PART II- Substation Grounding System – IEEE Grounding 2000

Introduction

The intent of this guide is to provide guidance and information pertinent to safe groundingpractices in AC substation design.The specific purposes of this guide are to

a) Establish, as a basis for design, the safe limits of potential differences that can exist ina substation under fault conditions between points that can be contacted by thehuman body.

b) Review substation grounding practices with special reference to safety, and developcriteria for a safe design.

c) Provide a procedure for the design of practical grounding systems, based on thesecriteria.

d) Develop analytical methods as an aid in the understanding and solution of typicalgradient problems.

Earthing System for Substation :

An effective substation earthing system typically consists of earth rods, connecting cables fromthe buried earthing grid to metallic parts of structures and equipment, connections to earthedsystem neutrals, and the earth surface insulating covering material. Current flowing into theearthing grid from lightning arrester operation, impulse or switching surge flashover ofinsulators, and line to ground fault current from the bus or connected transmission lines allcause potential differences between earthed points in the substation.A good grounding system provides a low resistance to remote earth in order to minimize theground potential rise. For most transmission and other large substations, the ground resistance ismostly specified .Unless proper precautions are taken in design, the maximum potential gradients along the earth’ssurface may be of sufficient magnitude during ground fault conditions to endanger a person in thearea. Moreover, dangerous voltages may develop between grounded structures or equipmentframes and the nearby earth. Without a properly designed earthing system, large potentialdifferences can exist between different points within the substation itself.

Safety in grounding Basic problemIn principle, a safe grounding design has the following two objectives:

a) To provide means to carry electric currents into the earth under normal and faultconditions without exceeding any operating and equipment limits or adverselyaffecting continuity of service.

b) To assure that a person in the vicinity of grounded facilities is not exposed to thedanger of critical electric shock.

An Effective Grounding System Has The Following Objectives:1. Ensure such a degree of human safety that a person working or walking in the vicinity of

grounded facilities is not exposed to the danger of a critical electric shock. The touch andstep voltages produced in a fault condition have to be at safe values. A safe value is onethat will not produce enough current within a body to cause ventricular fibrillation.

2. Provide means to carry and dissipate electric currents into earth under normal and faultconditions without exceeding any operating and equipment limits or adversely affectingcontinuity of service.

3. Provide grounding for lightning impulses and the surges occurring from the switching ofsubstation equipment, which reduces damage to equipment and cable.

4. Provide a low resistance for the protective relays to see and clear ground faults, whichimproves protective equipment performance, particularly at minimum fault.

Effect of magnitude and durationThe most common physiological effects of electric current on the body, stated in order ofincreasing current magnitude, are threshold perception, muscular contraction, unconsciousness,brillation of the heart, respiratory nerve blockage, and burning

whereIB is the rms magnitude of the current through the body in A

ts is the duration of the current exposure in s SB is the empirical constant related to the electric shock energy tolerated by a certain percent of a given population

Effective grounding (solid or direct)Effective grounding is usually obtained by a direct connection between system neutral and theearth with no intentional impedance. When the system neutral is effectively grounded, thetransient overvoltages are held to a minimum as compared to the other grounding methods. Thehigh fault current, if not cleared promptly, causes maximum fault damage and mechanical stresson equipment. It may also be sufficiently greater than three-phase currents to become aconsideration in the selection of circuit breaker interrupting ratings.

Importance of high-speed fault clearingConsidering the significance of fault duration both in terms of Equation (6) and implicitly as anaccident-exposure factor, high-speed clearing of ground faults is advantageous for two reasons

a) The probability of exposure to electric shock is greatly reduced by fast faultclearing time, in contrast to situations in which fault currents could persist for severalminutes or possibly hours.

b) Tests and experience show that the chance of severe injury or death is greatly reducedif the duration of a current flow through the body is very brief. The allowed currentvalue may, therefore, be based on the clearing time of primary protective devices, orthat of the backup protection

Duration formulaThe duration for which a 50 Hz or 60 Hz current can be tolerated by most people is related to itsmagnitude in accordance with Equation (6). it is assumed that 99.5% of all persons can safelywithstand, without ventricular brillation, the passage of a current with magnitude and durationdetermined by the following formula:

where, in addition to the terms previously defined in Equation (6)

Equation (8) results in values of 116 mA for ts = 1 s and 367 mA for ts = 0.1 s.

Users of this guide may select k = 0.157 provided that the average population weight can beexpected to be at least 70 kg.

Definitions:

Soil ResistivityThe electrical characteristic of the soil with respect to conductivity . The value is typically given inohmmeters.

Earth Current

The current that circulates between the grounding system and the ground fault current source thatuses the earth as the return path .

Ground Potential Rise (GPR)

The maximum electrical potential that a substation grounding grid may attain relative to a distantgrounding point assumed to be at the potential of remote earth. This voltage, GPR, is equal to themaximum grid current times the grid resistance.NOTE—Under normal conditions, the grounded electrical equipment operates at near zero groundpotential. That is, the potential of a grounded neutral conductor is nearly identical to the potentialof remote earth. During a ground fault the portion of fault current that is conducted by a substationgrounding grid into the earth causes the rise of the grid potential with respect to remote earth.

Grounding grid:A system of horizontal ground electrodes that consists of a number of interconnected, bareconductors buried in the earth, providing a common ground for electrical devices or metallicstructures, usually in one specific location.NOTE—Grids buried horizontally near the earth’s surface are also effective in controlling thesurface potential gradients. A typical grid usually is supplemented by a number of groundrods and may be further connected to auxiliary ground electrodes to lower its resistance withrespect to remote earth.

Influence of Potential Gradiant :Around every earth electrode a so called Potential Gradiant area develops during the flow of anelectric current .It is the soil resistivity that mainly affects the diameter of the potential gradiantarea .It must be assured that the testing probes is set outside the potential gradiant area .Normally , a distance of 20 m to the earth electrode and to the probes to each other is sufficient tominimize or eliminate the potential gradiant effect

Maximum grid current:A design value of the maximum grid current, defined as follows:

where

IG is the maximum grid current in ADf is the decrement factor for the entire duration of fault tf , given in sIg is the rms symmetrical grid current in A

Symmetrical grid current:

That portion of the symmetrical ground fault current that flows between thegrounding grid and surrounding earth. It may be expressed as

WhereIg is the rms symmetrical grid current in AIf is the rms symmetrical ground fault current in A

Sf is the fault current division factor

Symmetrical ground fault current:The maximum rms value of symmetrical fault current after the instant of a ground fault initiation.As such, it represents the rms value of the symmetrical component in the first half-cycle of acurrent wave that develops after the instant of fault at time zero. For phase-to-ground faults

Where

If(0+) is the initial rms symmetrical ground fault current I 0 is the rms value of zero-sequence symmetrical current that develops immediately after the instant of fault initiation, reflecting the subtransient reactances of rotating machines contributing to the faultThis rms symmetrical fault current is shown in an abbreviated notation as If , or is referred to onlyas 3I0. The underlying reason for the latter notation is that, for purposes of this guide, the initialsymmetrical fault current is assumed to remain constant for the entire duration of the fault.

Effective asymmetrical fault current:

The rms value of asymmetrical current wave, integrated over the interval of fault duration

WhereIF is the effective asymmetrical fault current in AIf is the rms symmetrical ground fault current in A

Df is the decrement factor

Fault current division factor:A factor representing the inverse of a ratio of the symmetrical fault current to that portion of thecurrent that ows between the grounding grid and surrounding earth.

where Sf is the fault current division factor Ig is the rms symmetrical grid current in A I0 is the zero-sequence fault current in A

NOTE—In reality, the current division factor would change during the fault duration, based on thevarying decay rates of the fault contributions and the sequence of interrupting device operations.However, for the purposes of calculating the design value of maximum grid current andsymmetrical grid current per definitions of symmetrical grid current and maximum grid current, theratio is assumed constant during the entire duration of a given fault.

Enclosure currents:

Currents that result from the voltages induced in the metallic enclosure by the current(s) flowing inthe enclosed conductor(s).

Mesh Voltage

The maximum touch voltage within a mesh of a ground grid .

Step VoltageThe difference in surface potential experienced by a person bridging a distance of 1 meter with hisfeet without contacting any other grounded object.

Touch Voltage

The potential difference between the ground potential rise and the surface potential at the pointwhere a person is standing while at the same time having his hands in contact with a groundedstructure.

Substation Equipments Grounding

Neutral Grounding:

In the normal case there is no current flow in or from the neutral point, but when an earth faultoccurs the previous case will be change and a large fault current flow in the system, this currenthas dangerous effect on power transformers and other equipments, so it must be eliminated orrejected out of the system, and that achieved by grounding the system neutral. The neutral pointof the three phase system can be grounded directly or through an impedance to limit the faultcurrent.

Substation Fence Grounding:The grounding of the substation fence is critical because the fence is generally accessible to thepublic. The substation grounding design should be such that the touch potential on both sides ofthe fence is within the calculated tolerable limit of touch potential.Earthing of fences can follow either of the following two procedures.

a) Independent earthing. This method has the advantage that a person on the outside of thefence can come into contact with a lower potential than that associated with method b).

b) Connection to the station earthing system. Where a fence is situated within the area of astation earthing system or cannot be separated from it by at least 2 m, the fence, its staysand anti-climbing fittings should be bonded to the earthing system at intervals not

exceeding 50 m with additional bonds at the fence corners and wherever an overheadconductor passes over the fence. Gate posts should be bonded together by a conductorlaid below ground. Metal gates should be bonded by flexible connections to adjacentfence sections. The substation fence should be connected to the main ground grid by theouter grid conductor which installed a minimum of 0.91 meter (approximately one arm’slength) outside the substation fence .

Metallic Cable Sheaths Grounding:

Metallic cable sheaths in power cable have to be effectively grounded to prevent dangerousvoltages resulting from insulation failure, electrostatic and electromagnetic induction, flow of faultcurrent in the sheath, and voltage rise during fault current flow in the substation ground system towhich the sheaths are connected. Cable sheaths should be grounded at two or more locations: atcable terminations and at splices and taps. Control cable shields are not intended to carrysignificant current and thus should only be grounded at one end.

Surge Arrester Grounding:Surge arresters are designed to pass surge energy from lightning and switching transients toground and so are frequently subjected to abnormal current flow to ground. They have to bereliably grounded to ensure protection of the equipment they are protecting and to minimize highpotential gradients during operation.

A. Soil Resistivity Measurements:

Before the design process can begin, soil resistivity measurements should be taken at thesubstation site. Make these at a number of places within the site. Substation sites where the soilmay possess uniform resistivity throughout the entire area and to a considerable depth are seldomfound.

For the new installations , it is recommended that a multi-layer model for apparentresistivity be generated. A two layer model yields significant benefits in both cost,accuracy and safety, these should identify the surface layer to about 1m and the averagedeep layer to the grid diagonal dimension. The multi-layer model is useful in providingmore accurate information regarding the presence of lower resistivity layers, and henceoptimizing rod driving depths. However, the two layer model is considered sufficientlyaccurate for modeling the behavior of grids in the majority of cases. If more than twolayers are identified, the lower layers are usually combined to form a two layer equivalentmodel. This is done because the surface potentials are closely related to the upper layerresistivity, whilst the grid resistance, which is primarily effected by the deeper layers, isnot usually adversely affected by this simplification.

The most-widely used method for the soil resistivity measurement is the Wenner four-pinmethod:The four-pin method obtains the soil resistivity data for deeper layers without driving the test pinsto those layers. No heavy equipment is needed to perform the four-pin test. The results are notgreatly affected by the resistance of the test pins or the holes created in driving the test pins intothe soil.

Figure 5-1: Wenner Four-Pin Method.

2222 421

4

baa

baa

aRa Equation - 1

Where:a = Apparent resistivity of the soil in -m.

R = Measured resistance in ohmsa = Distance between adjacent electrodes in metersb = Depth of the electrodes in metersIf b is small compared to a, as is the case of probes penetrating the ground only a short distance, Equation-1 canbe reduced to Equation – 2 :

aRa 2 Equation – 2

Testing several points or drawing up a curve will help the understanding of the area around theelectrode. It is always best to check results by using a different direction or a longer distance tothe test spikes. This will help to eliminate errors caused by nearby buried conductors and otherparts of the electrical system interfering with the results.

Grid Resistance Measurement in Operating Substations :

With Today's Increases in System Voltages and Power Plant Ratings, more attention is beinggiven to the safety of operational personnel in substations. When it comes to safety, the groundingsystem must be an important consideration in a substation's design. Following the installation of anew grounding system at a substation, testing is required to confirm the grounding resistancecomplies with the design value. In addition, the substation grounding system should be testedperiodically to make certain no changes are required. It is therefore advisable to check theresistance of an electrode both on installation and at regular intervals thereafter, preferably afterthe end of the driest period in the year. In this way comparison between records will show if thereis a trend which may require attention at some time in the future. It will also give a worst casevalue of resistance on which to check the proper functioning of circuit disconnection equipment.In some cases the voltages involved when testing earth electrodes may present a risk of shockand care should be exercised to take the necessary precautions. For existing installations it is notpermissible to disconnect earth electrodes unless the installation is also disconnected from allsources of power. The problem can sometimes be solved by installing multiple electrodes so that,with one disconnected for testing, the remaining electrodes provide an adequately low resistanceor use the modern Earth / Ground Tester which does not require any disconnection.NOTE : Danger can arise when removing a conductor connected to earth unless suitableprecautions are taken.

Some substations are located in cities , industrial zones, high rise commercial buildings andtowers making it increasingly difficult or impossible for the utility to find available space in morethan one direction to spread measuring wires so as to eliminate the coupling effect to haveaccurate grid resistance measurement also when the cables sheaths , feeding transformersneutrals are connected to the earthing grid .At substations, the coupling effect between the current and potential leads can be considerable,giving rise to measurement discrepancies of 100% or more.Typically mutual impedance precise value is dependant on the test lead spacing and it isrecommended that this effect should always be accounted for when measuring earthing systemswith a resistance of < 1 .Normally , the mutual coupling reaches a maximum value when the auxiliary potential test lead islaid in parallel with the auxiliary current test lead , and the coupling reduces as the spacingbetween the circuits is increased and falls to a minimum when the auxiliary potential test lead is a90º to the auxiliary current test lead .

GROUNDING TEST METHOD

Fall of Potential Testing :

The fall-of-potential method is the most-widely adopted test used on the majority of groundingsystems. By this method, the impedance measurements of a high-voltage substation groundingsystem are determined by passing an alternating current between the grounding grid and aremote current electrode. The potential electrode is placed at various positions between thecurrent electrode and grounding system. The ratio of voltage to current, known as the apparentresistance, is then plotted against the distance from the substation. The resultant curve, if themeasurements are accurate, has a relatively flat segment in the middle section of thischaracteristic. The required value of the grounding system resistance is then determined from thecharacteristic at a position that is approximately 50% to 70% of the current wire length.

The best method of measurement and the test connections is illustrated in Figure 11. A measuredcurrent is passed between electrode X, the one being tested, and an auxiliary current electrode Y.The voltage drop between electrode X and a second auxiliary electrode Z is measured and theresistance of the electrode X is then the voltage between X and Z divided by the current flowingbetween X and Y. The source of current and the means of metering either the current and voltageor their ratio are often, but not necessarily, combined in one device. . For single electrode earths,such as domestic earths and lightning conductors, the influence on the surrounding soil is limitedand current test spikes can be quite close (typically 10 to 20 m) to the electrode under test. It isusually quite easy to find a flat portion of the earth resistance curve which should be close to theresistance of the electrode.

Fig.4 shows an example of a small earth system with a test spike at 50 m. Using a potential spikedistance of between 10 and 40 m, a reading close to the earth resistance will be measured. Atdistances less than 10 m the influence of the electrode under test will affect the measurement.Above 40 m the "resistance area" of the current spike will give a higher measured value thanexpected.

This measured value consists of two components: The actual voltage difference between grounding system under test and the auxiliary

potential electrode The inducted potential due to alternating current flowing in the current test loop, known as

the coupling effect.If no available space in more than one direction to spread measuring wires so as to eliminate thecoupling effect and other interferences and a classic Earth / Ground Tester is used , and based onexperience , the measurement can be taken at the most convenient spacing distance between thecurrent and potential wires in this substation . The potential electrode is placed at variouspositions between the current electrode and grounding system. The ratio of voltage to current,known as the apparent resistance, is then plotted against the distance from the substation. Theresultant curve, if the measurements are accurate, has a relatively flat segment in the middlesection of this characteristic.The resistance value can be verified by taking same measurement arrangement at othersubstations where convenient spacing is available and more than one testing methodmeasurement can be taken and results can be compared and verified .This measured value consists of two components:

The actual voltage difference between grounding system under test and theauxiliary potential electrode

The inducted potential due to alternating current flowing in the current test loop,known as the coupling effect.

These forms of interference can be minimized or eliminated by the followings :1. Testing with modern Earth / Ground tester which has Automatic FrequencyControl Option which identifies existing interferences and chooses a measurementfrequency to minimize its effect.2. When the alternating current at a frequency different from that of the interferingpower currents and their harmonics. This is usually achieved by using a frequencygenerator with 60 Hz to 90 Hz frequency range .3. If there is enough available space , to repeat each measurement withrepositioned probes and only to regard a measurement as successful and accurate ifseveral subsequent measurements result in the same values .

B. Basic Design procedures of Substation Grounding System as per IEEE –Grounding 2000 :

Computer simulation :Most major engineering project companies and designers now have the in-house computer programs to design and evaluate different substation earthing arrangements .

The step by step approach to design a substation earthing system :

Ground Conductor Sizing

The ground conductor for both the grid and equipment connections should be sized according toEquation - 3 and the only difference will be the value of the current I , where I value will be the S/Sshort circuit current rating for the equipment grounding conductor and 60% of the S/S short circuitcurrent rating for the grid conductor .The required conductor size as a function of conductor current is given in the following equation :

Equation – 3Where:

I = Isc is the rms Switchgear rated short circuit current in kA for equipment conductor . = 0.6 – 0.8 Isc for Grid conductor sizing .Amm² is the conductor cross section in mm2Tm is the maximum allowable temperature in °CTa is the ambient temperature in °CTr is the reference temperature for material constants in °C

o is the thermal coefficient of resistivity at 0 °C in 1/°Cr is the thermal coefficient of resistivity at reference temperature Tr in 1/°Cr is the resistivity of the ground conductor at reference temperature Tr in µ -cm

Ko 1/ o or (1/ r) – Tr in °Ctc is the duration of current in s .( if not specified , the substation Switchgear rated duration time shall be considered ) which is normally 1sec. as per the new IEC standard . Since , some clients specify 3 sec as per the previous IEC Switchgear rating and some Clients accept 0.5 sec because of the use of the new Numerical protection relays .TCAP is the thermal capacity per unit volume from Table 1, in J/(cm3·°C)

Note : Only copper conductors are considered in the above table , for other materials please refer totable 1 of the IEEE – Grounding 2000 .

Alternate formulations :As per British Standard BS 7430 , The corresponding conductor cross-sectional area (Ac) in squaremillimeters can be simplified and given by Equation -4:

ktIAc Equation -4

whereI is the average fault current, in A r.m.s;t is the fault current duration, in s.k is the r.m.s. current density, in A/mm2 (given in Table 10).

Table 10 provides a guide to acceptable maximum fault current temperatures for bare earthingconductors, according to the environmental conditions and the type of connections used. For aconductor covered to provide corrosion or mechanical protection, or an insulated conductor, themaximum temperature may be limited by the covering or insulating material. The current densities (k)in r.m.s. amperes per square millimeter, for a 1 s duration, are given in Table 10 for copper, aluminumand steel conductors assuming an initial temperature of 30 °C.

For other durations the fault current capacity (I) in amperes r.m.s. can be calculated from one of thefollowing equations:

ort

kAI

Where ,I1 is the fault current for 1 s duration, in A r.m.s. (given in Table 11 and Table 12);

A is the conductor cross-sectional area, in mm²;k is the r.m.s. current density, in A/mm2 (given in Table 10).

t is the fault current duration, in s.

Table 10 — Earth fault current densities for 1 s duration for earthing conductorswith initial conductor temperature of 30 °C

Fault current capacities, for 1 s and 3 s durations, for a selection of standard sizes of copper andaluminum strips are given in Table 11 and Table 12.

Table 11 — Earth fault currents (in kA) for copper strip earthing conductors

For other initial and final temperatures the current density k for 1 s duration can be obtained from thefollowing equation:

Where ,T1 is the initial temperature, in °C;T2 is the final temperature, in °C; K and have the values given in Table 13.

Tolerable Touch and Step VoltagesThe tolerable touch and step voltages are the criteria that have to be met to ensure a safe design. Inmost cases the tolerable touch voltage will be the limiting factor. Figures5-5 through 5-8 are used toderive the equations for maximum touch and step voltage

Figure 5-5: Exposure to Touch Voltage.

Figure 5-6: Touch Voltage Circuit.

Figure 5-7: Exposure to Step Voltage.

Figure 5-8: Step Voltage Circuit.

The equations for the maximum touch and step voltages are as follows:

IRRE fBstep .2 Equation -5

IR

RE fBtouch .

2 Equation -6

Where:Estep = Step voltage in voltsEtouch = Touch voltage in voltsRB = Resistance of the human body to electric current. RB is generally estimated to be 1000 .Rf = Ground resistance of one foot

ssf CR 3 Equation -7 Cs is the surface layer derating factor based on the thickness of the protective surface layer spread above the earth grade at a substation. If no protective surface layer is used, then Cs = 1. s is the resistivity of the protective surface layer used at the substation in -m. If no protectivesurface layer is used, then s =resistivity of homogenous earth.

I is defined as:

tKI Equation -8

Where:I = Maximum grid current in A.t = Fault clearing time in s.k = Constant related to electric shock energy

For a person weighing 50 kg (110 lbs), k = 0.116 For a person weighing 70 kg (155 lbs), k = 0.157

Substituting for I, RB, Rf, and k in Equations -5, -6, and -7:

For body weight of 50 kg (110 lbs)

tCE ssstep

116.06100050 Equation-9

tCE sstouch

116.05.1100050 Equation -10

For body weight of 70 kg (155 lbs):

sssstep t

CE 157.06100070 Equation -11

tCE sstouch

157.05.1100070 Equation -12

Actual Mesh Voltage (Touch Voltage):

Mesh voltage (a form of touch voltage) is taken as being from a grounded structure to the center ofa rectangle of the substation grounding grid mesh. Mesh voltages represent the highest possibletouch voltages that may be encountered within a substation’s grounding system and thus represent apractical basis for designing a safe grounding system. The mesh voltage has to be less than thetolerable touch voltage for the grounding system to be safe.

LIKK

E imm Equation -13

Where:= Soil resistivity, -m.

Km = Spacing factor for mesh voltage.Ki = Correction factor for grid geometry.L = Effective length for mesh voltage in meters.

The geometrical factor Km is expressed by Equation -14:

128ln

482

16ln

21 22

nKK

dh

dDhD

dhDK

h

iim Equation -14

Where:D = Spacing between parallel conductors in meters.d = Diameter of grid conductors in meters.h = Depth of ground grid conductors in meters.n = Number of parallel conductors in one direction.Kii=Corrective weighting factor that adjusts for the effects of inner conductors on the corner meshKh=Corrective weighting factor that emphasizes the effects of grid depth.

For grids with ground rods along the perimeter, Kii 1

For grids with no ground rods or grids with only a few ground rods:

nii

nK 2

2

1 Equation -15

hK h 1 Equation -16

The correction factor for grid geometry, Ki, expressed by Equation -17:nK i 172.0656.0 Equation -17

Actual Step Voltage (Es):

Step voltages within a grid system designed for safe mesh voltages will be well within tolerable limits.This is because step voltages are usually smaller than touch voltages, and both feet are in seriesrather than parallel. Also, the body can tolerate higher currents through a foot-to-foot path since thecurrent does not pass close to the heart. The step voltage has to be less than the tolerable stepvoltage for the ground system to be safe.

LIKK

E iss Equation -18

Where:=Soil resistivity, -m.

Ks=Spacing factor for step voltage, simplified method.Ki=Correction factor for grid geometry, simplified method.I=Maximum grid current that flows between ground grid and surrounding earth in amperes.L=Effective buried conductor length in meters

The maximum step voltage is assumed to occur over a distance of 1 meter. For the usual burial depthof 0.25 m < h < 2.5 m, Ks is expressed by Equation -19.

25.01112

11 ns DhDh

K Equation -19

Determination of Maximum ground current:

The maximum grid current is the current that flows through the grid to remote earth, and is calculatedby Equation -20.

gf IDI Equation -20Where:

I = Maximum grid current in amperes.Df = Decrement factor to account for asymmetry of the fault current wave.Ig = rms symmetrical grid current in amperes.

Symmetrical Grid Current (Ig): The symmetrical ground fault current that flows between the groundinggrid and surrounding earth may be expressed by Equation -21.

ffg ISI Equation -21Where:

If = rms symmetrical ground fault current in amperesSf = Fault current division factor.

For the assumption of a sustained flow of the initial ground fault current, the symmetrical grid currentcan be expressed by Equation -22:

ofg ISI 3 Equation -22Where:

Io = Symmetrical rms value of Zero Sequence fault current in amperes

For grids with no ground rods, or grids with only a few ground rods scattered throughout the grid, theeffective buried length, L, is expressed by Equation -23:

RC LLL Equation -23Where:

Lc = Total length of grid conductor in meters.LR = Total length of ground rods in meters.

For grids with ground rods along the perimeter, as well as along the perimeter and throughout thegrid, the effective buried length, L, is expressed by Equation -24:

R

yx

rCM L

LL

LLL22

22.155.1 Equation -24

Where: Lr = Length of each ground rod in meters

Ground Potential Rise (GPR):

In all the above situations for step and touch voltage, the actual voltage potential encountered by theperson is related to the ground potential rise of the grounding system above remote earth.This fact stresses the importance of keeping that value as low as possible.Ground potential rise is the maximum electrical potential that a substation grounding grid may attain.

gGPR RIV Equation -25

Where:VGPR = Ground potential rise in voltsRg = Ground grid resistance in .

AhALRg /201

112011

Equation -26

Where:A = Area occupied by the ground grid in m2

Design of Substation Grounding System :The grid system would be extended over the entire substation switchyard and often beyondthe fence line. Multiple ground leads or larger sized conductors would be used where highconcentrations of current may occur, such as at a neutral-to-ground connection of generators,capacitor banks, or transformers.

The step by step approach to design a substation earthing system :Design Procedures : as shown in the following block diagram :

Example for Substation Grounding Calculation:

1. Outdoor Substation with Air Insulated Switchgear :

We want to design an earthing grid for 400/132KV , I sc = 50 KA ,1s – Outdoor AIS ( Air InsulatedSwitchgear ) substation by using the following data:

1. soil resistivity =75 .m2. resistivity at soil surface s = 3000 .m3. substation area =150m×200m=30000m2

4. max. grid current I = 50 KA5. fault clearing time t=0.5s6. 4 rods x 6 m each on corners = 24 m .7. 36 rods x 3 m each = 108 m

Grid and ground rod combinationsWhen a combination of grid conductors and ground rods are used in a grounding system, the numberand length of ground rods may have a great influence on the performance of the grounding system.For a given length of grid conductor or ground rod, the ground rod discharges much more current intothe earth than does the grid conductor. This current in the ground rod is also discharged mainly in thelower portion of the rod. Therefore, the touch and step voltages are reduced significantly compared tothat of grid alone.Only Copper conductor is considered in the calculations to be used for the grid:

1. spacing between conductors D=10m as the maximum recommended spacing .2. depth of ground grid conductors h=0.5m

Calculating Conductor Size:

As per IEEE - 2000

Tm = 1084 °C max. allowable fusing temp., see Table1 IEEE 80Ta = 35 °C ambient temperature (earth)

r = 0.00381 thermal coefficient of resistivity at reference temperature Tr at 20°C see Table1 IEEE 80r = 1.777u cm resistivity of the ground conductor at reference temperature Tr at 20°C

Ko = 242 see Table1 IEEE 80: 1/ o or (1/ r) – Tr in °C )tc = 0.50 s duration of currents ( if tc is not specified , switchgear rated duration time shall be taken.TCAP = 3.422 thermal capacity per unit volume from Table 1 IEEE 80, in J/(cm³ C°)

Amm² = 125.8 mm²Or ,As per BS 7430

1765.01050 3

ktIAc = 200 mm²

From Table 10 , the current density K = 176 for copper conductor with 250º C max. temperature,

=75 .m and for between 25-100 .m a 15% corrosion allowance is recommended .

So Ac must be increased by 0.15 to be equal:223015.0200200 mmAc

240 mm² copper conductor cable shall be used .

For the Grid conductors , the conductor size should be capable of carrying the maximum earth faultcurrent for 1s or 3s ( as specified ) .

Where multiple parallel paths are provided , each conductor should be capable of carrying 60% ofthe maximum earth fault current for the same period of time .

Conductor diameter could calculate as:

mmmAd C 3105.1749.1714.324044

Preliminary Design:

Let the grid cover all the station area:

A =150×200 = 30000m2

Total length of the used conductor to form the grid (L): L= 16×200+21×150 + 36 x 3 + 4 x 6 =6482mn = nc . nb . ne . nd

52.1870064822

p

ca L

L2n

01.1300004700

A4

Ln p

b

n = 18.52 x 1.01x 1 x1 = 18.71nc = 1 for square and rectangular grid.nd = 1 for square, rectangular and L- shaped grid.Lc = is the total length of the conductor in the horizontal grid in m.Lp = is the peripheral length of the grid in m.A = is the area of the grid m2.

Calculation of Grid Resistance:

AhALR

20111

2011

204.030000205.01

1130000201

6482175

The Conductor Length for Gradient Control:

116.05.11000....

s

im tIKKL

Ki = 0.644+0.148n=0.644+ (0.148×18.71) for n=18.71 = 3.413

Kii = 1 Kh = 225.15.011 h

128ln

482

16ln

21 22

nKhKii

dh

DdhD

DhDKm

3504.0

143.1828ln

225.11

101845.0

10181085.0210

5.01016100ln

21

33

2

mL

L

4970116.030005.11000

5.01050413.33504.075 3

Since the length of the conductor in preliminary design (6482m) is more than the conductor lengthrequired (4970m), for control of gradient, the design of the grid is safe from the consideration ofmesh voltage.

Actual and Tolerable Step Voltage:

Tolerable Estep=ts

116.0.61000

= V92.31165.0

116.0.300061000

Actual Estep= LIKiKs sc

25.0111211 n

DhDhKs

3806.05.01101

5.0101

5.0211 271.18Ks

Actual Estep=75×0.3806×3.413× (6482

1050 3

) =751.497

Actual Estep is less than the Tolerable Estep, so that the grid is safe from the consideration of stepvoltage.

Actual and Tolerable Touch voltage (mesh voltage):

Tolerable Etouch=ts

116.05.11000

= V26.9025.0

116.030005.11000

Actual Etouch= LIKiKm sc

=75×0.3504×3.413×(6482

1050 3

)= 691.87 V

Actual Etouch is less than the Tolerable Etouch, so that the grid is safe from the consideration of touchvoltage.

Ground Potential Rise (GPR):

GPR=Isc × Rgrid

= 50 ×103 × 0.204 = 10200 V

Fig (5-9) shows the final layout.

Now, it is certain that considerable and therefore dangerous potentials can arise between the soil, thefloors of buildings on one side, and the metallic parts of the plant during the time of faults. Potentialdifferences which may endanger cable insulation and low-voltage apparatus and facilities may beeliminated by metallic interconnection of equipment housing, sheaths of control and service cablesand their neutral conductors, and the construction parts in the control house.Therefore one must also consider the safety of operating personnel who in the course of their workmust touch such metal parts. For this purpose the operating position may be provided either with aninsulated floor capable of withstanding the high potential or with a metallic grid in the floor and tied tothe ground mat or provided with both. For protection of personnel at the danger points, narrowmeshed ground mats with mesh spacing of about 1 m will serveSuch metallic foot grids have been previously used for protection in some plants with ungrounded starneutral. They consisted of small meshed wire netting cemented into the floor and tied to thegrounding system, and provided absolute protection to persons standing thereon and graspingoperating controls in that a highly conducting shunt path was provided between hands and feet.The measurements show, as might be expected, that by using a fine mesh a considerable reductionin potential differences within the mat area can be obtained. Further, it is apparent that smallprotected areas can be produced by partial matting without completely matting the entire groundingarea. Practical application of such finer meshing can be found principally in outdoor stations in theneighborhood of accessible equipment where the hazard is greatest.

2. Indoor Type Substation with Gas Insulated Switchgear ( GIS ) :

For the Indoor Type Substations with GIS ( Gas Insulated Switchgear ) , the same shall be followedwith special care for the GIS switchgear which should have separate earthing grid to be followedstrictly as per the GIS manufacturer and to be connected to the above main grid .

GIS characteristicsGIS are subjected to the same magnitude of ground fault current and require the same low-impedance grounding as conventional substations. Typically, the GIS installation necessitates 10–25% of the land area required for conventional equipment.Because of this smaller area, it may be difficult to obtain adequate grounding solely byconventional methods. Particular attention should be given to the bonding of the metallic enclosuresof the GIS assembly, as these enclosures carry induced currents of significant magnitude,which must be confined to specific paths. In this respect, grounding recommendations by themanufacturer of a given GIS usually need to be strictly followed.As a result of the compact nature of GIS and its short distances, electrical breakdown in the insulatinggas, either across the contacts of a switching device during operation or in a fault that generates veryhigh frequency transients that can couple onto the grounding system. In some cases, these transientsmay have to be considered in the overall grounding design. These transients may cause highmagnitude, short duration ground rises and are also the source of electromagnetic interference (EMI)in the GIS. EMI is beyond the scope of this document.

Cooperation between GIS manufacturer and userUsually it is the GIS manufacturer who defines clearly what constitutes the main ground bus of theGIS and specifies what is required of the user for connecting the GIS assembly to the substationground..Usually the GIS manufacturer also provides, or is responsible for

a) Providing the subassembly-to-subassembly bonding to assure safe voltagegradients between all intentionally grounded parts of the GIS assembly and betweenthose parts and the main ground bus of the GIS.

b) Furnishing readily accessible connectors of sufficient mechanical strength to withstandelectromagnetic forces and normal abuse, and that are capable of carrying theanticipated maximum fault current in that portion of the circuit without overheating.

c) Providing ground pads or connectors, or both, allowing, at least, for two paths to groundfrom the main ground bus, or from each metallic enclosure and auxiliary piece of GISequipment designated for a connection to the substation ground if the main ground bus ofthe GIS assembly does not actually exist.

d) Recommending proper procedures for connections between dissimilar metals, typicallybetween a copper cable or a similar ground conductor and aluminum enclosures.

Touch voltage criteria for GISAlthough the GIS manufacturer generally designs the equipment to meet the alreadymentioned requirements for safe operation and usually performs most.In contrast to the general wisdom that a large ground connection necessarily equals a goodgrounding practice, the circulating currents generated in the GIS enclosures during a fault should alsobe taken into account.To be considered are:

1) Where these currents will circulate, and2) Where and to what degree the design engineer or GIS manufacturer, or both, prefer these

currents to circulate.Typically in a continuous enclosure design, the path of enclosure currents includes some structuralmembers of the GIS frame and the enclosures themselves. With each phase enclosure tied to theenclosures of adjacent phases at both ends, several loops are formed.For the hand-to-feet contact made by a person standing on a nonmetallic surface (for instance, aconcrete slab or the soil layer above the grounding grid), only a minor modification of the applicationcriterion of Equation (32) and Equation (33) of the IEEE – 2000 standard is required in order to takeinto account the maximum inductive voltage drop occurring within the GIS assembly.

The touch voltage criterion for GIS is

whereEt : is the maximum touch voltage, as determined for the point underneath a person’s feetE ' to max : is the (predominantly inductive) maximum value of metal-to-metal voltage difference on and between GIS enclosures, or between these enclosures and the supporting structures, including any horizontal or vertical members for which the GIS assembly is designed

In practical situations, as shown in Figure 16, a multiplicity of return paths and considerable cross-coupling occurs. As a rule, because of a great variety in possible physical arrangements of theGIS assembly, the GIS manufacturers perform detailed calculations for determining the basicdesign parameters, such as spacing and location of bonds.

Recommendations

The following recommendations should be considered for GIS installations:a) When applying the touch voltage criterion Equation (36), the following facts should be

considered. The case of an internal fault with ground return requires the addition of theresistive and inductive voltage drop to the resistive drop representing the difference ofpotentials between the substation ground and the point beneath a person’s feet. Thisgenerally is not necessary for faults external to the GIS. For an external line-to-groundfault, the voltages induced on the sheath should be checked for a hand-to-hand metal-to-metal contact, but the calculation of step and touch voltages at the earth’s surface isthe same as that for conventional installations [i.e., the inductive term E' to maxin Equation (36) is zero].

b) In evaluating the magnitude of induced voltages caused by faults external to the GIS,only the case of a close-in fault [case (B) in Figure 16] needs to be analyzed becauseremote external faults will cause less of a problem.

Figure - 16

Conclusions :

In general, a uniformly spaced grounding system consisting of a grid and ground rodsis superior to a uniformly spaced grounding system consisting only of a grid with the sametotal conductor length. The variable spacing technique discussed earlier might be used todesign a grounding system consisting of a grid only, with lower step and touch voltages thana uniformly spaced grid and ground rod design of equal length. However, this variablespacing technique might also be used to design a better grounding system using non-uniformly spaced grid conductors and ground rods. It shall be emphasized that this type ofdesign shall be analyzed using the detailed analysis techniques in the references.

The sheaths of the control cables provide a connection between the controlled apparatus inthe high-voltage bays and the control point. Thereby, a fault to ground in the station cancause a very large current to ow through the sheath and melt it. Communication cableswhich leave the plant will also conduct ground currents away since intentionally orunintentionally they come into contact with building construction parts. Thereby, the sheathsacquire the high potential of the station in their vicinity while the conductors approximatethe potential of the more-distant surroundings, so that insulation failures may occur.So likewise the cables of the low-voltage plant and the windings of control motorsamong others may be endangered by large potential differences. To this all cable sheathswithin the plant must also be connected; so likewise the control mechanisms in the switchingstation to which the control cables are connected.Basically the entire plant should be provided with a built-up ground mat for the ground-faultcurrent, to which all equipment parts in the plant are connected. So likewise, the existingneutral conductors of independent low-voltage systems should be tied to the ground mat. Bythis method there will be the least worry that signi cant potential differences will arisebetween the accessible metallic parts of the plant and the plant equipment so protected willbe safe from failure.

Ungrounded system : when an ungrounded system experiences a fault to ground,transient voltage on the un-faulted phases can exceed normal line-to-ground voltage.The sustained voltages to ground on the un-faulted phases will reach line-to-line values.Grounding reduces the deleterious effects of lightning surges in two ways. The groundingsystem will frequently help to dissipate and distribute the surge energy between the phases,thus reducing the severity of the insulation stress. In an indirect but more important manner,by holding system overvoltages down, the grounding system permits application of surge

arresters with lower sparkover values and a higher protective margin for the equipment. Theinfluence of lightning is minimal in the choice of grounding methods for station auxiliarysystems. The surges transferred through transformers from overhead lines are dissipated toa very low value among the multiple circuits emanating from station service buses.

Reference Documents :

1) IEEE – Grounding 2000 .2) BS 7430 .3) Engineer Al – Ghawanmeh , Ahmad Graduation Project .