Substation Grounding
Short Description
grounding...
Description
SUBSTATION GROUNDING
A Project
Presented to the faculty of the Department of Electrical and Electronic Engineering California State University, Sacramento
Submitted in partial satisfaction of the requi requirements for for the the degree degreeof
MASTER MA STER OF OF SCIENCE in Electrical and Electronic Engineering by Inna Baleva SPRING 2012
©2012 2012 Inna Baleva ALL AL L RIGHTS RESERVED RESERVED ii
©2012 2012 Inna Baleva ALL AL L RIGHTS RESERVED RESERVED ii
SUBSTATION GROUNDING
A Project by Inna Baleva
A pproved pproved by:
________________ _______________________ _______________ ___________, ___, Co Com mmitt ittee Cha Chair Tur Turan Gon Gonen
________________ _______________________ _______________ ___________, ___, Co Com mmitt ittee Cha Chair Sal Salah You Y ousif sif
________________ _______________________ ____________ _____ Date iii
Student: InnaBaleva
I certify that this student has met the requirements for format contained in the University format manual, and that this project is suitable for shelving in theLibrary and credit is to be awarded for the project.
__________________________, Graduate Coordinator Turan Gonen
Department of Electrical and Electronic Engineering
iv
___________________ Date
Abstract of SUBSTATION GROUNDING by Inna Baleva
Statement of Problem
Designing aproper substation grounding system is quite complicating. Many parameters affect its design. In order for a grounding design to besafe, it needs to provide a way to carry the electric currents into theground under both normal and faulted conditions. Also, it must provide assurance that a person in the vicinity would not be endangered.
The grounding portion of substation design will be explored. In order to properly plan and design the grounding grid, calculations of the following will be done: maximum fault current, grid resistance, grid current, safe touch and step voltages, ground potential rise, as well as expected touch and step voltage levels. Background information and guidelines to design a substation grounding grid will be provided. A set of equations will be presented to calculate whether the design is safe, and finally, an example will be provided that can be used as a template.
v
Sources of Data
IEEE Std. 80-2000
Conclusions Reached
A safe substation ground grid was designed.
_______________________, Committee Chair Turan Gonen _______________________ Date vi
TABLE OF CONTENTS Page List of Tables .............................................................................................................. ix List of Figures............................................................................................................... x Chapter 1. INTRODUCTION ..........……………………………………………………….. 1 2. LITERATURE SURVEY ....................................................................................... 3 2.1 Substation Grounding Overview................................................................. 3 2.2 Permissible Current Through a Human Body During the Fault ................. 4 2.3 Common Shock Situations ..........................................................................4 2.4 Design of a Substation Grounding System ..................................................5 2.5 Grid Connections ........................................................................................6 2.6 Material Selection .......................................................................................8 2.7 Soil Characteristics .....................................................................................9 2.8 Protective Surface Material .......................................................................10 2.9 Soil Resistivity Measurements ..................................................................12 2.9.1 Wenner’s Four-Pin Method .......................................................12 2.9.2 Schlumberger-Palmer Four-Pin Arrangement ...........................14 2.10 Ground Resistance ..................................................................................14 2.11 Design Procedures of a Grounding System ............................................15 2.12 Design Modifications ..............................................................................17 2.13 Construction of a Grounding System ......................................................18 2.13.1 Ground Grid Construction-Trench Method .............................18 2.13.2 Ground Grid Construction-Conductor Plowing Method .........19 2.13.3 Installation of Pigtails and Ground Rods .................................19 2.14 Computer Aided Design .........................................................................21 2.15 Special Danger Points .............................................................................21 2.15.1 Substation Fence Grounding ....................................................21 vii
2.15.2 Operating Handles ...................................................................22 2.15.3 Surge Arrestor Grounding ........................................................23 2.15.4 Control Cable Sheath Grounding .............................................23 3. THE MATHEMATICAL MODEL ...................................................................... 24 3.1 Introduction ...............................................................................................24 3.2 Tolerable Body Current Limits ..................................................................24 3.3 Circuit Equivalents for Common Shock Situations ..................................27 3.3.1 Resistance of the Human Body ..................................................27 3.3.2 Touch and Step Voltage..............................................................27 3.4 Addition of Surface Layer ........................................................................31 3.5 Tolerable Step and Touch Voltage ...........................................................32 3.6 Conductor Sizing ......................................................................................34 3.7 Asymmetrical Currents .............................................................................37 3.8 Soil Resistivity Measurements ..................................................................37 3.9 Ground Resistance ....................................................................................39 3.10 Maximum Grid Current ..........................................................................40 3.11 Fault Currents ..........................................................................................41 3.12 Ground Potential Rise (GPR) ..................................................................42 3.13 Computing Maximum Step and Mesh Voltages .....................................43 3.13.1 Mesh Voltage (Em) ...................................................................43 3.13.2 Step Voltage (Es) ......................................................................46 4. APPLICATION OF MATHEMATICAL MODEL ................................................48 4.1 Introduction ...............................................................................................48 4.2 Initial Design .............................................................................................49 4.3 Design Using Copper-Clad Steel ..............................................................59 5. CONCLUSION .......................................................................................................61 Appendix ................................................................................................................... 62 References................................................................................................................... 64 viii
LIST OF TABLES Tables
Page
1.
Basic Range of Soil Resistivity....................... .……………………………….10
2.
Typical Surface Material Resistivities................ ……………………………. 11
3.
Material Constants ..................... ………….…………………………………. 35
4.
Material Constants.................................................. …………………………. 36
5.
Typical Values of Df ...............................................…………………………. 38
6.
Soil Resistivity Data Summary............................... …………………………. 49
7.
Conductor Properties .............................................. …………………………. 63
ix
LIST OF FIGURES Figures
Page
1.
Basic Shock Situations..................................... .………………………………. 7
2.
Wenner’s Four-Pin Method................................ ……………………………. 13
3.
Schlumberger-Palmer Four-Pin Arrangement ................................................. 14
4.
Design Procedure Block Diagram ................................................................... 20
5.
Body Current vs. Time ..................................................................................... 26
6.
Exposure to Touch Voltage ............................................................................. 28
7.
Touch Voltage Circuit ...................................................................................... 28
8.
Exposure to Step Voltage ................................................................................. 29
9.
Step Voltage Circuit ......................................................................................... 29
10.
Cs versus hs......................................................................................................................................................... 32
11.
Rectangular Grid with 22 Ground Rods .......................................................... 54
x
1 CHAPTER 1 INTRODUCTION
Safety and reliability are the two major concerns in theoperation and design of an electrical power system. These concerns also pertain to the design of substations. To ensure that substations are safe and reliable, the substation must have a properly designed grounding system.
The two main design goals to be achieved by any substation ground system under both normal and fault conditions are: 1. To provide means to dissipate electric currents into the earth without exceeding any operating and equipment limits 2. To assure that a person in the vicinity of grounded facilities is not exposed to the danger of critical electric shock [4]. . This project provides necessary background information for substation ground design. It provides a set of guidelines that can be used, also it provides some design modification suggestions that might help to alter the preliminary design if the mesh and step voltages were greater than the tolerable voltages.
Also grounding system design was done for a transmission station using the IEEE Std. 80-2000 procedure as an example. Actual values from a transmission station were used
2 in the calculations, such as the measured soil resistivity, fault current, etc. Because copper theft is a major problem, calculations using copper-clad steel were done as well.
3 CHAPTER 2 L IT ERATURE SURVEY
2.1 Substation Grounding Overview Grounding is an important aspect of every substation. The function of a grounding system is: to ensure the safety of personnel and the public, to minimize hazard from transferred potential, to protect equipment, to provide a discharge path for lightning strikes, and to provide a low-resistance path to ground. A good grounding system has a low resistance to remote earth to minimize the ground potential rise (GPR) [2,4].
In order for a grounding design to be safe, it needs to provide a way to carry the electric currents into theground under both normal and faulted conditions. Also, it must provide assurance that a person in the vicinity would not be endangered. Because there is no simple relation between theresistance of the grounding systemand themaximumshock current a person can experience, a complete analysis must be done to consider many different aspects such as the location of the ground electrodes, soil characteristics, etc [6].
People assume that any grounded object can be safely touched, but that is not always the case. A low substation ground resistance doesn’t not guarantee safety [2-3]. There are no simple relation between the ground system resistance and themaximum shock current that a person might be exposed to [4].
4 2.2 Permissible Current Through a Human Body During the Fault Humans are quite sensitive to AC currents ranging from 50-60 Hz. The effects of the AC current going through a human body depend on the magnitude, duration, and also frequency [6]. The threshold of perception for the human body is about 1mA. Currents between 1-6 mA, often called let-go currents, usually do not impair a person from controlling his muscles and releasing the energized object they were holding. Higher currents ranging from 9-25 mA can cause pain and affect themuscle control so that the energized object is hard if not impossible to release [1]. Still higher currents between 2575 mA can affect breathing and may cause fatality. If current is even higher, it could result in ventricular fibrillation of the heart, which if not treated quickly, can result in death [6]. When currents reach 100 mA and higher, above the ventricular fibrillation level, it can cause burns, heart paralysis, and inhibition of breathing [1-3].
2.3 Common Shock Situations There are three main electrical shock situations that can occur when a person is around a substation. The first is a foot-to-foot shock which would involve thecurrent going through onefoot and then out theother. This is typically caused by an increase in ground potential rise which allows current to build up on the soil surface and then through objects on the surface. The foot-to-foot shock is the least dangerous of the threebecause the current does not go through vital organs such as the heart [4]. The second is hand-tofeet which involves touching something that is electrified with the hand and having the current pass into the ground through the feet. The final shock situation is a hand-to-hand
5 or metal-to-metal contact which would be touching something electrified with one hand and having the current go through the other hand that is touching something else. These shocks can usually be eliminated by connecting all the objects in the substation to the grounding grid [4]. The use of a thin layer of surfacematerial such as gravel around the substation can greatly reduce the chance and strength of electric shocks. The gravel can increase theresistance between the ground and a person thus making currents less likely to pass through them. Figure 1 shows the different shock situations.
2.4 Design of a Substation Grounding System The substation ground grid design is based on the substation layout plan. The following points serve as guidelines to start a grounding grid design: 1. The substation should surround the perimeter and take up as much area as possible to avoid high current concentrations. Using more area also reduces the resistance of the grounding grid. 2. Typically conductors are laid in parallel lines. Where it is practical, the conductors are laid along the structures or rows of equipment to provide short ground connections. 3. Typical substation grid systems may include 4/0 bare copper conductor buried 0.3-0.5 m (12-18 in) below grade and spaced 3-7 m (10-20 ft) apart in a grid pattern. The conductors should be securely bonded at cross-connections.
6 4. Ground rods may be installed at grid corners and junction points along the perimeter. They may also be installed at major equipment, especially near surge arresters. 5. The grid should extend over the entire substation and beyond the fence line [1-3]. 6. The ratio of the sides of the grid meshes are usually 1:1 to 1:3 [1, 4]. To get started on the preliminary design, the following steps can be taken: 1. Draw thelargest square, rectangular, triangular, T-shaped, or L-shaped grid that will fit on the layout drawing [1]. 2. Place grid conductors to produce square meshes, approximately 6.1-12.2 m(2040 ft) 3. Set the grid height equal to 0.4572 m (18 inches) 4. Set thickness of the surface material to 0.1016 m (4 inches) 5. Place ground rods around the perimeter [1].
2.5 Grid Connections Typically different sized conductors are used in linking the substation to the grounding grid. Any above-ground conductive material which could possibly become energized such as a metal structures, machine frames, and transformer tanks or any metal parts that could have a different potential from others should be tied together by the grounding grid [4].
7
. s n o i t a u t i S k c o h S c i s a B : 1 e r u g i F
. d e v r e s e r s t h g i r l l A . E E E I . 0 0 0 2 © t h g i r y p o C . 2 1 e r u g i F 0 0 0 2 0 8 . d t S E E E I . f e R
8 All other equipment that could be the source of a fault current must also be connected to the grid. Copper cable is often used for the connections, but in some cases the equipment and buildings can be used as the conductor link [4]. Usually thegrid connections are securely welded together to prevent any failure during high fault currents.
2.6 Material Selection Conductors can be of various materials including copper, copper-clad steel, aluminum, or steel. Each type of conductor has advantages and disadvantages.
Copper is the most commonly used material for grounding. Copper has high conductivity. Also, it is resistant to most underground corrosion because it is cathodic with respect o most other metals [4]. It also has good temperature characteristics and thermal capacity. The disadvantage of copper is that it is expensive and often stolen, leaving the equipment ungrounded.
Copper-clad steel is usually used for ground rods, and sometimes for grounding grids. Copper-clad steel has a fraction of the conductivity of copper, but it is adequate for use of grounding. It combines thestrength of steel with theconductivity of copper. Copper-clad steel is less susceptible to theft than copper because it is a bimetallic product and has virtually no recycle value.
9 Aluminumhas good conductivity, but not as good as copper. Aluminummay corrode in certain soils [4]. Aluminum costs less than copper, and theft is less of an issue. It’s fusing temperature is about half of copper and its thermal capacity is about two thirds.
Steel can be used for ground grid conductors and rods, but corrosion is an issue. Steel has good temperature characteristics and thermal capacity as well. Theft is not an issuefor steel.
2.7 Soil Characteristics The earth’s soil can be considered to be a pure resistance and thus is the final location that a fault current is dispersed. Soil resistance can contain a current up to a critical amount which varies depending on the soil and at this point, electrical arcs can develop on the surface of the soil that can electrify objects on the surface such as a person [4]. A soil’s resistivity can be affected by the flow of current through it by being heated which makes the soil dry out and becomemore resistive [4]. Wet soil has much less resistance than that of dry soil so ideally the grounding grid and rods should be located in moist earth. Typically soil resistance quickly increases when its moisture content is less than 15% of the soil weight and the resistance barely changes once the moisture content is at least 22% [4]. Table 1 shows a basic collection of soil resistivity depending on the moisture and type.
10 Table 1: Basic Range of Soil Resistivity Ref. IEEE Std, 80, Table 8. Copyright ©2000. IEEE. All rights Reserved Average Resistivity (Ω·m) Type of Earth Wet Organic Soil 10 Moist Soil 10 Dry Soil 10 Bedrock 10
Table 1 shows that wet or even moist soil have very small resistances so it is beneficial to keep the grounding soil as damp as possible. A common practice to help accomplish this is to use of a surface material layer such as gravel. Not only does a surface material greatly reduce the amount of soil evaporation, but it typically has a high resistance which reduces the magnitudes and chances of shock currents occurring [4]. Soil characteristics and the type of surface layer to be used vary depending on theareain the world in which the substation is located and what is required by the grounding system.
2.8 Protective Surface Material In order to greatly reduce theshock current and increasethe contact resistance between the soil and the feet of people in a substation, a thin layer of a highly resistive protective surface material just as crushed rock (gravel) is spread above the earth grade at a substation. Generally a layer of the surfacematerial is 3-6 inches and it extends 3-4 feet outside the substation fence. If it is not extended beyond the substation fence, the touch voltages becomedangerously high [1].
11 The resistivity values for the surface material layers vary. The range depends on many factors such as type of stone, size, condition of the stone, amount and type of moisture content, atmospheric contamination, etc [1]. Table 2 shows typical resistivity values for different types of surface materials. These values were measured by several different parties in different regions of the United States. These values are not valid for every type and size of stone in every region, thus tests need to be done for the resistivity in the region’s substation [1]. Table 2: Typical Surface Material Resistivities. Ref. IEEE Std, 80, Table 7. Copyright ©2000. IEEE. All rights Reserved Number 1 2 3
4 5 6
7 8 9
a
10 11
Resistivity of sample Ω ·m Dry Wet 6 140 x 10 1300(ground water, 45 Ω·m) 4000 1200(rain water, 100W) 6513(10 min after 45 Ω·m water drained) 6 6 #4 (1-2in) (0.025-0.05 m) 1.5 x 10 to 4.5 x 10 5000 (rain water, washed granite (Ga.) 100 Ω·m) 6 6 #3 (2-4 in) (0.05-0.1 m) washed 2.6 x 10 to 3 x 10 10 000 (rain water, granite (Ga.) 100 Ω·m) 6 Size unknown, washed limestone 7 x 10 2000-3000 (Mich.) (ground water, 45 Ω·m) 6 Washed granite, similar to 0.75 2 x 10 10 000 in (0.02m) gravel Washed granite, similar to pea 40 x 106 5000 gravel #57 (0.75 om) (0.02 m) washed 190 x 106 8000 (ground water, 45 Ω·m) granite (N.C.) 6 6 Asphalt 2 x 10 to 30 x 10 10 000 to 6 x 106 Concrete 1 x 106 to 1 x 109 a 21 to 100
Description of surface material (U.S. State where found) Crusher run granite with fines (N.C.) 1.5 in(0.04m) crusher run granite (Ga.) with fines 0.75-1 in(0.02-0.025 m) granite (Calif.) with fines
Oven dried concrete. Values for air-cured concrete can be much lower due to moisture content.
12 2.9 Soil Resistivity Measurements Before the design of the grounding system begins, soil resistivity measurements need to be taken at the substation[1]. Stations with uniform resistivity throughout theentire area are rarely found. Thus, measurements should be made at multiple locations within the site. Usually there are several layers, and each has a different resistivity. If there are large variations, more readings should be taken at these locations [4]. Lateral changes may occur as well, but in general the changes are gradual and negligible [4].
There are a number of measuring techniques. With two-point methods, rough measurements of the resistivity of undisturbed earth can be made. Three-point method or variation of depth method measured ground-resistancetest several times. Each time the burial depth of the test electrode is increased by a certain amount. But this method is not recommended if largevolume of soil needs to be investigated. Four-pin methods are the most accurate method of measuring theaverage resistivity of large values [5].
2.9.1 Wenner’s Four-Pin Method The Wenner’s four-pin method is the most common. This method is also called the Equally- Spaced Four-Pin method. [5]. In this technique, four probes are driven into the ground in a straight line to a depth b, at equal distances a apart. The voltage between the two inner probes is measured and is divided by the current of the two outer probes. This gives a value of the mutual resistance R. The Wenner’s four-pin method is shown in Figure 2 below [5].
13
Figure 2: Wenner’s Four-Pin Method Ref. IEEE Std. 81-1983 Figure 3(a). Copyright ©1983. IEEE. All rights reserved.
The resistivity measurement records should include temperature data and information on the soil moisture conditions at the time that themeasurements were done. Also record all data available on any buried conductors already known or suspected. Buried conductors in contact with the soil can invalidate readings if they are close enough by altering the test current flow pattern [4].
The Wenner four-pin method is popular for a number of reasons. This method obtains soil resistivity data for deeper layers without having to drive the test pins to those layers. Also, no heavy equipment is needed [1,3]. The results are not greatly affected by the resistance of the test pins or the holes created by driving the test pins into the soil [1].
A shortcoming of the Wenner method is that the magnitude of the potential between the two inner electrodes rapidly decreases when their spacing is increased to large values. And often times commercial instruments cannot measure such low potential values [5].
14 2.9.2 Schlumberger-Palmer Four-Pin Arrangement The Schlumberger-Palmer arrangement is another four-pin method. It is also called the Unequally- Spaced Four-Pin method [5]. This method is similar to the Wenner’s FourPin method. For this method, there is a larger spacing between the current electrodes. The potential probes are brought closer to the corresponding current electrodes. Doing this increases the measured potential value. Figure 3 shows the Schlumberger-Palmer arrangement [5].
Figure 3: Schlumberger-Palmer Four-Pin Arrangement Ref. IEEE Std. 81-1983 Figure 3(b). Copyright ©1983. IEEE. All rights reserved.
2.10 Ground Resistance The ground resistance for a substation needs to be very low to minimize the ground potential rise and increase the safety of the substation [2,6]. The ground resistance is usually 1 Ω or less for transmiss ion and other large substations [1-4] . In distribution
substations, the usual acceptable range is 1-5Ω [4]. Resistance primarily depends on the area to be occupied. Also resistance can be decreased for a given area by using ground
15 rods and adding more grid conductors. If it is impossible to reach a desired ground resistance by adding more grid conductors and/or ground rods, the soil surrounding the electrode can be modified.
Sodiumchloride, magnesium, and copper sulfates, or calcium chloride can be used to increasethe conductivity of the soil immediately surround theelectrodes. Another method is to place a ground enhancement material around the rod. Other methods are mentioned in IEEE Std. 80-2000 [4].
2.11 Design Procedures of a Grounding System The design process of a substation grounding system requires many steps. The following steps were established by the IEEE Standard 80-2000 for the design of the ground grid:
Step 1: The property map and general location plan of the substation should provide good estimates of the area to be grounded. A soil resistivity test will determine the soil resistivity profile and the soil model needed. Step 2: The conductor size is determined. The fault current 3I0 should be the maximum expected future fault current that will be conducted by any conductor in the grounding system, and the time, tc, should reflect the maximum possible clearing time (including backup). Step 3: The tolerable touch and step voltages are [to be] determined. The choice of time, ts, is based on the judgment of the design engineer. Step 4: The preliminary design should include a conductor loop surrounding the entire grounded area, plus adequate cross conductors to provide convenient access for equipment grounds, etc. The initial estimates of conductor spacing and ground rod locations should bebased on thecurrent, I G, and the area being grounded.
16
Step 5: Estimates of the preliminary resistance of the grounding system in uniform soil can be determined. For the final design, more accurate estimates of the resistancemay be desired. Computer analysis based on modeling the components of the grounding system in detail can compute the resistance with a high degreeof accuracy, assuming thesoil model is chosen correctly. Step 6: The current, I G, is determined. To prevent overdesign of the grounding system, only that portion of the total fault current, 3I 0, that flows through the grid to remote earth should beused in designing the grid. The current, I G, should, however, reflect theworst fault type and location, the decrement factor, and any future systemexpansion. Step 7: If the GPR of thepreliminary design is below the tolerable touch voltage, no further analysis is necessary. Only additional conductor required to provide access to equipment grounds is necessary. Step 8: The calculation of the mesh and step voltages for the grid as designed can be done by the approximate analysis techniques for uniform soil, or by the more accurate computer analysis techniques. Step 9: If the computed mesh voltage is below the tolerable touch voltage, the design may be complete (see Step 10). If the computed mesh voltageis greater than the tolerable touch voltage, the preliminary design should be revised (see Step 11). Step 10: If both the computed touch and step voltages are below the tolerable voltages, the design needs only the refinements required to provide access to equipment grounds. If not, the preliminary design must be revised (see Step 11). Step 11: If either the step or touch tolerable limits are exceeded, revision of the grid design is required. These revisions may include smaller conductor spacing, additional ground rods, etc. More discussion on the revision of the grid design to satisfy the step and touch voltage limits is given in [Section 2.12] Step 12: After satisfying thestep and touch voltage requirements, additional grid and ground rods may be required. The additional grid conductors may be required if the grid design does not include conductors near equipment to be grounded.
17 Additional ground rods may berequired at the baseof surge arresters, transformer neutrals, etc. The final design should also be reviewed to eliminate hazards due to transferred potential and hazards associated with special areas of concern [4, pp. 88-89].
The block diagram in Figure 4 illustrates the procedure to design the ground grid.
2.12 Design Modifications If the calculated grid mesh and step voltages are greater than the tolerable touch and step voltages, then thepreliminary design needs to be modified. The following are possible remedies: (a) Decrease total grid resistance: If the total grid resistance is decreased, the maximum GPR is decreased; hence the maximumtransferred voltage is decreased. An effective way to decrease thegrid resistanceis to increasethe area occupied by the grid. Deep driven rods or wells can be used also if area is limited. (b) Decrease grid spacings: Decrease the mesh size by increasing the number of parallel conductors in each direction. Dangerous potentials within the substation can be eliminated. For the perimeter, a ground conductor can be buried outside the fence, or increase thedensity of ground rods at theperimeter. (c) Increase the thickness of the surfacelayer: a practical limit may be 6 inches. (d) Limit total fault current: If feasible, limiting thetotal fault current will decrease the GPR and gradients in proportion. (e) Diverting greater part of the fault current to other paths
18 (f) Barring access to limited areas: if practical, can reduce the probability of hazards to personnel [1,4].
2.13 Construction of a Grounding System The method chosen for construction depends on the size of the grid, soil type, size of conductor, burial depth, equipment available, cost of labor, and physical or safety restrictions. There are two common ways to install the ground grid. These methods are the trench method and the cable plowing method. Both methods use machines. If the job site is too small or there is not enough spaceto move the machines around, then the ground grid is installed by hand digging [4].
2.13.1 Ground Grid Construction-Trench Method Markers are placed on theperimeter to identify the spacing between the parallel conductors. These markers serve as a guide for the trenching machine. The trench machine is used to dig trenches along the side having alarger number of parallel conductors to a specified depth, usually 0.5 m(1.5 ft). Conductors are then installed in thesetrenches and the ground rods are driven and connected to the conductors. Pigtails for the equipment grounds are also placed at this time. These trenches are then backfilled with dirt up the cross connections.
Cross-conductor trenches are then dug, again using markers as guides. Conductors are installed and any remaining ground rods are driven and connected to the conductors. Also
19 remaining pigtails are connected. Then cross-type connections are madebetween the perpendicular conductor runs. Finally thetrenches are filled with dirt [4].
2.13.2 Ground Grid Construction-Conductor Plowing Method This method is economical and quick when conditions are favorable and the proper equipment is available. This method plows the conductors in using a special narrow plow. This plow can be attached to, or drawn by, a tractor or a four-wheel drive truck. The conductor is laid on the ground either in front of the plow or a reel of conductor is fed into theground along the blade of the plow. For the cross conductors, they are plowed in at a slightly less depth in order to avoid damaging the previously laid conductors. The points of crossing and points where ground rods are to be installed are then uncovered and connections are made [4].
2.13.3 Installation of Pigtails and Ground Rods Pigtails are left for grounding connections to equipment or structures. Pigtails can be the same cable size as the underground grid, or a different size. This depends on the number of grounds per device as well as the magnitudeof the ground fault current. Ground rods are installed using ahydraulic hammer, air hammer, or other mechanical devices. Two ground rods are joined by either using a exothermic method or a threaded or threadless coupler [4].
20
Figure 4 : Design Procedure Block Diagram. Ref. IEEE Std. 80-2000 Figure 33. Copyright ©2000. IEEE. All rights reserved.
21 2.14 Computer Aided Design Computers are frequently used in designing substation grounding systems. Some reasons to use computer analysis are 1. The parameters exceed those of the simplified design equations. 2. A two-layer or multi-layer soil model is preferred due to significant variations in soil resistivity. 3. Uneven grid conductor or ground rod spacing. 4. Flexibility in determining local danger points 5. Presence of buried metallic structures/conductors that are not connected to the grounding system introduces complexity 6. Preliminary design can be optimized and analyzed [1,4].
2.15 Special Danger Points There are several danger points within a substation such as the fence, equipment operating handles, surge arrestors, etc. One has to make sure that they are properly grounded to ensure safety.
2.15.1 Substation Fence Grounding It is critical to ground the substation fence because the fence is generally accessible to the public. The touch potential on both sides of the fence needs to be within the calculated tolerable touch potential limit. The substation fence should be connected to themain
22 ground grid. An outer grid conductor should beinstalled a minimumof 0.91 m(3 feet) outside the fence. Connections to the outer grid conductor should be made at all corners posts and at line posts every 12.92-15.24 m(40-50 feet). The gatepost should be bonded securely to the fence. It is also recommended that all gates swing inward [1,4].
2.15.2 Operating Handles Equipment operating handles represent a significant concern if not adequately grounded because it requires the presence of an operator near a grounded structure. If a fault occurs, the operator may besubjected to an electrical shock. If the grounding system was designed with IEEE Std. 80, then the touch and step voltages near the operating handle should be within safe limits. But in most cases additional means are taken in order to provide a greater safety factor for the operator. Somepractices include connecting the switch operating shaft to a ground mat. The ground mat is directly connected to the ground grid and also the switch operating shaft. The operator stands of the mat when operating the switch. Using these techniques provides a direct bypass to ground [4].
Utilities use different practices to ground the switch operating shaft. About half of the utilities provide a direct jumper between the switch shaft and theground mat. The other half provided a jumper from theswitch shaft to the adjacent grounded structural steel and the steel is used as part of the conducting path. About 90% of utilities usea braid for grounding the switch shaft [4].
23 2.15.3 Surge Arrestor Grounding Surge arrestors need to be reliably grounded to ensure protection of the equipment they are protecting. They should be connected as close as possible to theterminals of the equipment it’s protecting and have as short and direct path to the grounding systemas possible and practical [4]. Also arrestor leads should be as free from sharp bends as practical [1].
2.15.4 Control Cable Sheath Grounding Metallic cable sheaths may attain dangerous voltage levels with respect to ground if not effectively grounded. All grounding connections should be made to provide apermanent low-resistance bond. Cable sheaths should be grounded at two or more locations [1,4].
24 CHAPTER 3 THE TH E MAT MA T HEM ATI AT I CAL M ODEL
3.1 I ntroduc ntroducti tion on In order to design design a proper and saf safe substa substation tion grounding grounding system, various various safety parameters must be found such as as the touch and step voltage vol tagelevels. E Each ach grounding grounding systemmust be unique uniquelly designe designed in in orde orderr to have the mesh esh and step voltage vol tages below below the the tolera tolerabl ble e touch and step voltage voltages of the personnel personnel that might beworki working ng at the si site when a fault fault occurs. This his chapte chapterr provide provi des the process process and equations tions to safely desi design gn a substati substation on grounding grounding system.
3.2 T oler olerable able Body Body Curr Cur rent L imits A human body at 50Hz or 60Hz can gave gave duration duration of the curren currentt le less than than the value value that can can causeventri ventricular cular fi fibril brillation tion of the heart. Ve V entricular ntricular fi fibril brillation tion is cause caused whe when the the body current current replace replaces the normal rhythmic rhythmic contracti contraction on of the hea heart and may causea lack lack of circulation and pulse [1-4,6].
Dalziel’s studies show that the no fibrillation current of magnitude, I B, at duration ranging from 0.030.03-3.0 3.0 s can be sim simply ply expressed expressed as:
IB =
k ts
where k = SB
(3.1)
25 and
IB
: rm rms magni agnitude tude of the curren currentt through the body (A (A )
ts
: durati duration on of the current exposure (s)
SB : shock energy energy related to electric ectric shock energy nergy k : constant rel
Base ased on Dalzi Dalzie el’s studi studies, 99.5% of people ople can saf safely withsta withstand themagni agnitude tudeof the current without ventricular fibrillation. Dalziel also found that the shock energy constant to vary with with weight [4]. [4]. For For a pe person wei weighing approxi approxim mate ately 50 kg (11 (110 0 lb) lb) k50 =0.116, thus theformul ormula for allowable all owable body current become comes: I B50 =
0.116
ts
(3.2)
For a person weighing approximately 70 kg (155 lb) k50 =0.157, thus the formula for all allowable owable body curren currentt becomes:
I B70 =
0.157
ts
(3.3)
This This equation ion is not value lued for for very short or very lon long duration ion.
Biegelmeier’s curve in Figure 5 shows the body current versus time. This curve has a 500mA limit for for tim ti mes up to 0.2 s, then thelimit decreases to 50 mA at 2 s and and beyond. This This figu figure also lso shows a compariso ison of the body current for for both a 50 kg and a 70 kg person.
26 I n modern odern operati operating ng practi practice ces, s, re recourse after ter a ground fau faullt is is comm common. In I n circum circumsta stance nces s where there are reclosures, reclosures, a person might experi experience encethe first fi rst shock without without permanent injury. njury. But then an automati atic reclosure reclosure can resul result in in anothe another shock less than than 0.33 seconds of the first shock. This second shock that occurs after a short interval of time before the person can recover from the initial can cause a serious accident [1,4].
Figure 5 : Body Current vs. Time. Ref. IEEE Std. 80-20 80-2000 00 Figure gure 5. Copyrigh Copyrightt ©20 2000 00.. IE I EEE. Al A ll righ rights ts reserved. rved.
27 3.3 Circuit Equivalents for Common Shock Situations 3.3.1 Resistance of the Human Body The human body can be approximated as a resistance for DC and 50 Hz or 60 Hz AC currents. The current path is considered from one had to both feet or from onefoot to the other. The internal resistance of a human body is approximately 300 Ω. The body resistance including skin ranges from 500-3000 Ω [4]. For simplicity, IEEE Std 80-2000 represents the resistance of a human body from hand-to-feet and also from hand-to-hand, or from one foot to the other as
RB = 1000 Ω (3.4)
3.3.2 Touch and Step Voltage The accidental circuit in Figure 6 is the result of hand-to-feet contact. The voltage found in this circuit is referred to as touch voltage because it results from someone touching an electrified object while the feet are in contact with theground. In most cases thelimiting factor for a grounding design is the tolerable touch voltage [1]. Figure 7 serves as a visual aid in displaying a typical hand-to-feet circuit through a person.
Another accidental circuit occurs as a result of foot-to-foot contact as seen in Figure 8. The voltage found in this circuit can be referred to as the step voltage because it would result from someone standing on soil which has current build up on its surface due to a ground potential rise [4]. Figure 9 serves as a visual aid in displaying a typical foot-tofoot circuit through a person.
28
Figure 6 : Exposure to Touch Voltage. Ref. IEEE Std. 80-2000 Figure 6. Copyright ©2000. IEEE. All rights reserved.
Figure 7 : Touch VoltageCircuit. Ref. IEEE Std. 80-2000 Figure 8. Copyright ©2000. IEEE. All rights reserved.
29
Figure 8: Exposure to Step Voltage. Ref. IEEE Std. 80-2000 Figure 9. Copyright ©2000. IEEE. All rights reserved.
Figure 9 : Step VoltageCircuit Ref. IEEE Std. 80-2000 Figure 10. Copyright ©2000. IEEE. All rights reserved. Using Figure 6 or Figure 8, the Thevenin equivalent circuit for the current through the body, I b , of a person is: Ib =
where:
V Th Z Th + RB
(3.5)
30
V Th : Thevenin voltage between terminal H and F (V) Z Th : Thevenin impedance from point H and F (Ω)
RB : body Resistance (Ω) The Thevenin equivalent impedance for the touch voltage accidental circuit is:
Z Th =
Rf 2
(3.6)
The Thevinin equivalent impedance for the step voltage accidental circuit is: Z Th = 2Rf
(3.7) where: Rf : ground resistance of one foot
In circuit analysis, a human foot is represented as a conducting metallic disc and resistanceof the shoes and socks are neglected.
The equation to calculate the ground resistance Rf is:
Rf =
ρ
4b
(3.8)
where: ρ :
earth’s resistivity (Ω·m) b : radius of a foot taken as a metallic disk (typically 0.08m)
Using a circular plate of approximately 0.08m, the equations for Zth are: For touch voltage accidental circuit
Zth = 1.5ρ (3.9) And for step voltage accidental circuit
31
Zth = 6ρ (3.10) 3.4 Addition of Surface L ayer When possible, substations placea layer of highly resistive material such as crushed rock. The addition of a surface layer changes the ground resistance, Rf . The new ground resistance becomes:
ρ Rf = s Cs 4b
(3.11)
The surface layer derating factor, Cs, can be calculated as:
0.09 1−
CS = 1−
ρ s ρ
2hs + 0.09
(3.12)
where ρ : resistivity of the earth (Ω·m) ρs : resistivity of surface layer material (Ω·m)
hs : thickness of surface material (m) Cs can also be approximated by first calculating the reflection factor between the different materials, K , and then using Table 10.
The reflection factor is calculated as: K =
− ρ s ρ + ρ s ρ
(3.13)
32
Figure 10 : Cs versus hs Ref. IEEE Std. 80-2000 Figure 11. Copyright ©2000. IEEE. All rights reserved. 3.5 Tolerable Step and Touch Voltage When designing a substation grounding system, the maximum tolerable voltages must be calculated in order to create a proper ground grid. These voltages depend on the soil resistivity, soil layer and the duration of the shock current. The maximum driving voltage of any accidental circuit shouldn’t exceed the step voltage and touch voltage limits.
For step voltage the limit is:
33 Estep = (RB + 2⋅ Rf ) ⋅ I B
(3.14) For a body weighing 50 kg
Estep50 = (1000+ 6⋅ Cs ⋅ ρ s )
0.116
ts
(3.15)
For a body weighing 70 kg
Estep70 = (1000 + 6⋅ Cs ⋅ ρ s )
0.157
ts
(3.16)
For touch voltage, the limit is
Rf Etouch = RB + ⋅ IB 2
(3.18)
For a body weighing 50 kg
Etouch50 = (1000 + 1.5⋅ Cs ⋅ ρ s )
0.116
ts
(3.19)
For a body weighing 70 kg Etouch70 = (1000+ 1.5⋅ Cs ⋅ ρ s )
0.157
ts
(3.20)
If no protective surfacelayer is used in thesubstation, Cs =1 and ρs=ρ .
If there is metal-to-metal contact, both hand-to-hand and hand-to-feet contact, ρs=0 since the ground is not included in this situation. In this case, the touch voltage limit equations are:
34 For a body weighing 50 kg
Emm−touch50 =
116
ts
(3.21)
For a body weighing 70kg
Emm−touch70 =
157
ts
(3.22)
3.6 Conductor Sizing The symmetrical current can be calculated based on the material and the size of the conductor used as:
I = Amm
2
TCAP ⋅10−4 K 0 + Tm ln t α ρ c r r K 0 + Ta
(3.23)
If the conductor size is given in kcmil, the equation becomes:
TCAP K 0 + Tm I = 5.07⋅10−3 Akcmil ln t α ρ c r r K 0 + Ta
(3.24)
Where I
: rms current (kA)
Amm2
: conductor cross section (mm2)
Akcmil
: conductor cross section (kcmil)
Tm
: maximumallowable temperature (oC)
Ta
: ambient temperature (oC)
αr
: thermal coefficient of resistivity at reference temperature T r (1/ oC)
: resistivity of the ground conductor at reference temperature T r (µΩ-cm) tc : duration of current (s) K 0 : equals 1/ α0 or (1/ αr)- Tr (oC) TCAP : thermal capacity per unit volume (J /2 ∙ ℃) ρr
35
Common values of αr, K 0 , Tm, ρr, and TCAP values can be found in Table 3. Table 3-Material Constants Ref. IEEE Std 80-2000 Table 1. Copyright ©2000. IEEE. All rights reserved Description
Copper,
o
TCAP thermal capacity [J/(cm3·oC)]
Material
αr factor at K 0 at
Fusinga
Conductivity
20oC
0oC
temperature (µΩ-cm)
(%)
(1/ oC )
(0oC ) Tm (oC )
100.0
0.00393
234
1083
1.72
3.42
97.0
0.00381
242
1084
1.78
3.42
40.0
0.00378
245
1084
4.40
3.85
30.0
0.00378
245
1084
5.86
3.85
20.0
0.00378
245
1084
8.62
3.85
64.0
0.00403
228
657
2.86
2.56
0.00353
263
652
3.22
2.60
0.00347
2268
654
3.28
2.60
0.00360
258
657
8.48
3.58
0.00160
605
1510
15.90
3.28
0.00160
605
1400
17.50
4.44
0.00320
293
419
20.10
3.93
0.00130
749
1400
72.00
4.03
ρr 20 C
annealed softdrawn Copper, commercial hard-drawn Copper-clad steel wire Copper-clad steel wire Copper-clad steel rodb Aluminum, EC grade Aluminum, 5005 alloy Aluminum, 6201 alloy Aluminumclad steel wire Steel-1020
a
53.5 52.5 20.3
10.8
Stainless-clad steel rodc Zinc-coated steel rod
9.8
Stainless steel, 304
2.4
8.6
From ASTM standards. Copper-clad steel rods based on 0.254 mm(0.010 in) copper thickness. c Stainless-clad steel rod based on 0.508 mm (0.020 in) No. 304 stainless steel thickness over No. 1020 steel core. b
36
The required area for a conductor given a current can be calculated as:
Amm = I 2
1
TCAP ⋅10 K 0 + Tm ln t α ρ c r r K 0 + Ta −4
(3.25)
or
Akcmil = I
197.4
TCAP K 0 + Tm ln t α ρ c r r K 0 + Ta
(3.26)
Equation (3.26) can be simplified as:
Akcmil = I ⋅ K f tc
(3.27)
where K f
: constant found in Table 4 which is based on the fusing and ambient temperature of the material
Table 4-Material Constants Ref. IEEE Std 80-2000 Table 2. Copyright ©2000. IEEE. All rights reserved Material Conductivity T ma (°C) K f (%) Copper, annealed soft-drawn 100.0 1083 7.00 Copper, commercial hard-drawn 97.0 1084 7.06 Copper, commercial hard-drawn 97.0 250 11.78 Copper-clad steel wire 40.0 1084 10.45 Copper-clad steel wire 30.0 1084 12.06 Copper-clad steel rod 20.0 1084 14.64 AluminumEC Grade 61.0 657 12.12 Aluminum5005 Alloy 53.4 652 12.41 Aluminum6201 Alloy 62.5 654 12.47 Aluminum-clad steel wire 20.3 657 17.20 Steel 1020 10.8 1510 15.95 Stainless clad steel rod 9.8 1400 14.72 Zing-coated steel rod 8.6 419 28.96 Stainless steel 304 2.4 1400 30.05
37 The following equation can be used to convert the conductor size from kcmil to mm2 :
Amm = 2
Akcmil ⋅1000
(3.28)
1973.52
The diameter of a conductor can be calculated as: dc(mm) = 2
Amm
2
(3.29)
π
3.7 Asymmetrical Currents If the effect of the dc offset is needed to beincluded in thefault current, the values of the symmetrical current is found by: I F = I f ⋅ D f
(3.30)
The decremental factor, Df , can be calculated as: −2t f Ta D f = 1+ 1− e Ta tf
(3.31)
where tf : time duration of the fault (s) X Ta = ω R
(3.32)
The typical decremental factors can also be found from Table 3.
3.8 Soil Resistivity Measurements The methods for soil resistivity measurements are discussed in 2.9. Since the Wenner’s four-pin method is the most common, only calculations for this method will be discussed.
38 Table 5-Typical Values of Df Ref. IEEE Std 80-2000 Table 10. Copyright ©2000. IEEE. All rights reserved Fault Duration, tf Decrement factor, Df Seconds Cycles at X/R =10 X/R =20 X/R =30 X/R =40 60 Hz 0.00833 0.5 1.576 1.648 1.675 1.688 0.05 3 1.232 1.378 1.462 1.515 0.10 6 1.125 1.232 1.316 1.378 0.20 12 1.064 1.125 1.181 1.232 0.30 18 1.043 1.085 1.125 1.163 0.40 24 1.033 1.064 1.095 1.125 0.50 30 1.2026 1.052 1.077 1.101 0.75 45 1.018 1.035 1.052 1.068 1.00 60 1.013 1.026 1.039 1.052
As mentioned in 2.9 the mutual resistanceR is determined by dividing the voltage between the two inner probes by the current of the two outer probes. Using the mutual resistanceR, the soil resistivity can be calculated as follows: 4π aR
ρ =
1+
2a
a2 + 4b2
−
a
(3.33)
a2 + b2
where ρ : soil resistivity (Ω ·m) R : measured resistance (Ω)
a : distancebetween adjacent electrodes (m) b : depth of the electrodes (m) If b
View more...
Comments
