Understanding the Zed-Meter® Instrument (Draft) Lightning Impulse Impedance of Transmission Tower Footings and Ground Electrodes 1015904
Understanding the Zed-Meter® Instrument (Draft) Lightning Impulse Impedance of Transmission Tower Footings and Ground Electrodes 1015904 Technical Update, December 2008
EPRI Project Manager F. Bologna
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, Inc.
This is an EPRI Technical Update report. A Technical Update report is intended as an informal report of continuing research, a meeting, or a topical study. It is not a final EPRI technical report.
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, EPRI, and TOGETHER…SHAPING THE FUTURE OF ELECTRICITY are registered service marks of the Electric Power Research Institute, Inc. Copyright © 2008 Electric Power Research Institute, Inc. All rights reserved.
CITATIONS This document was prepared by Kinectrics 800 Kipling Avenue Toronto, Ontario, Canada M8Z 6C4 Principal Investigators W. A. Chisholm E. Petrache This document 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: Understanding the Zed-Meter® Instrument (Draft): Lightning Impulse Impedance of Transmission Tower Footings and Ground Electrodes. EPRI, Palo Alto, CA: 2008. 1015904.
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PRODUCT DESCRIPTION Most utilities test transmission line tower ground resistance by isolating overhead groundwires; inserting low-frequency, battery-operated test equipment; taking readings; and restoring the connections. The resulting values are useful for power frequency grounding but less representative for lightning performance. Utilities can improve test productivity by switching to a Zed-Meter® instrument test technique that uses off-the-shelf equipment and injects a safe, lightning-like impulse signal into the tower base. Results and Findings The Zed-Meter instrument’s method for testing transmission line grounds relies on the injection of a safe, transient pulse into the tower base. The pulse is similar to lightning, and it delivers the results that utilities need for improving the lightning performance of lines. An additional benefit of this approach is that the surge impedance of the lead wires, rather than the galvanic resistance of driven rods, provides the connection to ground. This means that the Zed-Meter test leads need not be grounded; they need only be laid on the surface of the right of way. In addition, the overhead groundwires need not be isolated from the tower. These are important advantages that save test time, especially in frozen soil or rocky areas. Challenges and Objectives The objective of this report is to help users of the Zed-Meter instrument to apply it more effectively and to understand technical issues that might be encountered in the field. Application, Value, and Use The Zed-Meter instrument was designed to measure the potential rise at the base of a fourfooting high-voltage or extrahigh-voltage lattice tower without having to encircle the entire tower in a large, expensive sensor for the tower current. This report shows that the reaction electrode that is used to push current into the tower also provides a value of surge impedance that can be used to establish the soil resistivity near the tower. Other waveform features from this test can yield more information about the tower surge response, transfer impedance to nearby equipment, and the soil resistivity in the top layer of the soil. EPRI Perspective EPRI members have collectively used every existing method to measure transmission line grounding. Each has its strengths but none is completely appropriate for transmission line grounding—some require expensive equipment, and others take too long to set up at each tower. What separates the Zed-Meter instrument from the existing technologies is the right combination of test speed and accuracy for its intended purpose—lightning protection.
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Approach The goal of this report is to explain how to use the Zed-Meter instrument. It consists of the following sections: • • • • • • • • • • • • • •
Section 1, Overview of the Zed-Meter® Instrument Section 2, Zed-Meter® Instrument Bench Tests Section 3, Zed-Meter® Instrument Dipole Tests of Reaction Leads Section 4, Zed-Meter® Instrument Tests on Transmission Towers Section 5, Running the Zed-Meter® System Software Section 6, Typical Zed-Meter® Instrument Results Section 7, Post-Processing Section 8, References Appendix A, Frequently Asked Questions Appendix B, Specifications Appendix C Troubleshooting Guide Appendix D, Hardware Evolution Appendix E, The Current Reaction Lead Appendix F, Modeling Zed-Meter® Instrument Leads with NEC-4 Software
Keywords Lightning Surge impedance Test methods Transmission lines Zed-Meter instrument
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ABSTRACT The Zed-Meter® instrument’s method for testing transmission line grounds relies on the injection of a safe, transient pulse into the tower base. The pulse is similar to lightning, and it delivers the results that utilities need for improving the lightning performance of lines. An additional benefit of this approach is that the surge impedance of the lead wires, rather than the galvanic resistance of driven rods, provides the connection to ground. This means that the ZedMeter test leads need not be grounded; they need only be laid on the surface of the right of way. In addition, the overhead groundwires need not be isolated from the tower. These are important advantages that save test time, especially in frozen soil or rocky areas.
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CONTENTS 1 OVERVIEW OF THE ZED-METER® INSTRUMENT ..............................................................1-1 What is the Zed-Meter® Instrument? ...................................................................................1-1 How Does the Zed-Meter® Instrument Differ from Conventional Instruments?...................1-3 How and Why Do the Zed-Meter® Instrument’s Results Differ from Typical Resistance Measurements? ...................................................................................................................1-5 What Is the Basic Principle of Operation?............................................................................1-6 Labeled Diagram of Instrument............................................................................................1-7 Test Lead Arrangement .......................................................................................................1-9 Sequence of Zed-Meter® Operations ..................................................................................1-9 Operating Temperature Ranges ........................................................................................1-10 2 ZED-METER® INSTRUMENT BENCH TESTS .....................................................................2-1 Operational Test: Charging the Battery................................................................................2-1 Operational Test: Short Circuit.............................................................................................2-1 Connections and Process ..............................................................................................2-1 Results for High-Quality Short Circuit.............................................................................2-2 Results for Poor-Quality Short Circuit ............................................................................2-4 Operational Test: Fixed Resistance .....................................................................................2-5 Reference Result: 54-Ω Calibration Resistor .................................................................2-5 Reference Result: 500-Ω Resistor .................................................................................2-7 Practice Makes Perfect ........................................................................................................2-9 3 ZED-METER® INSTRUMENT DIPOLE TESTS OF REACTION LEADS ..............................3-1 Function of Reaction and Potential Leads ...........................................................................3-1 Dipole Impedance Test Method ...........................................................................................3-1 Variation of Dipole Impedance with Lead Orientation and Length .......................................3-2 Dipole Test Results, 90-m Versus 130-m Lead Length .......................................................3-3 Grounding of Reaction and Potential Leads ........................................................................3-4 Reasons for Grounding the Current Reaction Lead .......................................................3-4 Reasons for Grounding the Remote Potential Lead ......................................................3-4 Troubleshooting the Zed-Meter® Instrument’s Dipole Test Results ....................................3-5 Verifying a Safe Work Environment ...............................................................................3-5 Symptoms of Excessive Noise Level .............................................................................3-5 Symptoms of Problems in the Lead Layout ...................................................................3-5 Symptoms of Conductors Running in Parallel................................................................3-5 Symptoms of Problems in the Lead Terminations .........................................................3-6 4 ZED-METER® INSTRUMENT TESTS ON TRANSMISSION TOWERS ...............................4-1 Connection Diagram for Impedance Test ............................................................................4-1 Preferred Type of Tower Connection ...................................................................................4-2
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Preferred Location of Tower Connection .............................................................................4-4 Orientation of Reaction and Potential Leads........................................................................4-7 Dealing with Obstructions When Laying Out Leads...........................................................4-10 Twists and Turns: The Meander Line...........................................................................4-11 Using Shorter Potential Leads .....................................................................................4-13 Vehicles in Proximity to Leads .....................................................................................4-15 Considerations for Guyed Towers......................................................................................4-16 Considerations for Twin Steel-Pole Towers .......................................................................4-18 Overhead Groundwire Connections...................................................................................4-20 Comparison Tests with and Without Overhead Groundwires ......................................4-20 Calculation of Overhead Groundwire Impedance ........................................................4-22 Unparalleling the Effect of Overhead Groundwires ......................................................4-22 Which Is the Correct Value to Use? .............................................................................4-23 Tests on Towers with High Noise Level .............................................................................4-23 Tests on Towers with Buried Counterpoise .......................................................................4-24 Test Lead Orientation for Lines with Counterpoise ............................................................4-24 Which Leg Has the Counterpoise Connection? ...........................................................4-25 5 RUNNING THE ZED-METER® SYSTEM SOFTWARE .........................................................5-1 Overview ..............................................................................................................................5-1 Main User Interface..............................................................................................................5-3 Section 2 Elements ........................................................................................................5-4 Section 3 Elements ........................................................................................................5-7 Setting the Time Windows ...........................................................................................5-12 Setting the Alarm Thresholds .......................................................................................5-12 Pretrigger Interval.........................................................................................................5-12 Demo Mode........................................................................................................................5-13 How-To: Start the Measurement Process ..........................................................................5-14 How-To: Stop the Measurement Process ..........................................................................5-15 How-To: Save the Measurement Data...............................................................................5-15 Data File.......................................................................................................................5-15 Log File ........................................................................................................................5-16 How-To: Create Line Reports ............................................................................................5-16 6 TYPICAL ZED-METER® INSTRUMENT RESULTS..............................................................6-1 Effects of Overhead Groundwire..........................................................................................6-1 Reasons for Steadily Increasing Impedance Values............................................................6-1 Low-Frequency Versus High-Frequency Resistivity ......................................................6-2 Effect of the Time Window, 500 ns Versus 1000 ns.......................................................6-3 Special Case: Towers with Isolated Overhead Groundwires and High Resistivity.........6-4 Reasons for Steadily Decreasing Impedance Values ..........................................................6-6 Typical Responses of Buried Horizontal Wires ..............................................................6-6
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Reading Past the First Sweet Spot ................................................................................6-8 Reasons for High Impedance Values.................................................................................6-10 What Constitutes a High Reading? ..............................................................................6-10 Identifying Bad Connection to Current Leads ..............................................................6-11 Indications of Local Soil Resistivity from Dipole Test Results ......................................6-11 Reasons for Low or Negative Impedance Values ..............................................................6-11 Identifying a Bad Connection to the Potential Lead .....................................................6-12 Inserting Resistance in Series with Tower to Validate Connections ............................6-12 Indications of Local Soil Resistivity from Dipole Test Results ......................................6-12 7 POST-PROCESSING .............................................................................................................7-1 Organization of Data ............................................................................................................7-1 Converting Waveform Files to Excel Format........................................................................7-1 Producing Waveform Graphs in Excel .................................................................................7-1 Exporting Waveform Graphs from Excel to Portable Document Format..............................7-2 Establishing Confidence Level in Test Results ....................................................................7-2 Adaptive Filtering to Improve Confidence ............................................................................7-2 Offset Subtraction ..........................................................................................................7-2 Denoising Digital Artifact ................................................................................................7-3 Removing Noise Using Pretrigger Record .....................................................................7-4 Validating a Novel Test Lead Arrangement .........................................................................7-5 Validating Against What? ...............................................................................................7-5 NEC-2 for Simple Coupling Estimates ...........................................................................7-8 8 REFERENCES .......................................................................................................................8-1 A FREQUENTLY ASKED QUESTIONS .................................................................................. A-1 B SPECIFICATIONS ................................................................................................................ B-1 C TROUBLESHOOTING GUIDE ............................................................................................. C-1 D HARDWARE EVOLUTION ................................................................................................... D-1 E THE CURRENT REACTION LEAD ...................................................................................... E-1 Dipole Impedance Test Theory ........................................................................................... E-1 Theoretical Variation of Dipole Impedance with Height ...................................................... E-3 Theoretical Variation of Dipole Impedance with Soil Resistivity.......................................... E-4 Theoretical Variation of Dipole Impedance with Lead Orientation ...................................... E-5 Conductors Running in Parallel with Test Leads ................................................................ E-7 2003 Field Studies at American Electric Power .................................................................. E-9 2005 Current Reaction Lead Studies at the CN Tower in Toronto.................................... E-13 Dipole Test Results, 90-m Versus 130-m Lead Length .................................................... E-16 Dipole Test Results, 125-m Coaxial Cable Test Leads..................................................... E-16 Dipole Test Results, 300-m, 14-Gauge Solid Copper Wires............................................. E-19
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F MODELING ZED-METER® INSTRUMENT LEADS WITH NEC-4 SOFTWARE...................F-1 NEC-4 Software for Detailed Calculations Using Inverse Fast Fourier Transform ..............F-1 Other Modeling Software .....................................................................................................F-4 Finding an Experienced Practitioner ....................................................................................F-4
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OVERVIEW OF THE ZED-METER® INSTRUMENT What is the Zed-Meter® Instrument? The Zed-Meter® instrument is a test instrument that measures the impedance of transmission line grounds by generating an internal, lightning-like signal that is applied to the base of a transmission tower. The lightning impulse response of the local tower grounding system is monitored by tracing the rise and fall of tower base voltage as a function of the injected current for a short time after the signal is triggered. Rather than conducting this test with a single, largeamplitude pulse that has safety and weight issues, the Zed-Meter instrument averages results from repeated applications of lower-amplitude pulses from an electric fence shock generator that has been certified safe (although uncomfortable) for human and animal contact. The Zed-Meter instrument is a smart instrument that has benefited from improvements in computer technology to reduce its size, weight, and cost. Field trials have been conducted at Public Service Electric and Gas Company, Duke Energy, Tennessee Valley Authority, Georgia Power, Hydro-One, Bonneville Power Administration, Eskom, Manitoba Hydro, and National Grid. Each utility in its turn has seen improved versions of the Zed-Meter instrument that have been smaller, lighter, less expensive, and better at analyzing, presenting, and storing results. The utility input in the development cycle distinguishes the Zed-Meter instrument from the incremental improvements that have been incorporated in standard earth resistance testers during the same period. In addition to the instrument itself, the Zed-Meter kit contains the following components: •
A short cable and specially adapted clamp that feed the high-frequency test current into the tower • A pair of leads, usually coaxial cables of approximately 100-m length, that are used as highfrequency traveling wave antennas • An optional pair of ground rods and connectors for grounding the traveling wave antennas Table 1-1 shows how the Zed-Meter kit has progressed from a list of suitable parts to a selfcontained instrument with full control over pulse application and measurement with a Panasonic Toughbook laptop computer that represents more than half the overall cost.
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Table 1-1 Zed-Meter® instrument evolution: prototype to production 2004
2006
2008
Digitizer
Tektronix 3054B oscilloscope
Tektronix 2000 series oscilloscope, US$6000
Pulse generator
Lab grade 200 V, 50 Ω with 10-ns rise time
Farm grade, safe (Underwriters Laboratories [UL] approved) with wave shaping circuit
Current sensors
Wideband current transducers, sensitivity 1 V/A, 10-ns rise time
Cable reels
90 m RG58 (2)
90 m RG58 (2)
90–130 m (2)
Power
12 Ah battery, 117 V inverter
12 Ah battery, 117 V inverter
Internal rechargeable battery
Control and analysis
Manual setup of oscilloscope; manual control of pulser; comma-separated values (.csv) files on floppy disc to Microsoft Excel software
Macro setup of oscilloscope; manual control of pulser; comma-separated values (.csv) files through network to Microsoft Excel software
National Instruments Labview software control of digitizers, pulser, file transfer through USB, and wave analysis
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Acute DS-1102 200 MS/s USB digitizers, US$900 each
How Does the Zed-Meter® Instrument Differ from Conventional Instruments? The Zed-Meter instrument works strictly in the time domain, specifically about 100 ns to 2 µs, whereas most conventional earth resistance test instruments use one or more fixed frequencies around 100 Hz, making them low frequency-domain tools. Conventional, general-purpose, three- or four-terminal earth resistance testers also generate internal signals and perform the same basic functions as the Zed-Meter instrument. However, these testers use a low-frequency signal that gives the resistance of many towers in parallel. This result can indicate the contribution of individual towers to the low overall resistance only when expensive supplemental sensors are placed around each tower leg and guy wire. The lowfrequency, three-terminal methods also require the insertion of metal probes into the ground, achieving sufficient depth of penetration to allow injection of test signals. Typically, a probe resistance of less than 10 kΩ must be achieved. This is difficult in frozen soil, because the resistivity is about 100 times higher than unfrozen soil. In winter testing, probes must sometimes be pushed all the way through the frozen soil layer, which can be more than 1 m thick. Conventional clamp-on earth resistance testers generate internal signals at a medium frequency of approximately 2 kHz. The energy is magnetically coupled into a single ground lead through steel jaws that open and close. The power needed to drive test current into the grounding system can then be measured. If the multigrounded neutral system is in good condition with low impedance, most of the measured impedance comes from a local (single) ground rod. This method has limitations if there are two or more paths to ground on the same tower or if the connection from local tower to overhead groundwire (OHGW) or neutral is imperfect. In order to simulate what the Zed-Meter does, a standard earth resistance tester would have to be modified to produce much higher frequencies. This in itself is not unique. One earth resistance tester operated at a single frequency, 26 kHz, and a CIGRE working group recommended that 150 kHz would be a better choice [1]. However, a modified earth resistance tester simulating the Zed-Meter would have to read out a series of measured impedance values, one for every frequency in a series covering the range from 10 kHz to 4 MHz. It is possible to convert results between the time and frequency domains using Fourier transformation, as illustrated in Figure 1-1. A perfect impulse signal contains all frequencies. A lightning impulse has high-frequency and low-frequency roll-off. For the most important waveshape of the first negative downward return stroke, the associated time constants are approximately 1.2 µs on the front and 50 µs on the time to half value. This means that a suitable frequency-domain instrument for grounding tests would measure with several different Nyquist frequencies in the range of 0.5/120 ns = 4 MHz to 0.5/50 µs = 10 kHz. Most of the interesting effects occur at the sine-wave frequency of 124 kHz that has the same peak current (I=31 kA) and peak rate of current rise (dI/dt=24 kA/µs) as a median lightning flash. This characteristic frequency drops to about 80 kHz for very large currents because there is a strong correlation between peak current and rate of current rise.
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x 10
FFT So urce
-4
150
2 |Imp ulse|
Injected Impulse [V]
200
100
1
50
0 0
0.5
1 1.5 Time [microsecon ds]
2
0 -1 10
2.5
0
10 10 Fr equ ency [MHz]
1
10
2
Zed-Meter® instrument’s test signal in frequency domain
Zed-Meter® instrument’s test signal in time domain
Figure 1-1 Time and frequency domain equivalents for Zed-Meter® test signal
Before committing to the development of the Zed-Meter instrument, EPRI reviewed a wide range of other instruments, including the EPRI Smart Ground Multimeter, which is used mainly for substation grounding tests. The results of this evaluation are given in Table 1-2.. Table 1-2 Suitablity matrix for transmission tower ground impedance (Z) testers Measurement Approach
Cost
Setup Time
Signal
Strengths
Limitations
Clamp-on impedance meter
US$1000–2000
1 min
2 kHz
Ease of use, size, battery life, wide range of Z
Must fit downlead; needs bond to parallel towers through overhead shield wires
Time-domain reflectometer testers
US$1000–10,000
15 min
1-ns step or pulse
Size, battery life, wide time scale, useful for soil
Wrong readouts; narrow range of Z centers at 50 Ω
EPRI Zed-Meter® instrument
US$3000–10,000
15 min
1- to 3-μs pulse, 200 V, 50 Ω
Size, noise rejection, wide range of Z, does not require driving rods into frozen soil
125-m leads
Four-terminal earth resistance testers
US$4000–10,000
30–60 min
100– 140 Hz
Size, battery life, noise rejection, wide range of Z, useful for soil
Needs tower isolation from shield wires; 100m leads; many intermediate steps
ABB 26-kHz meter
US$40,000 (estimated)
15 min
26 kHz
Size, ease of use
Inaccurate for Z>25Ω; 70-m leads; no resistivity
EPRI Smart Ground Multimeter
US$100,000 (estimated)
60 min
500-Hz random square wave
Noise rejection, wide range of Z, useful for layered soil
Seven leads up to 60 m, all terminated with rods to give low resistance
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Table 1-2 lists a mixture of frequency-domain and time-domain instruments. With its internal analysis software, the EPRI Smart Ground Multimeter provides impedance readouts in the frequency domain using a Fourier analysis of a time-domain series of step pulses. This is promising, but the analysis extends only from low frequency to 500 Hz; therefore, the Smart Ground Multimeter is not suitable for measuring the lightning surge impedance of transmission tower ground electrodes. How and Why Do the Zed-Meter® Instrument’s Results Differ from Typical Resistance Measurements?
There is an important physical difference between the lightning impedance of a transmission tower grounding system and the impedance of the same system at power frequency. The lightning surge is so rapid that the peak stress on insulators occurs before adjacent towers have had a chance to react and share the surge current. The two-way propagation time at the speed of light to the nearest pair of towers, 300 m away, is about 2 µs. A test surge can also be injected and measured at a local tower before the signals can detect what is happening far away from the tower under test. This is the basic advantage of the Zed-Meter tester. In the frequency domain, the effect is described differently. The series inductance of the overhead groundwires has a higher inductive reactance as frequency increases. For a 300-m span with 300 μH series inductance, the inductive reactance will be 18.8 Ω at 10 kHz, 188 Ω at 100 kHz, and so on. At high frequency, the surge impedance, ZC (300 Ω in this example) appears in parallel with the local footing (see Figure 1-2 in the following subsection). The Zed-Meter test result indicates whether the tower under test is well grounded, but it reads a value that is higher than the power-frequency impedance of the OHGW system with multiple grounds or the continuous counterpoise system. In practice, the Zed-Meter test result tends to be a bit lower than the impedance of the isolated tower measured at low frequency. The lightning performance (number of flashovers) of transmission lines is related to the values of the tower footing resistances along the line length. Low-frequency methods for measuring the resistance of transmission tower ground electrodes lose accuracy when OHGWs are connected to the towers, and they do not provide a result that is related to lightning performance. Accurate measurement of each individual tower resistance, whether tested at low or high frequency, can be obtained only if one or more of the following conditions are true: • The towers are temporarily isolated from parallel connections to remote earth • Sensitive current sensors can be placed around ground leads • The measurement frequency is raised to approximately 150 kHz [1] If the safety of a temporary working ground system relies on having low power-frequency impedance, the Zed-Meter tester is not the right choice for proof tests. When a power system fault occurs, many towers can share the fault current if they are connected together by an OHGW. The 60-Hz impedance of a multigrounded OHGW system can typically be approximately 2 Ω if each individual footing resistance is approximately 20 Ω. The same effect occurs for continuous counterpoise electrodes. The Zed-Meter tester does not resolve these remote effects, and it is not reasonable to assume that any fixed reduction factor will relate the local Zed-Meter test result to the power frequency impedance for many similar towers in
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parallel. This is a case in which a properly applied low-frequency earth resistance test instrument should be used. What Is the Basic Principle of Operation?
The basic operating principle of the Zed-Meter instrument is the following: •
Two leads, typically 90–125 m long, are laid in straight lines in different directions away from a tower leg. One lead is used for current injection, and the other is used to measure the remote potential rise. The angle between the leads should be at least 90°. • A lightning-like transient current is injected into the tower base through a current sensor. • The potential rise at the tower base relative to the remote potential is measured. • A time-varying impedance profile, Z(t), is derived from the potential rise response to the injected current. • Analysis identifies the desired features of Z(t), which, simply stated, are the median and the standard deviation of the values taken between two programmed time intervals. The impedance measurement vector, Z(t), is valid after the effects of the tower surge response have rung down and before the effects of the adjacent towers, or the far end of the current injection lead, have time to affect the reading. As a quality control measure, the current into the reaction lead is also measured and compared with the current into the tower. These are initially different when the measurement is not useful, but they should stabilize after some time (about 200 ns) to the same, constant value if the measurement leads are correct. In 2005, a CIGRE working group issued a brochure that reviewed methods for measuring the earth resistance of transmission towers equipped with earth wires [1]. This study considered all the options reviewed for EPRI in the EPRI reports The EPRI Zed-Meter: A New Technique to Evaluate Transmission Line Grounds (1008734), Field Testing of EPRI Zed-Meter: Transient Impedance of Transmission Line Grounds (1010235), and Summary of Zed-Meter Field Tests: Transient Impedance of Transmission Line Grounds (1012314), which are shown in Figure 1-2 [2, 3, 4]. The most promising option for the development of a suitable tower footing resistance test in frozen soil uses impulse test methods, including a pair of propagation lines. These lines have been tried in various configurations such as the reflection method shown in Figure 1-2.
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Figure 1-2 Equivalent circuit for voltage impulse test method
A good understanding of the role of the propagation lines in Figure 1-2 is helpful (see Section 3, Zed-Meter® Instrument Dipole Tests of Reaction Leads). This understanding will lead to good decisions about the test lead length, orientation, and configuration in difficult situations on crowded or remote rights of way. One unique feature of the Zed-Meter tester is that the propagation lines, typically 90–125 m long, need not be terminated in ground rods. The surge impedance, Z, of the lines couples them to ground potential rather than to a remote ground rod. The lines behave as a surge impedance for only the time that it takes an electromagnetic surge to propagate to the end and to return. The propagation speed is a fraction of the speed of light, c, or 300 m/μs. At a typical speed of 0.6 c, the 125-m lead has a reflection disturbance at about 1.4 μs. If the end of the propagation line has an open circuit, the returning voltage wave will be double the initial value, and the current wave will drop from its initial value to zero. Labeled Diagram of Instrument
The Zed-Meter tester’s method for measuring the impedance of transmission tower grounds does not require disconnecting the ground wire, and it is ideally suited for evaluating the lightning response or footing impedance of transmission towers, including towers in frozen soil. Photographs and a block diagram of the instrument are shown in Figures 1-3 and 1-4.
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Internal components
External view Figure 1-3 Photographs of the Zed-Meter® instrument
Pulse Generator
Wave Shaping Circuit
12-V Power Supervisor Circuit
100 MS/s Digitizer Two Channels
100 MS/s Digitizer Two Channels
USB-2 Interface to Laptop Computer
Figure 1-4 Block diagram of the Zed-Meter instrument
Figure 1-5 shows the Zed-Meter lead arrangement, with the impulse source and measurement equipment at the tower base.
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Figure 1-5 Equivalent circuit for Zed-Meter instrument’s impulse test method in frozen soil
Test Lead Arrangement The test lead arrangement in Figure 1-5 is symmetrical. With suitably short leads, there should be little difference in the results if the connections are reversed so that the current is injected into the left-hand lead and the potential is measured in the right-hand lead. In any test that involves straight leads, the leads should be reversed as a standard practice. Also, when the leads form a dipole antenna on the surface of the earth, a dipole or antenna impedance should be measured between the two wires. Generally, obtaining a constant value in the proof test of the propagation line impedance is a good first step to obtaining a reliable value of tower footing impedance. A proof test of the propagation line impedance should produce a signal that is initially of low amplitude and then rises to a constant level and stays at that level until the reflections from the end of the propagation lines arrive. Practically, there will often be some initial oscillations that damp down in approximately 100 ns if the connections are short and secure. The time after the current waves in each direction are equal and before the reflections arrive from the remote ends is used to establish the current flowing into the tower. The voltage rise in response to this current, measured over the same interval, is divided by the current to establish the value of the dipole surge impedance in the proof test.
Sequence of Zed-Meter® Operations Under control of the National Instruments Labview program running on the laptop computer, the supervisor circuit initiates a series of high-voltage pulses by controlling the supply voltage of the pulse generator. The wave-shaping circuit flattens the crest of the pulses and sharpens the front time to make a rectangular current pulse (see Figure 1-5) between the two current sensors.
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The injected currents, I1 and I2, are fairly constant regardless of the value of the tower footing resistance, RT, and the parallel OHGW impedance, ZC. The currents in each lead of the pulse generator are converted to voltage by a pair of current transducers. If there are no connections linking the “Tower Lead” and “Current Lead” terminals in Figure 14, there is no current flow. If there is a resistive connection, the currents should be equal. Two 2-channel digitizer modules running at 100 megasamples per second simultaneously measure the following: • • •
Current I1 in the tower lead Current I2 in the current lead Voltage rise V relative to the tower lead
Under the control of the Labview program, waveforms from the digitizer are transferred to the laptop computer using the USB interface cable. The waveforms are 8000 points long, corresponding to a time of 80 μs. The quiet period before each impulse is recorded with the use of a pretrigger. The pretrigger duration is presently fixed at approximately 1.5 μs or 150 points. The waveforms are accumulated by the Labview program, and the process is repeated a specified number of times to collect average values of each of the three parameters. After the averaging is completed, two vectors are generated: 1) the voltage rise V relative to the tower lead, divided sample by sample by the current I1 in the tower lead, and 2) the voltage rise V relative to the tower lead, divided sample by sample by the current I2 in the current lead. Two preprogrammed time intervals are used to calculate median impedance values. The first time interval starts after the tower oscillations have damped down and ends when the reflection from the remote end of the current reaction lead arrives back at the test point. This interval is valid for both grounded and ungrounded lead configurations and also for most practical transmission line span lengths. The second time interval starts after the current in the reaction lead has settled down to a new, stable, large value and normally ends when reflections arrive from adjacent towers, at about 2 to 3 μs. If the span length is short (100 ns) on the voltage or current measurements is noted in the results, the connection method should be reevaluated.
Operational Test: Fixed Resistance The Zed-Meter instrument’s reading of a fixed resistance should reach the low-frequency value in about 10 μs, depending on the characteristics of the resistor. The better the wiring configuration, the faster the Zed-Meter will settle to this value. It is recommended that high-quality low-inductance resistors be purchased or constructed for calibration of the Zed-Meter instrument. The value should be in the range of 10–500 Ω. One possibility is standard, matching 50-Ω or 75-Ω resistors, which typically have a constant impedance (or low-voltage standing wave ratio) at frequencies up to 1 GHz. Alternately, a lowinductance resistor can be constructed by soldering 10 or more metal film resistors in parallel. Generally, wire-wound resistors will have internal inductance that will make them unsuitable for the calibration process. The selected resistor should ideally be mounted in an adapter with a pair of insulated BNC male bulkhead connectors, using only the ground leads. Reference Result: 54-Ω Calibration Resistor
Bench measurements of a Zed-Meter instrument, using a 1.3-μs pulse width, were conducted on a 54-Ω resistor mounted in a small coaxial box. The setup and test results are shown in Figure 2-5.
2-5
Measured voltage, 105 V after 100 ns
Test setup (2006 version)
Measured current, 2 A
Calculated Z(t) and standard deviation over 16 samples
Figure 2-5 Zed-Meter® instrument test setup and results for a 54-Ω calibration resistor
In this case, the voltage across the resistor stabilized to a constant value with a time constant, τ, of about 50 ns. This time constant, multiplied by the measured resistance of 54 Ω, gives the inductance of the measurement loop, consisting of two 1-m wire leads, as shown in the test setup. The inductance works out to about 2.7 μH, a little higher than the expected 1 μH/m for straight leads. Figure 2-5 also shows a plot of the calculated impedance profile, Z(t), from each of the measured currents. Using logarithmic scales, it is also possible to show the standard deviation of the overall result. The standard deviation is a function of several factors, including the inherent noise in the measurement, the degree of agreement between the two current sensors, and the settling time of the wiring. In this case, the standard deviation is initially high, but it falls below 1 Ω at about 200 ns, and it remains less than 1 Ω to the end of the pulse waveform at 1200 ns.
2-6
The time period at which the standard deviation of the reading falls below a threshold and remains relatively constant is called a sweet spot with high signal-to-noise ratio. In original iterations of the Zed-Meter instrument, the sweet spot was identified by eye, sometimes with different results, depending on which eye was used. Modifications to the instrument have focused on making the sweet spot as wide as feasible, consistent with retaining a rapid test time. On the bench, the sweet spot should be quite long. Reference Result: 500-Ω Resistor
The reference resistor test is set up by fitting coaxial cable–to–banana jack and binding post adapters to each of the two current outputs. The calibration resistor is attached between the ground (black) terminals of the binding posts. A BNC tee is used to provide a parallel path to connect the potential measurement lead to the current reaction lead terminal using a short length of coaxial cable, terminated in BNC male connectors. Alternately, a third coaxial to banana jack/binding post adapter can be used with a short length of standard wire, connecting the ground (black) end of the potential measurement binding post to the ground (black) binding post of the current reaction lead. The use of a coaxial cable and BNC tee is preferred. The voltage and current signals should be averaged for 16 impulses. The results can be stored as text (.txt) files for analysis. For example, Figure 2-6 shows the measured voltages and currents for a 500-Ω wirewound resistor.
Figure 2-6 Measured voltage and currents into wirewound 500-Ω resistor showing oscillations
The standard deviation of the measured impedances in Figure 2-7 was approximately 10 Ω when both signals were considered, and the standard deviations reduced to 1–3 Ω when they were computed individually. It took about 500 ns for the oscillations in the currents to damp down sufficiently to start the sweet spot of the record. This is a function of the choice of resistor value
2-7
(500 Ω) and its internal capacitance of about 50 pF, giving a resonant frequency of about 17 MHz with the 2-μH wire loop.
Short-term response with standard deviation
Detail of long-term response, showing current transformer droop
Figure 2-7 Zed-Meter® instrument measurement of wirewound 500-Ω resistor on two time scales
The detail of the long-term response of the Zed-Meter instrument’s records in Figure 2-7 shows two defects. First, the impedance seems to increase with time. This is a result of the lowfrequency limitations of the selected current transformers. They exhibit a droop, or signal loss, of about 0.1% per microsecond. This means that, compared to the initial value, the measured current drops by about 8% at a time of 80 μs. For a constant applied source voltage provided by the Zed-Meter instrument, a decrease in measured current corresponds to an increase in impedance—from 480 to 530 Ω for one current transformer and from 500 to 545 Ω for the other. Each current transformer has a slightly different droop that can be compensated in software. Second, each current transformer indicates a slightly different value of current, leading to a 20-Ω difference in the measured value of the reference resistor. The transient impedance of the tested resistor at late time varied from 480 Ω to 530 Ω using Z1, and it was 3% higher using Z2. It is expected that these two readings will be within 1% of each other. The software process for correcting the droop and scale factors for the Zed-Meter instrument is under development. When comparing the results in Figure 2-7 with those in Figure 2-5, remember that both setups have approximately the same lead inductance. The L/R time constant is far less of a factor for the 500-Ω resistance than for the 54-Ω resistance, and the lead inductance dominates the response of the short-circuit test.
2-8
Practice Makes Perfect The Zed-Meter instrument can be connected to the tower and current leads in a number of different ways. This section has shown that the type of wire, the type of connectors, and the instrument settings can all affect the quality of the results. A familiarization period on the bench, testing impedance of resistors with good and bad wiring practice, is helpful to establish good habits in the field. This is especially important when measuring towers with low impedance. If the connection leads are too long, the Zed-Meter instrument will simply report the dynamic impedance of the leads themselves. It is possible to compensate for sloppy wiring practice near the tower by extending the length of reaction leads and analyzing the Z(t) values at later times. However, it is better to start with a good waveform.
2-9
3 ZED-METER® INSTRUMENT DIPOLE TESTS OF REACTION LEADS Function of Reaction and Potential Leads The Zed-Meter® instrument relies on the fact that the surge impedance of an insulated wire, laid close to the ground, is constant and has a value in the range of 400–700 Ω. Two propagation lines are used. The first propagation line is a current reaction lead. The impulse source is placed between the tower base and this lead. The current injected into the tower base will be a faithful copy of the current launched down the reaction lead after some initial oscillations up and down the tower have decayed away. If a tower has a resonant structure with guy wires, it can take a bit longer for this to occur, and the propagation lines might have to be extended to 125 m or more. The second propagation line is the potential lead. The tower base potential is measured by this insulated wire, which is also coupled to ground by its surge impedance. This lead impedance plays less of a role in the accuracy of the measurement because the input impedance of the measuring circuit is high. However, any ac or high-frequency noise picked up by this horizontal antenna must be rejected by the measurement circuit. This is done by averaging many impulses, with the expectation that the noise signals are not correlated to the test wave. If the surge impedance of the current reaction lead is too high (as can be the case over highresistivity grounds like snow and ice), less current is injected, and the potential rise at the tower base becomes too low. In addition, if there is a high noise level on the potential reference lead, more impulses must be averaged. In severe cases, potentials on the ungrounded leads might be too high (>50 V) to be handled without personal protective equipment. The configuration and selection of reaction and potential lead layouts has had an extensive period of development. Appendix D, Hardware Evaluation, summarizes the options that have been considered.
Dipole Impedance Test Method Although it adds to the field test time, it is recommended that a dipole test be conducted on the leads every time, before taking a measurement of the tower footing impedance. The dipole impedance of the two test leads can be measured well, and it should be approximately constant whether the leads are oriented at 90° or 180°. The results make the most sense if both leads run in straight lines and both leads have the same type of termination, either driven thin rods or open circuit. The dipole test setup connects the Zed-Meter instrument’s voltage (potential measurement) terminal to the current reaction lead using a BNC-type tee and a short coaxial cable. The current lead terminal is also connected to the current reaction lead. The central terminal feeds the potential lead (see Figure 3-1).
3-1
90-125 m
90-125 m
Figure 3-1 Connection diagram for dipole impedance test
It should be possible to reverse the red and blue leads in Figure 3-1 and still obtain the same impedance result. This is worth checking, especially in areas in which there is considerable induced noise on the cables or in cases in which it was easier to ground one cable. Induced noise will probably be measured on the pretrigger record of the digitizers. Ideally, everything before the pulse fires should read zero. In practice, the meter often picks up currents of a few mA resulting from AM radio broadcasts. Averaging multiple test shots should reduce this interference to a low level compared to the 400-mA test signal. If the dipole impedance values fluctuate from one test series to another, or if there is a large difference when the leads are reversed, it might be appropriate to increase the number of samples being averaged or to terminate the far ends in ground rods if they are floating in the first test series. Depending on the vegetation, the reaction and potential reference leads can be near the surface of the soil, or they can be suspended a meter or more off the ground. A lead close to the ground will have lower and more constant surge impedance as well a- a slower propagation time. These factors will give a higher quality measurement. For that reason, the coaxial cable should be placed as close to the ground as possible. If possible, it is best to walk back along the lead to force it close to the ground. If this is not feasible, the lead length should be increased.
Variation of Dipole Impedance with Lead Orientation and Length The angle between the two test leads can vary from 180° to 45° without making much change in the dipole impedance. Therefore, the lead orientation will not have a major effect on the results if 3-m leads running parallel to one anohter are avoided.
3-2
Lead lengths depend on soil resistivity (see Appendix E, The Current Reaction Lead, for details). In order to have a measurement with a period of constant injected current, lead lengths should be increased in areas with higher soil resistivity or in areas where the lead must be draped in vegetation well above the earth surface. Observe the following guidance for required length for the current reaction lead: • •
•
Use a minimum 90-m lead length for soil types with resistivity 100 < ρ < 500 Ωm Lead lengths of approximately 125 m should be considered in any of the following circumstances: — High soil resistivity ( ρ >500 Ωm) is anticipated. — Soil is frozen. — It is not feasible to lay the lead close to the ground. — The tower has guy wires. — The tower is isolated from the OHGWs by insulators. A configuration with two or more possible issues—for example, a guyed tower on frozen soil—might work better with lead lengths of 150 m.
Dipole Test Results, 90-m Versus 130-m Lead Length The bench tests in Section 2 show some important aspects about Zed-Meter instrument testing. When measuring low values of resistance, the effects of series test lead inductance take some time to damp out before the sweet spot with constant voltage and current enables a valid calculation. Measuring resistances of ≥500 Ω can also lead to high-frequency oscillations. The advantage of a 130-m cable over a 90-m cable was evaluated in a test series in freezing conditions, which would be the worst case, with extremely high surface resistivity in the frozen soil and snow layer. Pulse G enerator Me asure s curr ent to the right wire
Measures current to the left wire Insulated 90 m Wire
Insul ated 90-m Wire
Snow and/o r Frozen S oil La yer
Pulse G enerator Measures current to the l eft wire
Me asure s cur rent to the right wire
Insulate d 130 m Wire
Insulated 1 30-m Wire
Snow and/or Frozen Soil L ayer
Figure 3-2 Comparison of 90-m and 130-m RG-58 coaxial cable laid on surface of frozen soil
3-3
Because it is pointless to drive ground rods into frozen soil, the leads were left floating. This meant that the traveling wave reflections from the ends of the cables were marked by clear reductions or reversals of current. In Figure 3-2, the sweet spot of constant and equal current in each leg starts at about 500 ns in all cases. The suitable evaluation time continues to 800 ns for the 90-m leads, corresponding to a propagation velocity of 0.75 c. The sweet spot increases to 1100 ns for the 130-m leads, also corresponding to 0.75 c. This is a worthwhile gain for an extra minute or two of walking time in each direction.
Grounding of Reaction and Potential Leads Test experience has shown that, in most cases, there is no need to ground the ends of either the current reaction lead or the reference potential lead. Both wires are “earthed” through their surge impedance to ground, which is typically about 500 Ω. It can be fairly difficult to drive a temporary rod into the local soil to obtain a resistance lower than 500 Ω. If the termination resistance is higher or lower than the wire surge impedance, a negative or positive current reflection coefficient will appear at the end of the current reaction lead. This means that, after two-way travel time along the wire, the current will drop or increase to a new value. Several reflections can occur before the current settles to a new, constant value given by the pulse generator output voltage divided by the ground termination resistance. Reasons for Grounding the Current Reaction Lead
The initial reading of impedance is taken while the test current is constant and limited by the surge impedance of the current reaction lead. If the current reaction lead is terminated in a ground rod, the test current will settle down to a new value after a few round trips of the traveling waves. It is then possible to take a second reading of impedance in a different, slower time window. The degree of agreement between the two values, one in the 0.5–1 μs range and another in the 2–5 μs range, will help to characterize the soil better and thus improve modeling. In order to obtain a useful second reading, the resistance of the remote termination of the current reaction lead must typically be 90 Meter Lead
Figure 4-1 Connections to measure voltage potential on leads
If the potential exceeds 50 Vrms, most utilities call for the use of insulating gloves or other countermeasures. The Zed-Meter instrument itself can generate good results even if the induced pickup exceeds 100 V, because the current transducers are dielectrically isolated from the leads.
Connection Diagram for Impedance Test The basic layout of the current reaction and remote potential leads was described in Section 3, Zed-Meter® Instrument Dipole Tests of Reaction Leads, along with a recommended method for validating the impedance of the configuration. Deviations from the recommended practice, 125-m leads in two directions, 180° apart along the right of way, are described in “Orientation of Reaction and Potential Leads” in this section. When a good-quality result is obtained in the dipole test for the leads, say 400–700 Ω with 20-Ω standard deviation, the Zed-Meter is ready for connection to the tower (see Figure 4-2).
4-1
90-125 m
90-125 m
Figure 4-2 Connection of Zed-Meter® to reaction lead, remote potential lead, and tower leg
Some interesting things should happen when the dipole test is changed to a tower test. First, the current injected into the tower should be about twice the current into the dipole. The surge impedance of one leg of the dipole is twice that of the full dipole. This impedance is typically much greater than the impedance of the tower under test. Second, the potential rise of the tower base relative to the ground reference potential lead can initially be quite high, but it soon settles down to a relatively constant value. The Zed-Meter instrument is fast enough to see the effects of currents traveling up and down tower legs and guy wires. This also means that the connection of the tower lead to the base of the tower steel should be as short as possible, and it should make a good electromagnetic connection at high frequency. It might be necessary to initiate the auto-range function of the Zed-Meter to adjust to the new signal levels (see “How-To: Start Measurement Process” Section 5). This is done by changing the line name or structure number when starting a test. The software uses a change in either field to initiate the auto-range sequence.
Preferred Type of Tower Connection A number of methods have been explored for making a good high-frequency connection from the Zed-Meter instrument to the transmission tower. Figure 4-3 shows the following three examples: • • •
An alligator clip with 0.5-m wire lead and banana plug A citizens band (CB) radio antenna clamp and coaxial cable (intended for truck mirror mount) A welding clamp fitted with a swaged BNC female connector
4-2
2003 (American Electric Power), alligator clip to lattice tower leg—defect: overshoot
2004 (Public Service Electric and Gas), CB radio clamp-on antenna mount
2008 (Manitoba Hydro), modified welding clamp Figure 4-3 Improvements in radio frequency connection of Zed-Meter® to transmission towers, 2003–2008
4-3
Generally, the best results are obtained when the two measured currents are in close agreement and not oscillating too much after the initial rise. Figure 4-3 shows that the alligator clip is poor in this respect, with 100% overshoot; the CB radio antenna clamp is good, and the modified welding clamp is quite good. Although the high-frequency performance of the CB antenna radio mount is theoretically better than the modified welding clamp and its cost is much lower, in practice the device could be loosened and tightened only about 50 times before it broke in half. The welding clamp (see Figure 4-4) is likely to give longer service life and is recommended as the best choice at this time.
Connection to lattice tower
Connection to stud on steel pole
Figure 4-4 Close-ups of modified welding clamp with swaged BNC female connector
Preferred Location of Tower Connection Numerical simulations have suggested that the height of current reaction plays a role in the quality of the results. The effect of lead height on the propagation velocity and impedance of the reaction lead was detailed in Section 3, Zed-Meter® Instrument Dipole Tests of Reaction Leads. A dipole test establishes whether the leads are long enough to compensate for this effect. The numerical simulations also found a second factor. The height and length of the connection from the instrument’s tower lead to the tower plays a considerable role in the quality of the results. This seems to be more important than the height of the reaction lead further away from the tower. An example of a desirable, tight connection, with short leads and close to the ground, is shown in Figure 4-5.
4-4
Figure 4-5 Preferred Zed-Meter® instrument connection to lattice tower
Figure 4-6 shows that, when the wiring is tight, there is little difference in the results with a connection to the tower 0.1 m off the ground. Sim# I1-C-L01-GR-GW-50
Sim# I1-C-L1-GR-GW-50 BIS
40
40
h=0.1 m: 5Ω h=1 m: 4Ω
Vmeas / I meas
35
Measured Impedance: Vmeas / I meas [Ohm]
ρ=50 Ωm
Measured Impedance: Vmeas / I meas [Ohm]
Vmeas / I meas
30 25 20 15 10 5 0
Median 450-650 ns = 5 Ω 0
0.5
1 1.5 Time [microseconds]
2
35 30 25 20 15 10 5 0
2.5
Median 450-650 ns = 4 Ω 0
Sim# I1-C-L01-GR-GW-1000
h=1 m: 43 Ω
90 80 70 60 50 40 30 20 10 0
2
2.5
100
Vmeas / I meas Measured Impedance: Vmeas / I meas [Ohm]
46 Ω
Measured Impedance: Vmeas / I meas [Ohm]
h=0.1 m:
1 1.5 Time [microseconds]
Sim# I1-C-L1-GR-GW-1000 BIS
100
ρ=1000 Ωm
0.5
Median 450-650 ns = 46 Ω 0
0.5
1 1.5 Time [microseconds]
2
2.5
Reaction lead 0.1 m above ground
Vmeas / I meas
90 80 70 60 50 40 30 20 10 0
Median 450-650 ns = 43 Ω 0
0.5
1 1.5 Time [microseconds]
2
2.5
Reaction lead 1 m above ground, except 0.1 m at tower
Figure 4-6 Effect of reaction lead height on measured impedance with 90-m leads using good (tight) wiring practice as a function of soil resistivity ρ and height above ground h
4-5
In contrast, Figure 4-7 shows simulations for a detailed electromagnetic model of a lattice tower. These simulations were performed for three values of soil resistivity—extremely low (50 Ωm), high (1000 Ωm), and extremely high (20,000 Ωm).
Bottom leg of lattice tower geometry in NEC-4 model
Sim# I1-C-L01-GR-GW-50
Sim# I1-C-L1-GR-GW-50
40
40
Loose: 9 Ω
Vmeas / I meas [Ohm]
35
35 30
/I 25
meas
meas
/I
meas
30
Measured Impedance: V
Good: 5 Ω
Measured Impedance: V
ρ=50 Ωm
meas
[Ohm]
Vmeas / I meas
20 15 10 5
25 20 15 10 5
Median 450-650 ns = 5 Ω 0
0
0.5
1 1.5 Time [microseconds]
Median 450-650 ns = 9 Ω
2
0
2.5
0
0.5
Sim# I1-C-L01-GR-GW-1000
V
meas
[Ohm]
90
/I
meas
meas
80
meas
/I
70 60
Measured Impedance: V
[Ohm] meas
/I meas
Measured Impedance: V
Loose: 43 Ω
50 40 30 20 10 0
Median 450-650 ns = 46 Ω 0
0.5
1 1.5 Time [microseconds]
2
90
60 50 40 30 20 10
Median 450-650 ns = 43 Ω 0
0.5
2.5
[Ohm]
300
meas
/I
meas
400
meas
400
/I
[Ohm]
2
V
meas
300
Measured Impedance: V
meas
meas
/I
1 1.5 Time [microseconds]
Sim# I1-C-L1-GR-GW-20000 /I
meas
meas
Measured Impedance: V
meas
500 V
Loose: 219 Ω
/I
70
Sim# I1-C-L01-GR-GW-20000
Good: 261 Ω
meas
80
0
2.5
500
ρ=20000 Ωm
2.5
100 V
Good: 46 Ω
2
Sim# I1-C-L1-GR-GW-1000
100
ρ=1000 Ωm
1 1.5 Time [microseconds]
200
100
0
200
100
0
Median 450-650 ns = 261 Ω -100
0
0.1
0.2
0.3 0.4 0.5 Time [microseconds]
0.6
0.7
Good (tight) wiring practice
Median 450-650 ns = 219 Ω 0.8
-100
0
0.1
0.2
0.3 0.4 0.5 Time [microseconds]
0.6
Poor (loose) wiring practice
Figure 4-7 Effect of wiring practice at tower base on measured impedance with 90-m reaction leads
4-6
0.7
0.8
As Figure 4-7 shows, there is an extensive and large disturbance in the Z(t) profiles during the sweet spot (about 450–650 ns) when using poor wiring practice, with a connection 1 m above the ground. This effect is greatest when the soil resistivity is low; the variation in Z(t) seems to have a constant amplitude. In practice, the differences between clamp locations are somewhat less than those suggested by simulations. Even so, minimizing the length of the tower lead is probably as important as using a high-quality clamp with low impedance at high frequency for obtaining satisfactory results.
Orientation of Reaction and Potential Leads The normal procedure is to run the current reaction lead in one direction along the right of way and to run the remote potential lead in the other direction (see the right side of Figure 4-8). The calculation of tower base voltage with this configuration gives an estimate of tower-base potential that is closest to the true situation of a vertical lighting flash to the tower top. It is also the most practical orientation in a typical right of way because permission to run the leads through adjacent properties is not necessary. If access to the right of way along one direction is restricted, perhaps by a road, the alternative of running the current reaction lead at right angles to the line direction, 90° to the potential lead, can also be used (see the left side of Figure 4-8). This configuration is more common for grounding tests at a single, fixed frequency because it reduces steady-state mutual coupling effects among leads. The effect of lead orientation is most noticeable when testing towers with low footing impedance. The differences amount to 1–2 Ω, which is a considerable fraction of a 5-Ω result but can be neglected if the tower impedance is measured to be 25 Ω.
4-7
Current reaction lead 90° to line; potential lead along line (toward)
Current reaction lead along line (away); potential measurement along line (toward) Sim# I1-C-L01-GR-GW-50
Sim# I1-A-L01-GR-GW-50
40
40
Vmeas / I meas Measured Impedance: Vmeas / I meas [Ohm]
Measured Impedance: Vmeas / I meas [Ohm]
Vmeas / I meas 35 30 25 20 15 10 5 0
Median 450-650 ns = 6 Ω 0
0.5
1 1.5 Time [microseconds]
2
35 30 25 20 15 10 5 0
2.5
Z(t) in 50 Ωm soil, leads at 90°
Median 450-650 ns = 5 Ω 0
0.5
1 1.5 Time [microseconds]
2
2.5
Z(t) in 50 Ωm soil, leads at 180° along right of way
Figure 4-8 Numerical modeling of Zed-Meter® instrument result with 90° and 180° lead orientation
In some cases, it is desirable to run the leads at right angles to one another. A preferred alternate configuration calls for the current lead to run at right angles to the OHGW direction and the voltage lead to run along the right of way. Figure 4-8 shows that there is a slight increase of about 1 Ω when the leads run at 90°, compared to the reference case. This amounts to an increase of about 20% for a well-grounded tower with a 5-Ω impedance. Measurements with test leads at each orientation were conducted on a pair of towers, Manitoba Hydro DC 1/2 tower 1966 and DC 3/4 tower 1966. The impedances were low, and the best standard deviation values of just under 1 Ω were achieved from 500–900 ns (see Figure 4-9).
4-8
Figure 4-9 Typical Zed-Meter® instrument result: 2.9 Ω for Manitoba Hydro guyed-V DC 3/4 bipole tower 1966
The effect of lead orientation was tested in the presence of induced dc voltages that were surprisingly low, considering the line voltage of ±500 kVdc (see Tables 4-1 and 4-2). Table 4-1 Effects of lead orientation on Zed-Meter® instrument results for DC 1/2 tower 1966 Leads at 180° Normal
Impedance (Ω)
Leads at 90°
Reversed
2.64
2.49
2.13
Normal
2.14
2.04
2.50
Reversed
2.32
2.1 Ω
Median
2.59
2.5 Ω
Table 4-2 Effects of lead orientation on Zed-Meter® instrument results for DC 3/4 tower 1966 Leads at 90° Connect to Tower Steel Normal
Impedance (Ω) Median (Ω)
4.39
Leads at 180° Connect to Tower Base
Reversed
4.03
3.31 3.7
3.27
Normal
3.45 3.5
4-9
3.09
Reversed
2.93
2.71 3.0
3.11
The results of this test series confirm the numerical modeling in Figure 4-8. The measured impedance with leads in the reference configuration (180° along right of way) was about 20% lower than the results with leads at 90° in both cases. In addition to changes in lead orientation and position, the influence of the connection to the base plate rather than the tower steel was checked. A connection to tower steel, 1 m above the base plate (see Figure 4-10), gave results that were about 0.2 Ω higher than the desired connection with tight wiring practice to the base plate of the tower.
Typical tower base
Typical overall view, DC 1/2 tower 1967
Figure 4-10 Detail of preferred Zed-Meter® instrument connection to tower
High-voltage dc lines have static voltage at ground level that might influence test procedures in ways that are different from ac electrostatic fields. These tests were needed to establish whether the static pick-up was low enough for normal handling of the leads and proper operation of the equipment.
Dealing with Obstructions When Laying Out Leads The first obstruction that will be noted when laying out potential and current reaction leads at the preferred 180° angle is that one of the leads can run past a tower leg that is not being connected to the Zed-Meter instrument. The lead should be offset from the tower by a distance of >1 m. The tower will tend to reduce the measured potential rise, so it is better if the current lead runs past the unenergized leg. To check the influence, however, it is good practice to take measurements with leads reversed, by interchanging the coaxial connectors for the current and voltage leads. The two results should be in good agreement; the degree by which that they differ indicates the uncertainty in the selected test layout.
4-10
Twists and Turns: The Meander Line
In some situations (for example, where the leads are oriented at 90° to the line direction) it can be impossible to continue laying the leads beyond the edge of the transmission line right of way. In these cases, simulations show that a meander configuration with one or more 90° bends still gives good results. Possible meander configurations are shown in Figure 4-11, and the resultant waveforms are shown in Figures 4-12 and 4-13.
ZZ1
ZZ2
ZZ3
ZZ4
Figure 4-11 Four possible meander configurations of current reaction and remote potential leads
Measured Impedance: Vmeas / Imeas [Ohm]
40 ZZ1-L01-GR-GW-50: 5 Ω ZZ2-L01-GR-GW-50: 4 Ω ZZ3-L01-GR-GW-50: 5 Ω ZZ4-L01-GR-GW-50: 5 Ω
35 30 25 20 15 10 5 0
0
0.5
1 1.5 Time [microseconds]
2
2.5
Figure 4-12 Effect of meander-line configurations ZZ1 to ZZ4 on Zed-Meter® instrument impedance profile Z(t) for 50 Ωm soil resistivity, lattice tower 4-11
Measured Impedance: Vmeas / I meas [Ohm]
100 ZZ1-L01-GR-GW-1000: 47 Ω ZZ2-L01-GR-GW-1000: 48 Ω ZZ3-L01-GR-GW-1000: 47 Ω ZZ4-L01-GR-GW-1000: 46 Ω
90 80 70 60 50 40 30 20 10 0
0
0.5
1 1.5 Time [microseconds]
2
2.5
Figure 4-13 Effect of meander-line configurations ZZ1 to ZZ4 on Zed-Meter® instrument impedance profile Z(t) for 1000 Ωm soil resistivity, lattice tower
Table 4-3 shows the errors introduced when the potential leads (ZZ1), current leads (ZZ3), and both leads (ZZ2 and ZZ4) have the meander or zigzag layout. Even for regions with high soil resistivity, where the inherent measurement uncertainty is high, all four zigzag patterns give results that are practically the same as the reference configuration. Table 4-3 Computed Zed-Meter® instrument results (450–650 ns) for 100-m meander configurations 50 Ωm
1000 Ωm
20,000 Ωm
Reference configuration
5Ω
46 Ω
261 Ω
ZZ1
5Ω
47 Ω
262 Ω
ZZ2
4Ω
48 Ω
252 Ω
ZZ3
5Ω
47 Ω
261 Ω
ZZ4
5Ω
46 Ω
248 Ω
Soil resistivity
4-12
Using Shorter Potential Leads
If one lead must be shorter than the other, it makes sense that the short lead should be the one used to provide the remote zero potential reference. If the current reaction lead is shortened, the entire measurement process can be compromised. An early return of the reflection from the far end can occur before the initial oscillations have damped out, leading to a sweet spot of zero length. If, instead, the tower base potential rise is measured relative to a nearby ground, a large fraction of the total voltage rise will still be recorded. Numerical simulations have confirmed that, if one lead must be shorter than the other, it is better that the short lead be used to measure potential. The potential falls off rather quickly—for example, to a level of about 40% at a distance that is one diameter away from a four-leg footing. This can even be compensated exactly if the equivalent radius of the ground electrode is known. It a short potential lead is used, it should be terminated as well as possible in a ground rod. Four configurations were tested in the numerical models with NEC-4, using a 100-m current reaction lead. The reference case has a 100-m potential lead at 180°, oriented along the transmission right of way. A tight wiring geometry was assumed at the tower base, reflecting the extra care that should be taken in all other measurement aspects when compromising in one aspect. The results in Figure 4-14 are quite encouraging. As expected, the potential measured on the shortened lead is less than that on the 100-m long reference, but it is not reduced by much. The most sensitive case is when the soil resistivity is high. It would be feasible to derive a correction table as a function of soil resistivity for short potential leads that could be applied, for example, to the 25-m case relative to a 100-m case, or even a case in which a vertical lightning flash strikes the same tower and grounding system. However, in order to apply this table, one would need to measure the soil resistivity. At present, this measurement is a time-consuming process. Additional development of the Zed-Meter instrument’s dipole test could lead to a successful method that separates the effects of soil resistivity from the effects of lead height over ground. This could eventually supply the missing link needed to implement this improvement.
4-13
Soil resistivity and resultant Impedance
Reference: 100 m potential lead
Results with 75 m, 50 m, and 25 m
40
40
All: 5 Ω
meas
/I
C-PL75-L01-GR-GW-50: 5 Ω C-PL50-L01-GR-GW-50: 5 Ω C-PL25-L01-GR-GW-50: 5 Ω
meas
Measured Impedance: Vmeas / I meas [Ohm]
50 Ωm
Measured Impedance: Vmeas / I meas [Ohm]
V 35 30 25 20 15 10 5
35 30 25 20 15 10 5
Median 450-650 ns
Median 450-650 ns = 5 Ω 0
0
0
0.5
1 1.5 Time [microseconds]
2
50 m 45 Ω 25 m 43 Ω
90
meas
/I
meas
Measured Impedance: Vmeas / I meas [Ohm]
75 m 46 Ω
Measured Impedance: Vmeas / I meas [Ohm]
V
100m 46 Ω
80 70 60 50 40 30 20 10 0
Median 450-650 ns = 46 Ω 0
0.5
1 1.5 Time [microseconds]
2
50 m 238 Ω 25 m 202 Ω
meas
/I
2.5
300
200
100
0 Median 450-650 ns = 261 Ω 0
0.1
0.2
0.3 0.4 0.5 Time [microseconds]
0.6
80 70 60 50 40 30 20 10
Median 450-650 ns 0
0.5
1 1.5 Time [microseconds]
2
2.5
500
400
-100
2
C-PL75-L01-GR-GW-1000: 46 Ω C-PL50-L01-GR-GW-1000: 45 Ω C-PL25-L01-GR-GW-1000: 43 Ω
C-PL75-L01-GR-GW-20000: 257 Ω C-PL50-L01-GR-GW-20000: 238 Ω C-PL25-L01-GR-GW-20000: 202 Ω
meas
Measured Impedance: Vmeas / I meas [Ohm]
75 m 257 Ω
Measured Impedance: Vmeas / I meas [Ohm]
V
100m 261 Ω
1 1.5 Time [microseconds]
90
0
2.5
500
20,000 Ωm
0.5
100
100
1000 Ωm
0
2.5
0.7
400
300
200
100
0 Median 450-650 ns
0.8
-100
0
0.1
0.2
0.3 0.4 0.5 Time [microseconds]
Figure 4-14 Effect of shortened remote potential lead on Zed-Meter® instrument result for three soil resistivities
4-14
0.6
0.7
0.8
Vehicles in Proximity to Leads
At a wood-pole gulf-port structure (Manitoba Hydro CW8 tower 219), a comparison test evaluated sensitivity to local conducting objects. In this case, a large utility vehicle (a Chevrolet Suburban) was parked on the current reaction lead. Table 4-4 shows all the test results for this case. Table 4-4 Results of Zed-Meter® instrument tests on CW8 tower 219 Impedance, Ω 400–920 ns
Leads Normal Trial 1
Leads Reversed
Trial 2
Trial 1
Trial 2
Median (Std Dev)
Tower 219
12.5
12.8
12.5
12.9
12.6 (0.2)
Tower 219, 90°
13.2
12.4
12.0
12.4
12.4 (0.5)
These are all highly consistent results, even for the test in which the Suburban was parked on the test lead (marked in bold font). It was apparent where the vehicle was located, indicated by a extra peak in the Z(t) profile (see Figure 4-15).
Results without truck on lead
Results with truck parked on reaction lead
Figure 4-15 Comparison of Zed-Meter® instrument test results for CW8 tower 219, 90° lead orientation
The vehicle caused an extra peak of more than 100 Ω at 100 ns in Figure 4-15, but the median impedance settled down to the same value, 12.4 Ω, as in the other tests. Nevertheless, this is not a recommended practice. By way of historical interest, one of the first methods evaluated to “park” the test current was to use the capacitance of the line truck rather than the constant surge impedance of the reaction line. Unfortunately, there was not enough current flow into the truck to obtain a valid measurement.
4-15
Considerations for Guyed Towers Guyed transmission structures, such as the Georgia Power 230-kV structure shown in Figure 416, have a resonant response associated with the distance up the tower and out to each of the individual guy anchors. The guy anchors can provide some or most of the grounding for the tower, depending on how many there are and how deep the guy anchors are driven. Each guy wire is a straight, simple path connected to the tower, leading to an electromagnetic response that is poorly damped.
Figure 4-16 Guyed H-frame 230-kV tower 72 on Georgia Power Cedartown–Portland 230-kV line
The Zed-Meter instrument’s test waves will also bounce back and forth several times before oscillations decay to acceptable levels. This response is illustrated in Figure 4-17 for two conditions, one before installing the radial electrodes and a second test afterward to see the performance improvement.
4-16
Tower base potential, injected current and measured impedance Guyed H-frame 230-kV tower 73. Georgia Power Cedartown-Portland Median, 500–800 ns: 8.0 Ω untreated; 3.8 Ω with four 38-m buried radial crowfoot electrodes in parallel
Figure 4-17 Typical Zed-Meter® instrument results for H-frame tower with 16 guy wires
Figure 4-17 illustrates that the guyed structure has a tower base voltage that takes almost 500 ns to stabilize, and the effects of the tower resonant frequency are even seen to some extent in the measured currents. In this test series, relatively high soil resistivity led to a fairly rapid propagation velocity along the current reaction lead. This reduced the sweet spot for valid measurements from both directions, leaving a fairly narrow 300-ns window from 500–800 ns. The current reaction lead was 90 m long, and it would have been helpful with this tower type to use a longer, 125-m length, which would have extended the reflection from the far end of the reaction lead to about 1100 ns. The leads in this case should be laid from the tower base, not from one of the guy wires. The leads should bisect the guy wires, and because the oscillation time is longer, a minimum length of 125 m should be used. The test results in Figure 4-17 showed that the Georgia Power grounding specification, calling for installation of four buried, 38-m radial wires, was effective in reducing the surge impedance of the tower by a factor of two. However, we could have ensured greater confidence in the results if we had used a longer, 125-m current reaction lead to extend the duration of the sweet spot. This is especially important when measuring towers with low footing impedance, because the voltage swings below zero volts give negative values to the Z(t) vector before the beginning of the sweet spot. 4-17
Considerations for Twin Steel-Pole Towers Each leg of a twin steel-pole tower such as the one illustrated in Figure 4-18 will have slightly different impedance. The lightning response will be the parallel combination of the two impedances, adjusted for the effects of their mutual resistance. Measurement of the lightning response with the Zed-Meter instrument should eventually give the same impedance. The question about how long to wait has a relatively complicated answer. There is a smooth path from one leg, up to the cross-member, across to the adjacent pole, and back down to the adjacent concrete base. This smooth path means that the tower is susceptible to resonance at a frequency that is related to the total path length. The pole height (11 m) and separation (6 m) can be scaled from the tower photo, using the length of the 12-unit insulator string. The total path length from foundation to foundation works out to 28 m, which has a travel time at the speed of light of 94 ns. Figure 4-18 shows that the calculated impedance profile has an oscillation with a period of about 90 ns, which persists to about 700 ns.
4-18
Tower base and ground lead
Overall view
4.2 Ω with denoising process
5.9 Ω on other pole, longer 300-m leads
Figure 4-18 Typical Zed-Meter® instrument measurement results for each leg of twin steel-pole tower, Manitoba Hydro S65R tower 67
One way to compensate for problems with persistent oscillations caused by simple or guyed towers is to extend the length of the current reaction lead. This in turn will extend the current’s pulse width past the time when the tower oscillations have damped down. Although it is not feasible in every case, it was quite simple to lay out a 300-m reaction lead on the frozen snow in winter and to repeat the measurement in Figure 4-18 to obtain better results at a later time after 800 ns.
4-19
Overhead Groundwire Connections OHGWs generally have constant surge impedance, with a good estimate for individual wires given by Z=60 ln (2h/r), where h is the OHGW height (in meters) and r is the wire radius (also in meters). Lines with a single OHGW have a surge impedance that is one-half the value of the wire in a single direction. Lines with two or more OHGWs have some mutual coupling among wires that tends to increase the value slightly from the rough estimate of the surge impedance of a single wire, divided by the number of OHGWs heading away from the tower top. If the measured footing impedance is less than about 20 Ω, the effect of four parallel OHGWs (two in each direction) can generally be ignored. For higher resistance values, a correction can be calculated from the overall impedance of the OHGW, given by the OHGW diameter, height above ground, and separation. These values should be recorded at the tower under test, for use as described in the following subsection, Comparison Tests with and Without Overhead Groundwires. Some utilities insulate their OHGWs from towers using one or more disc insulators. This is done to improve AM re-radiation characteristics, to limit loop flow current, or for other technical reasons. In these cases, the transmission tower itself appears as a capacitance in parallel with the footing resistance. After the tower is charged up to the test potential, it does not exert any more influence. This process takes a time roughly equal to four times the tower height, divided by the speed of light. In this case, the influence on the Zed-Meter instrument’s measurement occurs early in time, and the case should be treated in the same way as the oscillations of a guyed tower. Comparison Tests with and Without Overhead Groundwires
The twin H-frame towers (Manitoba Hydro S65R 16 and 17) were tested with and without OHGWs attached to the pole bonds. Table 4-5 Zed-Meter® instrument test results, twin H-Frame with and without overhead groundwires Tower 17 With OHGW
Tower 16
No OHGW
With OHGW
No OHGW
Impedance (Ω), offset correction
4.98
4.81
4.20
2.58
2.17
Impedance (Ω), denoising process
5.10
4.19
4.43
2.67
2.65
The Zed-Meter instrument measures the parallel combination of the footing impedance and the OHGW surge impedance in two directions. With the typical 1-Ω standard deviation in each result, there is no significant difference in the readings with or without OHGWs. In this case, with low footing impedance, the high parallel impedance of the twin OHGWs of about 150 Ω would make a negligible change in the expected result of the parallel combination of the two values.
4-20
Tests with and without a connection were also conducted on a lattice tower with a single OHGW. The isolation was carried out live-line, as shown in Figure 4-19. A specialized highreach bucket truck was used to work between the phases of this double-circuit line in high wind conditions.
Isolation with bucket and boom between live phases
High wind conditions and blowing snow
Figure 4-19 Isolation of overhead groundwires and Zed-Meter® instrument testing on Tower RY 6-7 #11
This setup would not have been feasible on a line with a traditional double OHGW because the upper phase would block access to the outboard OHGW from the inside, and the center phase would block access from the outside using a vertical lift. The test results with and without OHGW connection are given in Table 4-6. Table 4-6 Results of Zed-Meter® instrument tests on lattice towers with and without single overhead groundwire connection Trial 1
Trial 2
RY 6-7 #11 without OHGW
1.33
1.54
RY 6-8 #11 with OHGW
1.17
0.44
1.06
1.35
1.1 (0.4)
RY 6-7 #10 with OHGW
1.70
1.61
0.62
0.89
1.2 (0.5)
RS 51 #10 with OHGW
6.51
7.16
8.22
6.55
6.8 (0.8)
Impedance (Ω), 400–910 ns
4-21
Trial 3
Trial 4
Median (Std. Dev)
1.4 (0.1)
There was no significant difference between the test results on RY 6-7, tower 11, with and without the OHGWs. Other test results were obtained on steel pole towers as shown in Table 4-7. Table 4-7 Results of Zed-Meter® instrument tests on steel poles with and without single overhead groundwire connections Trial 1
Trial 2
Trial 3
Trial 4
Trial 5
Median (Std. Dev)
RS 51 #15 with OHGW
1.56
1.35
1.98
1.87
1.89
1.9 (0.3)
RS 51 #15 without OHGW
1.70
1.84
1.97
2.06
RS 51 # 16 with OHGW
6.47
6.22
6.56
6.5 (0.2)
RS 51 #16 without OHGW
7.02
7.13
7.24
7.1 (0.1)
Impedance (Ω) 400–910 ns
1.9 (0.2)
The presence or absence of the OHGW had a negligible effect on the test result for tower 15 because its footing impedance was low relative to the surge impedance of the OHGW in parallel. The median result without OHGW for tower 16 was 8% higher, and a 3% increase would be expected from theory. Calculation of Overhead Groundwire Impedance
If desired, the effect of the parallel impedance of OHGWs can be corrected in the Zed-Meter instrument’s result. Equation E-2 gives the surge impedance of a single OHGW in one direction. To obtain ZGW, this impedance is divided by 2 at the tower top because effectively there is a wire in each direction. With Zaa equal to 543 Ω for a 7-mm radius wire at a 30-m height, ZGW will be 272 Ω. With two OHGWs in each direction, the single-wire impedance of Zaa=60 ln (2h/r) is divided by 4 and then corrected for the mutual impedance, Zab. This makes use of Equations E-3 and E-4 along with the knowledge that the currents and voltages in each OHGW will be equal. For a 5-m separation between OHGWs, the mutual impedance Zab is 149 Ω, and the total surge impedance of 0.25(Zaa+Zab) results in ZGW of 173 Ω. Unparalleling the Effect of Overhead Groundwires
After the value of the surge impedance of all OHGWs in parallel, ZGW, has been obtained, the true impedance of the footing is calculated from Equation 4-1:
Z footing =
1 1 Z Meas
−
Equation 4-1
1 Z GW
If the measured impedance is greater than the calculated impedance of OHGW, it implies that there is no conductive path from tower to the OHGW, and it also indicates that the tower is quite poorly grounded.
4-22
Which Is the Correct Value to Use?
The Zed-Meter instrument reading accurately reflects the response of the footing in parallel with the OHGWs. This is the same value that is used to calculate the tower potential rise in lightning calculations. For decisions about which towers would benefit from treatment, the Zed-Meter instrument’s value with OHGW in parallel should be used. The estimate of critical current for each tower should be based on the insulation level, the Zed-Meter instrument’s result, and the coupling coefficient from OHGW to phase conductors. Most sophisticated programs for lightning analysis consider the effects of the OHGWs in parallel. If the Zed-Meter instrument’s result will be entered in these programs, tower by tower, the recorded dimensions of OHGW height, separation, and radius should be used to unparallel the result.
Tests on Towers with High Noise Level At one tower, Manitoba Hydro S65R 63, all the test leads near the steel-pole towers were found to have high noise levels of up to 80 Vrms. The high noise levels led to considerable remaining dc offset in the averaged waveforms. The induced potentials depended on lead orientation. Table 4-8 lists the test results. Table 4-8 Typical variations in Zed-Meter® instrument test results, S65R towers Tower
63
63, other pole
68
69
70
Offset (V)
+62
+7
-5
+8
-6
-26
+24
+14
+30
Impedance (Ω) with offset correction
4.47
6.01
6.29
8.13
5.52
2.07
1.26
3.15
2.95
Impedance (Ω) with denoising process
4.89
6.78
6.82
8.66
5.97
2.66
1.76
3.49
3.36
Tower 69 was re-tested with 300-m leads of solid, insulated, 14-gauge wire rather than 125-m RG-58 coaxial cable sheath (see Figure 4-20). The improvement in standard deviation was significant with the denoising process and the longer leads, as noted for tower 69.
4-23
1.3-Ω impedance with offset correction
1.8-Ω impedance with denoising process
Figure 4-20 Zed-Meter® instrument measurement results for twin steel pole, S65R tower 69, with 300-m leads
Tests on Towers with Buried Counterpoise When the soil resistivity is high, a continuous buried or surface wire is sometimes used to connect the bases of transmission towers. This wire can have a finite length and be terminated in a ground rod, or it can continue all the way to the next tower. It provides a path with low impedance at power system frequency, but the connection has high inductance and surge impedance to transient lightning currents and Zed-Meter instrument test signals. One cross-calibration series on towers with continuous counterpoise connection to a substation was carried out at Duke Energy. The Zed-Meter instrument’s impedance values were, on average, 2.5 times as high as the reference values of low-frequency resistance obtained using an EPRI Smart Ground Multimeter tester.
Test Lead Orientation for Lines with Counterpoise Calculations showed little influence of parallel conductors above perfectly conducting ground on the surge impedance of the reaction leads. The presence of the counterpoise does not affect the injection of test current into the tower and grounding system. After the current is launched into the counterpoise, it propagates at a speed of about 0.3 c, compared to a speed closer to 0.6 c in the Zed-Meter instrument’s leads on the earth’s surface. The problem is that the counterpoise takes on the same tower potential that we are trying to measure against the remote potential lead. Any coupling of potential from the counterpoise system to the remote potential lead reduces the measured voltage, giving an incorrect (low) estimate of the actual impedance. For this reason, if it is at all feasible, the Zed-Meter instrument’s test leads should be oriented at right angles to the counterpoise to reduce coupling.
4-24
Which Leg Has the Counterpoise Connection?
In some grounding test methods that measure the current splits into each tower leg, the readings for one leg are quite low relative to the other three. In some cases, the readings of the three ungrounded legs can even be negative. When taking the parallel impedance of all values, this should still lead to a positive result. A feature of this method is that, if one leg has a low resistance, it is a good indication that it still has an intact connection to buried counterpoise. Simulations were carried out to evaluate whether the Zed-Meter instrument can also conduct this function. Results for 1000-Ωm soil are most relevant because this is the type of soil in which buried counterpoise would normally be effective. Two burial depths, 0.6 m and 1 m, were simulated with 38-m length in each direction from a single tower leg, running along the line right of way. With the recommended lead orientation at 90° to the direction of the counterpoise, the results in Figure 4-21 show that the two results do deviate, especially after the 450–650 ns sweet spot used to read out the impedance values. If the Zed-Meter instrument’s test leads must be laid along the right of way, where they are more closely coupled to the counterpoise, there is a much more notable effect. In this case, if the test leads are run along the right of way, the leg that is known or suspected to have a counterpoise connection should be the one that is tested.
4-25
Leg does have counterpoise
Leg does not have counterpoise 100
CA1-06-38-B-L01-GR-GW-1000: 24 Ω CA1-10-38-B-L01-GR-GW-1000: 24 Ω
90
Measured Impedance: Vmeas / I meas [Ohm]
Measured Impedance: Vmeas / Imeas [Ohm]
100
80 70 60 50 40 30 20 10 0
0
0.5
1 1.5 Time [microseconds]
2
80 70 60 50 40 30 20 10 0
2.5
CA2-06-38-B-L01-GR-GW-1000: 21 Ω CA2-10-38-B-L01-GR-GW-1000: 20 Ω
90
0
0.5
1 1.5 Time [microseconds]
2
2.5
Preferred leads for counterpoise test, 180° apart, 90° to right of way 100 CA1-06-38-C-L01-GR-GW-1000: 27Ω CA1-10-38-C-L01-GR-GW-1000: 25 Ω
90
Measured Impedance: Vmeas / I meas [Ohm]
Measured Impedance: Vmeas / I meas [Ohm]
100
80 70 60 50 40 30 20
CA2-06-38-C-L01-GR-GW-1000: 11 Ω CA2-10-38-C-L01-GR-GW-1000: 11 Ω
90 80 70 60 50 40 30 20 10
10 0 0
0
0.5
1 1.5 Time [microseconds]
2
2.5
0
0.5
1 1.5 Time [microseconds]
Reference leads, 180° apart, running along right of way Figure 4-21 Effect of counterpoise connection on tested leg using the Zed-Meter® instrument
4-26
2
2.5
5 RUNNING THE ZED-METER® SYSTEM SOFTWARE Previous sections have described the Zed-Meter® instrument and presented results from several versions of the instrument. The contents of those sections will remain essentially fixed, because the principles of applying a safe transient pulse to the tower base with a pair of long leads placed on the ground are what define the concept of the Zed-Meter instrument. Software, as opposed to hardware, is subject to continuous improvement. This section, which can change with each improved version, describes the operation of the existing Zed-Meter instrument using a laptop computer and the Labview program.
Overview The program is launched by double-clicking the icon on the computer screen, or it can run automatically, depending on the installation. On startup, the instrument checks that its components—a pair of digitizers and a special controller—are available for interrogation on the USB port. An interface screen is then presented. This allows the user to enter the specific data for the tower to be tested. Test results can all be given a common line name, in order to be grouped later into a line report. After the data are entered, the user should press the Start button. The instrument will apply its test impulses, transfer the results to the laptop, and perform some analysis. It will select an appropriate sweet spot in the records, record the median impedance and standard deviation, and read these out to the user in a large display. The user has the capability to save the data, repeat the test, or exit. Figure 5-1 shows the overall block diagram from the Labview program, and Figure 5-2 shows the details of the acquisition control loop that actually applies the sequence of pulses.
5-1
Figure 5-1 Block diagram of the main loop in the Labview program
Figure 5-2 Block diagram of the data acquisition control loop in the Labview program
5-2
Main User Interface The main user interface can be separated into the following three sections (see Figure 5-3): • • •
Section 1. This section contains the display selection control, which allows the user to navigate between the meter and graph displays (see Section 2). Section 2. This section shows the initialization screen and then toggles between the meter and graph displays. Section 3. This section contains the menu for the measurement and data storage screens.
Figure 5-3 Three sections of the user interface
5-3
Section 2 Elements
Section 2 is composed of three main elements: • • •
The initialization screen The meter display The graph display
Initialization Screen
When the software is launched, the first active display is the initialization screen in Section 2. The initialization screen contains a progress bar, which indicates the status of the software and hardware initialization.
Figure 5-4 Initialization screen in section 2
5-4
Meter Display
After initialization, the second display is the meter display (see Figure 5-5). The meter display is designed to display the test parameters entered by the user (1); an impedance graph (2); the calculated impedance (3); which also includes the total measurements taken and the impedance standard deviation; and two limit indicators (4). The top limit indicator warns the user if the measured impedance exceeds the target impedance. The bottom limit indicator warns the user if the impedance standard deviation has exceeded a set limit.
Figure 5-5 Meter display in section 2
5-5
Graph Display
The graph display (see Figure 5-6) allows the user to view the voltage, current, and impedance measurements (1). The graph selector (2) allows the user to select specific measurements to view.
Figure 5-6 Graph display in section 2
5-6
Section 3 Elements
Section 3 contains the menu for the measurement and data storage processes (see Figure 5-7), which consists of the following options: • • • • • • • • •
Start. Starts the measurement process (see “How-To: Start the Measurement Process” in this section.) Stop. Stops the measurement process (see “How-To: Stop the Measurement Process” in this section.) Save Data. After a successful measurement, this control can be used to save the data (see “How-To: Save the Measurement Data” in this section.). Line Report. Generates individual reports based on the line names stored in the log file (see “How-To: Create Line Reports” in this section). Data Viewer. Allows saved data files to be viewed graphically (see “Data Viewer” in this section). Setup. Allows the user to set parameters for the Zed-Meter instrument (see “Configuration (Setup) Window” in this section). Help. Displays the help file. About. Displays the software information and version. Exit. Closes the software.
Figure 5-7 Menu in section 3
5-7
Data Viewer
Press the Data Viewer button on the menu to access this function. The data viewer (see Figure 5-8) allows the user to view saved data files. The user interface for the data viewer is similar to that of the graph display. A graph exists to display the saved data, and the user can also select a specific type of data to view by using the graph selector. The data file path indicator, located to the right of the graph selector, points to the directory of the saved data files. Left-click the rectangular graph display to change plot characteristics.
Figure 5-8 Data viewer
The user can also manipulate the data that is displayed on the graph. The graph palette (in the lower left corner) allows the user to move or zoom in and out of the graph. The graph palette provides the following tools: — The cursor movement tool (left button) moves the cursor on the display. — The zoom tool (middle button) zooms in and out of the display. — The planning tool (right button) moves the plot around on the display. The plot legend (upper right corner) allows the user to edit the plot characteristics, such as plot colors.
To load a saved data file, follow these instructions: 1. 2. 3. 4.
Press the Data Viewer button. Press the Open button on the Data Viewer interface. Select the appropriate data file and click the OK button to view the file. Use the graph selector to view specific data.
5-8
Configuration (Setup) Window
Press the Setup button on the menu to access this function. The configuration window allows the user to set critical parameters for the Zed-Meter instrument. The configuration window consists of a category selection box, which allows the user to select a specific parameter to edit, and the following three buttons: • The OK button allows the user to save changes made in the configuration window. • The Default button sets all parameters to the default state. • The Cancel button exits the configuration window without saving any changes. The following parameters for the Zed-Meter instrument are controlled by the configuration options: • Measurement parameters • File path parameters • Impedance standard deviation limit • Target impedance Measurement parameters. The measurement parameters (see Figure 5-9) have three inputs. The first input, Number of Pulses, sets the total number of pulses (current and voltage) to be measured by the Zed-Meter instrument, which are then averaged. Setting a higher pulse count helps to remove unwanted noise. The Impedance Measurement Start (ns) and Impedance Measurement Stop (ns) inputs set the time interval on which the median impedance and impedance standard deviation are calculated.
Figure 5-9 Measurement parameters
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File path parameters. The Data File Path field (see Figure 5-10) allows the user to specify a folder to which all data files, including line reports, are saved when the Save Data button is pressed.
Figure 5-10 File path parameters
To set the data file path, follow these instructions: 1. Click the folder icon to the right of the Data File Path field (see Figure 5-10). 2. Navigate to the preferred folder (see Figure 5-11). 3. Click the Current Folder button (see Figure 5-11) to accept the folder as the save path.
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Figure 5-11 Pop-up screen for setting data file path
Impedance Std Deviation Limit. This screen (see Figure 5-12) allows the user to set a limit for the impedance standard deviation. If the calculated impedance standard deviation is higher than the limit specified by the user, it can indicate a connection problem between the Zed-Meter instrument and the structure under test.
Figure 5-12 Impedance standard deviation limit
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Target impedance map. This screen (see Figure 5-13) allows the user to set a target impedance for a specific line voltage. The target impedances specified in the map are then used by the ZedMeter instrument as impedance limits. To save changes to the map, press the Save button; pressing the OK button has no effect on the target impedance map.
Figure 5-13 Target impedance map
Setting the Time Windows
The lead configuration should be set up so that the minimum sweet spot duration is 200 ns. The default time window of 500–700 ns is practical in most cases because it allows for the decay of oscillations in guyed towers, yet ends well before reflections are expected back from the end of the current reaction lead. The time window start and end points can be adjusted in the Measurement Parameters window of the configuration menu (see Figure 5-9) after viewing the test waveforms. Setting the Alarm Thresholds
Many utilities have internal standards for the low-frequency ground resistance to be achieved with an isolated tower. This value can be used as a guide to the high-frequency resistance limit. However, the lightning impulse response of the majority of tested towers has been lower than the low-frequency value. This means that a tower that achieves a 10-Ω impedance in a valid ZedMeter instrument test could very well have a low-frequency resistance of 20 Ω. Alarm thresholds can be set in the Target Impedance Map screen (see Figure 5-13) Pretrigger Interval
The pretrigger interval is currently fixed at 1500 ns. This allows a few cycles of AM radio interference to be recorded as input to the denoising process that might be necessary.
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Demo Mode If any of the three hardware devices cannot be detected, the software enters Demo Mode (see Figure 5-14). In this mode, the user does not have the ability to acquire and save measurement data. In order to resume normal operation, the user must check all hardware connections to the computer and restart the software.
Figure 5-14 Demo mode
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How-To: Start the Measurement Process To start taking measurements, ensure that the hardware is properly connected and the software is not running in demo mode. 1. Press the Start button in the menu (see Figure 5-7). The Test Parameters window displays (see Figure 5-15).
Figure 5-15 Test parameters window
2. Enter the necessary information in the Test Parameters window (see Figure 5-15): — Operator name. Select the name of the current user. — Date and time. The date and time are entered automatically, based on the computer system time. — Line name. Enter or select the line name, a location-specific parameter based on the line. — Structure type. Enter or select the structure type, a location-specific parameter based on the structure. — Structure number. Enter or select the structure number, a location-specific parameter based on the structure. — Line voltage and target impedance. Select a proper line voltage for the line being measured. The corresponding target impedance is automatically selected based on the line voltage. This information is gathered from the target impedance map, which is accessed through the configuration (setup) window. 5-14
— Comments. Enter a brief description of the lead layout—for example “90 degree, voltage on ROW” or “180 degree, cross to ROW for counterpoise.” 3. Click the OK button to start the measurement. 4. If the test parameter line name or structure type is different from the previous input, the software performs an auto range (see Figure 5-16). This is where the proper hardware settings for the digital storage oscilloscope are set.
Figure 5-16 Zed-Meter® instrument auto range
5. Monitor the Pulse Count indicator on the meter display (see Figure 5-5). 6. After the Pulse Count indicator reaches the number of pulses set in the Measurement Parameters screen of the configuration window (see Figure 5-9), the software calculates and displays the line impedance and the standard deviation of the impedance. If the line impedance or the standard deviation of the impedance is too high, the limit indicators light up. To view a graphical display of the measured data, select the Graph option in the display selector (see Figure 5-3).
How-To: Stop the Measurement Process To stop a measurement that is currently in progress, press the Stop button in the menu (see Figure 5-7). The Stop button is active only when a measurement is taking place. Otherwise, this button is disabled.
How-To: Save the Measurement Data Data can be saved only after a successful measurement. Otherwise, the Save Data button is disabled. The data are saved in a text (.txt) file. The file name is composed of the line name, followed by an underscore, and then the tower number. To save the current measurements, press the Save Data button. The software creates a data folder and a log folder inside the folder (data file path) that you specified in the configuration window (see Figure 5-10). Data File
The data folder contains the text files, in tab-delimited format, of data for each of the lines measured. These files can also be opened in Excel for further analysis. The data file (see Figure 5-17) contains several lines showing the identification data, followed by the sweet spot start and stop times. The measured value of impedance and its standard deviation follow on the next lines. Next is a heading with the time (ns), remote voltage, lead current, structure current, lead impedance, and structure impedance, followed by 8000 readings starting at -1500 ns.
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Figure 5-17 Example of data file for test of National Grid line 4VW_234 structure
Log File
When measurements on a particular line are finished, pressing the Line Report button (see Figure 5-7) causes the software to create a second text file (see Figure 5-18), the log file. The log folder contains the log file, in tab-delimited format, for all the lines measured. The difference between the log file and the data file is that the information collected in the log file is reserved for creating line reports. The log file can also be opened in Excel for further analysis.
Figure 5-18 Example of a log file
How-To: Create Line Reports Line reports can be created only if a log file is present. 1. Press the Line Report button (see Figure 5-7). 2. If a log file is present, the software creates a line reports folder and the necessary line report files. The line reports folder is located inside the folder (data file path) that you specified in the configuration window (see Figure 5-10). 3. When the “Line reports created” prompt (see Figure 5-19) displays, click OK.
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Figure 5-19 Line report created prompt
4. A line report (see Figure 5-20) is generated for each line that was measured. These files are quite similar to the log file, except they are for each individual line. The line reports can also be opened in Excel for further analysis. 5. After the waveforms have been reviewed, you can change some of the default settings for analyzing the results.
Figure 5-20 Sample line report
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6 TYPICAL ZED-METER® INSTRUMENT RESULTS The “s” on the end of the section title, Results, is deliberate. The impedance of a ground electrode is not a constant value. The Zed-Meter® instrument’s impedance profile can decline with increasing time, suggesting that the grounding system has long buried wires (counterpoise). In the more common case, Z(t) can increase slightly with time as a result of the changes in resistivity of common earth materials and also because the ground has capacitance that takes some time to charge up. The time window selected for analyzing the impedance profile will affect the result in each case. Experience has shown that a good sweet spot of 500 to 800 ns can be achieved in most tests, and this is used as a reference in the production Zed-Meter instrument. However, additional information can gained by evaluating the impedance at different time intervals in postprocessing. For this reason, at least one waveform file should be saved for every test.
Effects of Overhead Groundwire The test result should increase by a small amount when the OHGWs are disconnected at the tower top. The high (170–250 Ω) surge impedance of all the OHGWs appears in parallel with the low (5–30 Ω) impedance of the footing. The amount of the increase in the test result depends on the value of footing impedance. For well-grounded towers, the increase should usually be less than 10% when OHGWs are isolated. For this reason, and also to save test time, isolation of the OHGW is not recommended as part of the normal test process. The OHGW heights and separation should be recorded if a correction for the parallel impedance will be made after the measurement. On towers with insulated OHGWs, there is often a strong benefit to carrying out conventional low-frequency measurements of resistance, using an oblique (45–60°) orientation of the potential profile in the three-point test. This lead geometry gives the low-frequency tower resistance, R, the local average resistivity, ρ, and effective ground electrode perimeter, ρ/R, with the same effort usually given to obtaining only the resistance value. Tests of this sort formed an important component of the Zed-Meter instrument’s cross-calibration program at several utilities.
Reasons for Steadily Increasing Impedance Values In many of the test results, there is a slight trend for the impedance profile with a wide sweet spot to increase with time. The median impedance in the time range from 500 ns to 700 ns forms the reference. Nearly all tower and lead configurations produce good results in this window. It takes some extra time to lay out the extra lead length needed to obtain an impedance profile that extends to more than 1000 ns, especially on soil with high resistivity. However, when this is done, processing of the results in two separate time windows will often yield different impedance values.
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Low-Frequency Versus High-Frequency Resistivity
The first physical reason for an increase in impedance with increasing time is related to the role of the dielectric response of the soil. Many natural soil materials have relative dielectric permittivity values, εr, in the range of 5 to 10. Especially for soils with high resistivity (ρ>1000 Ωm), the initial impedance of the soil will be capacitive, and it will charge up with a time constant to the eventual resistive value observed at lower frequency. Figure 6-1 gives one indication of how much difference there should be when Zed-Meter instrument values are compared to low-frequency measurements, based on the difference in measured soil resistivity at 100 Hz and 100 kHz.
Figure 6-1 Expected ratio of Zed-Meter® instrument readings to low-frequency readings of compact electrodes
If the soil around the tower is sand, the Zed-Meter instrument’s result should be 70–85% of the low-frequency value. The actual readings will show a large change with moisture content, meaning that readings taken days or hours apart can be quite different if there has been recent rain. The factors relating Zed-Meter test results to low-frequency values are much larger for other types of soil. Till is soil smeared into place by glaciers, containing a wide range of particle sizes from boulders (>256 mm) and pebbles through sand to silt (0.0625–0.004 mm). Till retains moisture much better than sand and supports hardwood forest or farming. For towers in this common soil type, Zed-Meter instrument readings should be 50–70% of the values found in lowfrequency measurements at 100 Hz.
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Clay has the smallest particle size (
