9. M. Darveniza, "Further Experience with the Integration Method of Estimating Non-Standard Impulse Insulation Strength," Proceedings of Sixth intl. Symposium on High Voltage Engineering, paper 22.15, New Orleans, LA, 1989. 10. A.M. Mousa, "The Soil Ionization Gradient Associated with Discharge of High Currents into Concentrated Electrodes," lEEE Trans. on Power Delivery, Vol. 9, No. 3, July 1994, pp. 16691677. 11. K.H. Weck, "Remarks to the Current Dependence of Tower Footing Resistances," CIGRE Working Group Report, No. 33-85 (WGO1)IWD. 12. A.V. Korsuncev, "Application of the Theory of Similitude to the Calculation of Concentrated Earth Electrodes," Elektrichestvo, No. 5, May 1958, pp. 31-35. 13. F. Popolansky, "Generalization of Model Measurement Results of Impulse Characteristics of Concentrated Earths," CIGRE Working Group Report, No. SC33-80 (WGO1) IWD, August 1980. 14. ANSI/IEEE 62.11, Standard for Metal Oxide Surge Arresters for AC Power Circuits. 15. J. Williams, "Transmission Line Arrester Energy Study for Lightning Surges," Distributed at the EPRI TLSA Workshop, July 1994. 16. E.R. Whithead, "CIGRE Survey of the Lightning Performance of Extra-High Voltage Transmission Lines," Electra, No. 33, March 1974. 9-2
References
17. C.H. Shih, B.J. Ware, J.G. Anderson, J.J. LaForest, "The Effect of Metal Oxide Arresters on Switching Overvoltages on EHV Systems," CIGRE Paper 33-03,1982. Tarasiewicz, E., Rimmer, F. and Morched, A.S., “Transmission Line Arrester Energy, Cost, and Risk of Failure Analysis for Partially Shielded Transmission Lines”, IEEE Trans. PWRD Vol. 15 No.3, Jul 2000, p.919 International Telecommunications Union. Recommendation ITU-R P.368-7, Ground-Wave Propagation Curves for Frequencies between 10 kHz and 30 MHz. 1992. International Telecommunications Union. Recommendation ITU-R P.832-2, World Atlas of Ground Conductivities. 1999. Kenji TSUGE, Hiroaki YAMADA, “Application Technology of Lightning Arrester for 275kV Transmission Line”, ICLP 2006 Teru ARAYA, Naoto TAMURA, Kazuo KUMATA, “Effective Partial Installation of Line Surge Arresters for 275kV Transmission Lines in winter lightning area”, ICLP 2006 Kundu, Debu, “Lightning Protection and Its Effects on Industrial Plants and Electrical Systems”, May 8-9, 2000 Seminar, Toronto, Ontario Jim Sanders and Kevin Newman, “Polymer Arresters as an Alternative to Shield Wire”, 24th Annual Transmission & Substation Design and Operation Symposium, EU1413-H, 1992 B. RICHTER, W. SCHMIDT K. KANNUS, K. LAHTI, V. HINRICHSEN C. NEUMANN W. PETRUSCH, K. STEINFELD, “LONG TERM PERFORMANCE OF POLYMER HOUSED MO-SURGE ARRESTERS”, CIGRE 2004 A3-110 Loudon, D, Kjell Halsan, Jonsson, Karsson, Stenstrøm and Lundquist, “A compact 420 kV line utilising line surge arresters for areas with low isokeraunic levels”, CIGRE 1998, 22/33/36-08 In addition, it is noted that the v1.0 gridded satellite lightning data (see Figure 4-3) were produced by the NASA LIS/OTD Science Team (Principal Investigator, Dr. Hugh J. Christian, NASA / Marshall Space Flight Center) and are available from the Global Hydrology Resource Center.
9-3
A TLSA MECHANICAL PERFORMANCE TESTS
To better understand the mechanical limitations for the installation of TLSA, an evaluation of the components comprising a TLSA was conducted. Two manufacturers, noted as A and B, provided disconnectors and arrester bodies for mechanical evaluation. This evaluation included static tension and bending tests of arrester bodies, static tension and shear tests of arrester disconnects, and fatigue testing of disconnects. The goal was to develop strength data for the arrester components and thus the arrester assembly. This will allow utility engineers to compare arrester strength with line design parameters to determine if a sufficient level of reliability exists for the anticipated arrester installation.
Line Disconnector Testing
Figure A-1 Shear and Tension Tests on TLSA Disconnects
A-1
TLSA Mechanical Performance Tests
Arrester Body Testing
Figure A-2 Arrester Body Bending Test Setup Table A-1 Observed TLSA Failure Loads Manufacturer
Tension
Bending
A
(5085 lb)
(330 lb)
B
(5594 lb)
(540 lb)
Evaluation of Different Arrester Installation Configurations
A-2
B TLSA ENERGY WITHSTAND TEST DATA
Definition of Withstand Criteria Tests of 63-mm TLSA to Destruction Tests of 8.4-kV MCOV Samples to Thermal Runaway
B-1
C TRANSMISSION LINE LIGHTNING PERFORMANCE CASE STUDIES
C-1
D MECHANICAL FORCE ANALYSIS FOR GAPLESS TLSA INSTALLATIONS
Figure D-1 Example of Typical Conductor to Pole Suspension
D-1
Mechanical Force Analysis for Gapless TLSA INstallations
Figure D-2 Example of Typical Conductorto Tower Mast Suspension
D-2
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The Electric Power Research Institute (EPRI), with major locations in Palo Alto, California, and Charlotte, North Carolina, was established in 1973 as an independent, nonprofit center for public interest energy and environmental research. EPRI brings together members, participants, the Institute’s scientists and engineers, and other leading experts to work collaboratively on solutions to the challenges of electric power. These solutions span nearly every area of electricity generation, delivery, and use, including health, safety, and environment. EPRI’s members represent over 90% of the electricity generated in the United States. International participation represents nearly 15% of EPRI’s total research, development, and demonstration program. Together…Shaping the Future of Electricity
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