Cbip-publication-223.pdf 676v59

MANUAL ON SUB-STATION

~R

Chapter on DESIGN OF EARTHING MAT FOR HIGH VOLTAGE SUB-STATION

PUBLICATION

No. 223

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NEW

DELHI

CENTRAL BOARD OF IRRIGATION AND POWER Malcha Marg, Chanakyapuri,

, 1Iif.)

New Delhi-l10021.

New Delhi

January 1992

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AMENDMENT NO.1 FOR CBI&P PUBLICATION NO.223 (JANUARY 1992) ON ~DESIGN OF EARTHING MAT FOR HIGH VOLTAGE SUB-STATION. 1. 2. 3.

.

Page Page Page

6, line 19, read 'IG' for 'IG' 8, line 3 under figure 4, delete line 4 under figure 4, delete 9, line 8, delete 'following'.

4.

t. Km= 5.'

as follows'

line 21, In

id+2h)2 8 [(~: h.d + 8 D.d' - L\ 4 d) + Kii' Kh I n. JI:(2n-1) Page 10, line 21, insert 'increasing' before 'mat depth', line 35, shift 'For ground mat depths ~Jt

between J.25 and about th~ second und~r the second Where

'h'

=

depth

of ground

mat

6.

Page 11 line 26 & 28, insert

7.

Page

12,

lines 1 & 2, insert

8.

Page

14,

line

1, read

l~ne

25,

insert

33,

read

1O.

Page 16, line Page 17, line

11.

line

10,

9.

'determined 'Cs'

2.5 meters' just equation and add equation;

the

J

following

in meter

'=' after

'R ' 1 '=' after 'R2' and 'R12' 'figure 8' for 'figure 2'

1, insert

'R g ' and

'transformers' after 'part' for 'card' '=' after

'Y/Y'

'-2750'

3250 read

(1.) 'figure

10'

for

(figure

3' '+' for "(=) I between '7x76' & '7x69' design fi~ures giien in the sketch.

(ii)

12.

Page.21,

13.

Page 22, Bd:i' '*'q:{1.1:ifte CT in the ligure

14. 15. 16.

disregard

and before ':nsulating point of enclosure' . -4' Page 23, read (i) '1.6x10 for '1.6 x 10' in line item 2 of the Example.

Page 29, read Page 17, read

'5.21'

for

(+) for

'5.51'

(-) before

as the kii

value

in the

7 of

of .IG' equation

20.

for Km. Page 18, h' should equal yd,h r:;-r:kh Page 16, Read 'E tou~h' for 'F touch' in last line. Page 17, Read 'E touch' & 'E step' for 'F touch' & 'F step' Page 18, Read 'E mesh' & 'E step' for 'F mesh' & 'F step'

21.

Page 19, (i) Delete

17. 18. 19.

.

3rd (ii)

Read

(iii)

Read

(iv)

-"

the

Read Read Read Read Read Read ReaJ

(v) (vi) (vii) (viii) (ix) (x) (xi) Reaci

'gh' line.

in the

first

line,

'R12' for '~1 R2' in 19th '=' for 'I' in 9th line. 'R2' 2.2932 2.2932 2.7518 ?~932 ].3604 1.134 1.1996

for

and

line

'R ' in 15th line. z for 2,237 in 19th line for 2.3222 in 25th line for 2.70756 in 26th line for 3.5222 in ~6th line for 1.39524 in 26th line for 1.163 in 26th line for 1.1997 in 27th lin0.

'h' in the

1

CONTENTS

Earthing

2.

Purpose of Sub-station Earthing System

3.

Earthing System

4.

Parameters Affecting the Design of Earthing Mat

5.

Design Procedure

6.

Construction and Installation of Earthing Mat

7.

Earthing Mat and Perimeter Fence Connection

Annexure A :

Annexure B :

.I

Page 1

1.

Estimation of Mesh and Step Potentials by Graphical Method

Example Showing Division of Fault ,Current Between the Overhead Earthwire and Earthing Grid

1 2 15 15

25

28

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Design of Earthing Mat for High Voltage Substations

I !.

,

1.

Earthing

Provision of adequate earthing in a substation is extremely important for the safety of operating personnel as well as for proper system operation. By earthing we mean connecting the electrical equipment to the general mass of the earth which has a very low resistance. 2. Purpose of Substation Earthin~ System The object of an earthing system in a substationis to provide under ami around the suhstation II surface which shall be at a uniform potential and near zero or absoluteearth potentialas possible.The provision of such a surfaceof uniform potential under and around the substation ensures that no human being in the substation is subject to shock or injury on the occurrence of a short circut or development of other abnormal condition,,>in the equipment inslalled in the yard. The primary requirements of a good earthing system in a sub-station are: (i) It should stabilise circuit potentials with respect to ground and limit the overall potential rise. (ii) It should protect life and property from over-voltage. (iii) It should provide low impedance path to fault currentS to ensure prompt and consistent operation of protective devices during ground faults. (iv) It should keep the maximum voltage gradient along the surface inside and around the substation within safe limits during ground faults.

I

!

3. Earthing System 3.1 The earthing system meeting the above requirements comprises an earthing mat buried horizontally at a depth of about half-a metre below the surface of the ground and ground rods at suitable points. All the non-current carrying parts of the electrical equipment in substation are connected to the earthing mat. Under the normal conditions, the ground rods contribute little towards lowering the ground resistance. However, these are helpful in lowering mesh potentials and maintaining low values of resistance under all weather conditions. 3.2 The earth mat is connected to the following in a substation: (a) The neutral point of each system through its own independent earth. (b) Equipment framework and other non-current carrying parts. (c) All extraneous metallic framework not associated with equipment. (d) The earth point of Lightning Arresters, Capactive Voltage Transformers, Voltage Transformers, Coupling Capacitors and the lightning down conductors in the substation through their permanent independent earth electrode. (e) Substation fence.

I

1

3.3

The earthing system installation shall strictly comply with the requirements of latest edition of Indian Electricity Rules. relevant Indian Standards and Applicable Codes of Practices. 4. Parameters Affecting the Design of Earthing Mat Several variable factors are involved in the design of earthing mat conductor. Therefore, earthing '!1at for each substation has to be designed individually usually. The earthing mat has to be designed for the site conditions to have a low overall impedance and a current ca.-rying capacity consistent with the fault current magnitude. The parameters listed below influence the design of earthing mat: (e) Shock duration; (a) Magnitude of fault current; (f) Material of earthing mat conductor and (b) Duration of fault; (g) Earthing mat geometry. (c) Soil resistivity;

, I

iI i 1

(d) Resistivity of surface material;

,,0

~

-'-'

5.

Design Procedure The following steps are involved in the design of earthing mat: (i) The substation layout plan should be finalised before the design of earthing mat is taken up. From the proposed layout of the substation, detennine the area to be covered by the eanhing mat. ' (ii) Detennine the soil resistivity at the substation site. The resistivity of the earth varies within extremely wide limits, between 1 and 10,000 ohm-metres. The resistivity of the soil at many station sites has been found to be non-unifonn. Variation of the resistivity of 'the soil with depth is more predominant as compared to the variation with horizontal distances. Wide variation of resistivity with depth is due to stratification of eanh layers. In some sites, the resistivity variation may be gradual, where stratification is not abrupt. Highly refined techniques for the determination of resistivity of homogeneous soil is available. To design the most economical and technically sound grounding system for large stations, it is necessary to obtain accurate data on the soil resistivity and on its variation at the station site. Resistivity measurements at the site will reveal whether the soil is homogeneous or Ilon-unifonn. In case the soil is found uniform, conventional methods. are appli~ble for the computation of earth resistivity. When the soil is found non-unifonn, either a gradual variation or a two-layer model may be adopted for the computation of earth resistivity. The resistivity of earth varies overa wide range depending on its moisture content. It is, therefore, advi~able to . conduct earth resistivity tests during the dry season in order to, get conservative results. Measurement of Earth Resistivity I

'

,

Tcst Locations ,

In the evaluation of earth resistivity for substations and generating stations, at least eight test directions shall be chosen from the centre of the station to cover the whole site. This number shall be increased for very large station sites of it, the test results obtained at various locations show a significant difference, indicating variations in soil fonnation. Principle of Tests Wenner's four electrode method is recommended for these types of field investigations. In this method, four electrodes arc driven imo the earth along a straight line at equal intervals. A CUffem J is ed through the two outer

electrodes and the eanh as shown in Figure

~

and the,voltage difference V, observed between the two inner electrodes.

The current J Oowing into the eanh produces an electric field proponional to its density and to the resistivity of the soil. The voltage V measured between the inner electrodes is, therefore, proponional to the field. Consequently, the resistivity will be proportional to the ratio of the voltage LOcurrent, i.e., R. The following equation holds for:

p

4SnR

= I +

~ where p S = I{

S! ~ -I c'

5

~

P1 '1

S'

I,

III \Ililll:;,

(~

Potential electroqe

//

..

;1I1l1

depth or burial of electrode in lIletres, If the depth of burial of the electrodes in the ground is negligible compared to the spacing betwecn the electrodes, then

p=2n5R

er

( I)

(1

1l'.\I:;I:lII\\'

Me

("

resistivity of soil ill olllll-lllelre, distance between two successive electrodes ill metres, Ratio of voll:II'.,' to (,111'1\'11101 l'1\'('(I\Hk

e

2 5

,

(2)

~s Figure

"'I-

s

"I-

S --I

~

1: Connections for a Four- Terminal Megger

Test Procedure At the selected test site, in the chosen direction, four electrodes arc driven into the eanh along a straight line at equal intervals, 5, The depth of the electrodes in the ground shall be of the order of IO to 15 em. The megger is placed on a steady and approximately level base, the link between terminals PI and CI opened and the four electrodes connected 2

.IJ4~

to the instrument terminals as shown in Figure 1. An appropriate range on the instrument is thus selected to obtain clear readings avoiding the two ends of the scale as far as possible. The readings are taken while turning the crank at about 135 rev/min. Resistivity is calculated by substituting the value of R thus obtained in the Equation (2). In case where depth of burial is more than 1120th of spacing, Equation (1) should be used instead of (2). Correction for Potential Electrode Resistance In case where the resistance of the potential electrodes (the two inner electrodes) is comparatively high a correction of the test results would be necessary depending on its value. For this purpose, the instrument is connected to the electrodes as shown in Figure 2. The readings are taken as before. The correction is then effected as follows: Let the readings of the megger be Rp with as shown Figurecircuit 2 andofthethe electrode spacing pI the andconnections the resistance of the in voltage instrument usedintometres. obtain If the uncorrected value of soil resistivity is R (as indicated inside the scale cover of the meter) is Rv, the corrected value of the earth resistivity would be: p = pi x (Rv + Rp)/Rv

I t

!~

(i) Testing of Soil Uniformity During the course of above tests, it would be desirable to get information about the horizontal and vertical variations 1 earth resistivity over the site under consideration for the correct computation of the resistivity to be used in the design calculations. The vertical variations may be detected by repeating the tests at a given location in a choosen direction with a number of different electrode spacings increasing from 2 to 250 metres or more, preferably in the steps 2,5,10,15,25 and 50 metres or more. If the resistivity variations are within 20 to 30 percent, the soil in the vicinity of the test location maybe considered uniform. Otherwise a curve of resistivity versus electrode spacing shall be plotted and this curve further ~alyz7d to deduce stratification of soil into two or more layers of appropriate thickness or a soil of gradual resistivity 'variation. The horizontal variations are studied by taking measurements iQvarious directions from the centre of the station. (ii) Computation of Earth Resistivity of Uniform Soil When the earth resistivity readings for different electrode spacings in a direction are within 20 to 30 percent, the soil is considered to be uniform.

P,

l'

1--

Figure

s

-+-S

P2

Megger

...

\

It

s --1

2: Test Connection to Measure the Sum of The Potential Electrode Resistances

3

Figure

3: Polar

Curve

"\I

When the spacing is increased gradually from low values, at a stage, it may be found that the resistivity readings is more or less constant irrespective of the increase in the electrode spacing. The resistivity for this spacing is noted and taken as the resistivity for that direction. In a similar manner, resistivities for at least eight equally spaced directions from the centre of the site are measured. These resistivities are plotted on a graph sheet in the appropriate directions choosing a scale. A closed curve is ploued on the graph sheets ting all the resistivity points plotted to get the polar resistivity curve. The area inside the polar resistivity, curve i~ measured and equivalent circle of the same area is found out. The radius of this equivalent circle is the average resistivity of the site \'Oder consideration. The average resistivity thus obtained may be

used for the design of the earthing grid and other computations and the results will be reasonably accurate when

the soil is t.omogeneous

(see Figure

3). f

The methodology for non-homogeneous soil is dealt with in '6',(xiii). (iii) Determine the Maximum Ground Fault Current Fault current at the substation is determined from the system studies. A correction factor is 2lpplied to the fault current thus determined to take care of the future growth of the system. Value of this correction factor is usually of the order of 1.2 to 1.5. However, in practice 40 KA for 400 kV system and 31.5 KA for 220/132 kV systems are generally adopted for design purposes. (iv) Duration of Fault For the design of earthing mat, the practices regarding assumption of duration of fault differ from country to country. Thus in the USSR, the duration of fault is assumed as 0.2 second. In the USA, it is assumed as 4.0 seconds which is equal to the duration on which the short time rating of the switchgear is based. In India, the short time rating of most of the equipment is based on 1.0 second duration of fault. Therefore, 1.0 second may be adopted as the duration of fault in the calculations to determine the size of conductorfor earthingmat. For the purposeof determining,the safe step and mesh potentials a duration of 0.5 second may be adopted. However, it may be ensured on the basis of the protective gear and protective schemes provided in each case that fault is cleared in the period not exceeding of 0.5 seconds. Where the fault clearing time exceeds 0.5 seconds, this duration may be taken equal to fault clearing time. (v) Determine the size of Conductor for Earthing Mat (a) Size of conductor based on Thermal Stability: The size of conductor for earthing mat based on thermal stability is determined with the help of the approximate formula as per IEEE 80-1986 given below: I

:I( Where, A I .;,., t::l r

pr

)

~Cf~,l0~ c

ex r

Pr

= Conductor

In

cross

( Ko Ko

+ 'Tin + Ta ,)

= section

in mm2

= rms value of current in kilo amps. (KA) = thermal co-efficient

of resistivity at reference temperature Tr of earthing mat conductor at reference temperature Tr, in ,~cm3

= resistivity

I ...

Ko

tc Tm T. Tr

=~

ar

-'I'

r

I I I

.-

= time of current flow, in second = maximum allowable temperature in degrees celcius (CO) = ambient temperature in degrees celcius (CO) = reference temperature

for material constants in degrees celcius (CO)

TCAP

= Thermal capacity factor in j/cm3fC = 4.184 SH. SW Where SH is specific heat in caIlgm;oc, and SW is specific weight in gm/cm3 of conductor 4

material

-~~~~("'t';":'-':

~.

li~.i:.'i!.;;&_J.~,14~"'J;;;,,,,"""i,,'i.~.,

....

The values of the various constents in the above equation applicable to steel are given below. at 20°C cif = 0.00423 ., . 1.0 second t. =

K

0

=

1

-- 20 = 216

0.004'23 SH SW TCAP

= 0.1 14 = 7.86

Pre Tm

= 15 micro-ohm/cm3 = 620°C for welded Ls = 310°C for boILedts = 40°C .

= 3.749

..' k Idcd .' A f bo th I the above va ue In e equauon given a ve or we ts wor s-out as 0n I x 12.30 or 12.30 I mm2' I x 15.13~ and A for bolted ts works-out as or 15.13 I mm2

~'. subsututiOn 0f

I

~

Mechanical Ruggedness of Conductor From the consideration of mechanical ruggedness. and easy installation. The maximum width to thickness ratio of steel flats for ground mat conductor should be 7.5 such that thickness of the flat is not less than 3 mm. Ground mat conductor comprising steel rod having a diameter not less than 5 mm. The standard sizes of conductor as per IS : 17301989 are as follows: (ii) 20 x 6 mm2 (i) 10 x 6 mm2 (iv) 40 x 6 mmi (iii) 30 x 6 mm2 (vi) 60 x 6 mm2 (v) 50 X 6 mm2 (viii) 65 x 8 mm2 (vii) 50 x 8 mm2 (ix) 75 x. 12 mm2 (c) Corrosion: On an average steel corrodes about six time as fast as copper when placed in soil. The extent of corrosion depends upon the properties of soil. Many a time, soils have conflicting properties. some of which indicate that the soil is corrosive and others indicate the opposits. Despite this, a very fair degree of correlation has been found between electrical resistivity of soil and corrosion. The generally accepted correlation between the electrical resistivity'of soil and its corrosivity is as indicated in the Table below:

TABLE Soil Resistivity and Corrosion (Class of soil)

Range of soil resistivity (Ohm-metre) Severely corrosive Moderately corrosive

Less than 25 25-50 50-100 Above 100

Mildly corrosive Very mildly corrosive

5

'~'_."_"'

The following methods are available to safeguard conductor against excessive corrosion:(a) Provide cathodic protection. (b) Use current conducting, corrosion resistant coating on steel (e.g. zinc coating). (c) Use steel conductor with large cross-section having allowance for corrosion. The first two methods are expensive and find application in special cases. The third method is much simpler and relatively less costly and therefore finds wide application. Based on the'results of the field studies on rates of corrosion, the following allowances in cross-sectional area of the earthing- conductor are retommanded (Refer CBI&P Publication Technical Report no 5) to take the effect of corrosion into : (a) In the case of conductors to be laid in soils having resistivity greater than 100 Ohm-metre~-No allowance. (b) In the case of conductors to be laid in soils having resistivity from 25 to 100 Ohm-metre-15 percent allowance. (c) In the case of conductors to be laid in soils having resistivity lower than 25 Ohm-metre or where treatment of soil around electrodes is carried out --30 percent allowance. For the purpose of detcnnining the allowance to be made for corrosion, the minimum resistivity of the soil encountered at the location of grounding electrodes should be considered. The resistivity will be the minimum in wet and hot weather. Thus, for very mildly corrosive soils, steel conduct(}fs meeting the thermal stability and mechanical requirements are adequate. However, the steel conductors in the soils of other types, should be atleast 6 mm thick if steel flat and have a diameter of atlcast 16 mm if in the form of steel round. (vi) Determine the Maximum Grid Current The design value of the maximum grid current (10) is given by the following equation: 10

= .Dr I.

= Maximum grid current in Amperes Df = Decrement factor for the entire duration of fault Typical values of Df are given in the following table.

Where

Ie;

Fault duration (S)

Decrement factor Dc

0.008 0.1 0.25 0.5 or more

=



I&

=

Where I& = Sf

=

1.65 1.25 1.10 1.0

Corrective projection factor for the relative incrcase of fault currents during the station. life span. For zero future growth of the system, C p =1 An example showing the method for determining the value of 10

as a ratio of the maximum fault current is given in Annexure-B

Sf ( 3 10) rms value of the symmetrical grid current in Amp. Curn~nt divisioll factor relatillg to the magnitudc of faull currcllt to that of its portion flowing between thc t~anhillg mal and surrounding earth.

Sf is dependent on the following parameters: (i) Location of fault. (ii) Magnitude of station earthing mat resistance. (iii) Buried pipes and cables in the vicinity of or directly connected, or both, to the station earthing system. (iv) Overhead ground wires or neutral conductors. Sf is computed by deriving an equivalent representation of the overhead ground wires, neutrals, etc., connected to the earthing mat and then solving the equivalent to determine the fractions of the total fault current which flow between

6

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-"--

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~,'.'..',{>- ."

the mat and eanh and through the ground wire or neutrals. For calculating Sf' the following formula is used: Sf

=

Combinedeq. resistanceof overheadstatic wire networkas seen from fault point Combined eq. resistance of overhead static ground wire network(as seen from fauItpoint) + stltion ground resistance to remote earth.

10 = . Zero sequence fault current For line to line ground and line to ground faults, the values of 10are given by the relations given ~Iow.

10 for line to line-ground

fault

E~

=

X1(Xo +X2)+X2. Xo

10 for line to ground fault

E

=

X1+~+Xo

= phase

to neutral voltage in volts. The values of Xl' X2, Xo' the sequence reactances are computed looking into the system from the point of fault. (vii) Resistivity of Surface Layer (p) Crushed rock is used as a surface layer in substations for the following reasons:

Where E

(a) It provides high resistivity surface layer.

.

,

(b) It serves as impedment to the movement of reptiles and thereby helps in minimising the hazards which can be caused by them. (c) It prevents the formation of pools of oil in the event of leakage of oil from oil insulated and oil cooled electrical equipment. .

(d) It discouragesthe growth of weeds.

.

(e) It helps retention of moisture in the underlying soil and thus helps in maintaining the resistivity of the subsoil at lower value. (f) It discourages running of persons in the switchyard and saves them from the risk of being subjected to possible high step potentials. In tropical countries like India. where the population of reptiles is large, it is advantageous to surround the electrical equipment and the structures ing conductors by a surface layer of about 1.Pcm of crushed rock up to a distance of about two metres in all directions. Such surface layer around the metallic equipment and structures. besides minimising the hazards caused by reptiles. provides a high resistivity layer below the feet of human beings approaching the equipment! structures and enables them to withstand higher touch potentials. If step potential without crushed rock is well within safe limits, it is not necessary to spread crushed rock over the complete switt>hyardarea. However, if it exceeds the safe limits crushed rock of 15 to 20 mm size may be spread to cover the earth in the entire switchyard area. The resistivity of rock depends on the types of rocks, as wi1\be seen from the table (Refer CBI&P Publication Review No.1) given below: Type of rock

Range of resistivity (Ohm-metre)

Morain gravel Boulder gravel Lime stone Primary Rock (Griess, Granite etc.)

1000 to 10,000 3000 to 30,000 10,000 to 50,000

Average vaulue of resistivity (Ohm-metre) 3,000 15,000 5,000 25,000 -

7

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,"",

;~{~li

If the type of rock to be used is known the lower value of resistivity for that type of rock may be adopted in the design. Otherwise, in conformity with the design practicesfollowed by most of the electric utilities, an average resistivity yalue of 3,000 Ohm-metre may be adopted for the purpose of earthing mat design. (viii) Determine the Tolerable To'uch and Step Potentials The values of these potentials depend on,the bodylweights, thickness and resistivity of surface layer,and duration of shock current. The relations between the above factors for persons with average weight of 50.kg are given below. E touch

+ 1.5 Cs (hs' K) ps)

= (1000

0.116

~ E

step

= (1000 + 6Css (h,

K) PS)

0.116

-Jt Where Cs

s

= 1 for crushed

soil, reference Ps

rock having resistivity equal to that of soil. If crushed rock resistivity does not equal that"of may be made to Figure 4 for obtaining the value of C

= resistivity

s

.

of surface layer in Ohm-metre.

p

= resistivity of

K

=

soil in Ohm-metre.

P - Ps .

P+

Ps'

t. = Duration of shock current flow in seconds. h. = Surface layer thickness in metre. k=O

"

1.0

0.8

(,

Lh. 0

0

0.12

0.16

0.20

0.21.

hs (Mltlrs)

Figure 4 : Reduction factor C, as a Function of Reduction Factor K and Crushed Rock Layer Thickness h where

,

s

C, = reduction factor for derating the normal value of surface layer resistivity determined as follows C, = 1 for crushed stone resistivity equal to soil resistivity 8

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: - T':Q.'U';;:r".m::fLT,;~:;:\:,~:;::::~

,:;:::1.:"

(ix) Develop Preliminary

Arrangement

of Earthing Mat

A prelim~nary earthing mat arrangement is developed on the basis of an assumed spacing between two parallel conductors. In this arrangement a continuous conductor should be assumed as surrounding the switchyard and the conductor within it should be located at reasonably uniform spacing parallel to each other along the rows of the structures, equipments etc. From the arrangement so arrived at the number of parallel and cross conductors and the total length of conductor constituting ;he earthing mat are determined for use in the further design calculations.

(x) Determine the LikEly Mesh and Step Potentials The values of the expected maximum mesh and step potentials are calculated with the help of the following formulae given below. Several simplifying assumptions are made in derivation of these formulae. These assumptions may result in incorrect results for comparison

with the results obtained by computer analysis, for some cases. For determining the

inaccuracies for practical purposes, these formulae may be used with the following limits for square grids or for rectangu!ar grids having the same numtcr

of conductors

in both directions:

(ii) 0.25 m ~ h ~ 2.5 m (iv) D > 2.5 m

(i) n ~ 25 (iii) d < 0.25h These symbols are defined below

Mesh potential on the earth's surface above the centre of a corner mesh: p.

Km' Kj'

Where,

K.I

= =

10

=

10

volts

Em =

L

Corrective factor which s for the increase in current density in the grid extremities. 0.656 + 0.172 n Maximum grid current in Amperes. D2

1

[In (

(d+2W +

K..

h

8 ) +K"h -

In

]

Km =

2;t

K..II

1 for grids with earthing rods along the perimeter or for rods in the mat comers as well as along the parimeter and throughout the grid area,

=

1

=

Kh

(2n) 1Jn

=,/1~

16 h.d

4 d"

8 D.d

for grids without earthing rous or for grids with only a few earthing rods, none located on the perimeter or in the comers.

+~ h 0

D

= = =

Soil resistivity in ohm-metre. 1 metre (reference depth of earthing mat) Spacing between parallel conductors, in metres.

n

=

--lnA

nA

=

The number

nB

=

The number of parallel conductors in longitudinal direction

P h0

. nB

7t (2n-l)

for calculating

Em

of parallel. conductors

in transverse direction

9

= = =

depth of earthing mat conductor in metres. diameter of earthing mat conductor in metres L. + L, for eanhing mat without earthing rods or with only a few rods located within the mat, but away from the perimeter.

= = =

Lc + 1.15 L, for earthing mat with ground rods predominantly along the perimeter. Total earthing mat conductor length, in matres; and Total earthing rod length, in metres.

h d L

Lc

L,

Step potential E = p. Ks. K. lJL volts step Where K, = 0.656 + 0.172 n I

and K, =

~

L, hand

-

I

[ 2h

]

-+D+h

I

D

(l-0.5n.2)

]

D being the same as defined earlier and n being larger of nA and nB for calculating E..

The value of expected mesh voltage and step indicated below:

voltage should be determined for the following conditions

in the order

- without ground rods - with uniformly distributed ground rods - with ground rods only in the perimeter If the computed value of mesh voltage is less than the tolerable touch voltage, the design of earthing mat is completed. However, if the computed mesh voltage is found to exceed the tolerable touch voltage the design will require inclusion of ground rods or revision. Similarly, the computed step voltage should also be less than the tolerable step voltage. If either the step or touch voltage are found to exceed the tolerable voltages, the eanhing mat design will have to be revised .by including additional earthing rods, mat depth reducing spacing, etc. Additional earthing rods should be provided at the base of lightning arresters and transformers neutrals. In order to facilitate checking of the mesh and step potential the graphical method for estimation of mesh and step potential is given in Appendix 'A'. (xi) Determine the Station Ground Resistance For ground mat depths less than 0.25 metres; The value of the substation grounding resistance in uniform soil can be estimated by means of the following formula.

Rg = -;.J++~ where R = station ground resistance in Ohm (.0) p = average earth resistivity in .o-m A = area under earthing mat in square metres (m2) L = [he [olal lengrh of huried conductors in metre (m) R, = Starion ground resistance in ohm (.0) Rg

=

P

[~+ L

-L{l+ ./2OA

l+~}] A

The station ground resistance for ground mat with £round rods is determined with the use of Schwarz formula given bel()w . For ground mat depths between 0.25 and 2.5 metres:

10

'-

--.--

.Ii;.~;;....

.~\ii~W,f""~1t;,g;~i.i;...t:"~;;;'''';

"

-.

RI R"

R& =

-R

2

12

-

RI + R" 2RI2 Where RI = resistance of ground mat conductors

~ = resistance RI2

:::.:

of all ground rods.

mutual resistance between the group of grid conductors and group of ground rods.

The value of RI' R2' and RI2 can be determined with help of the formulae given in (xiii) with the assumption that for the uniform soil PI - P. (xii) Determine the Ground Potential Rise The value of the likely ground potential rise is given by the product of the maximum grid current, 10 (see item 5 (vi) and the estimated station ground resistance, RI . If the value of this prOlluct is below the tolerable touch voltage, no further analysis is necessary and only thc additional conductor required to connect the mat to equipmcnt grounds has to be provided. Otherwise the earthing mat arrangement will require revision till the above condition is met. (xi' Design Philosophyfor Non-homogeneousSoil , fhe methodology covered under Clause x, xi and xii pertains to uniform soil conditions. Normally the apparent resistivity values obtained by Wenner's 4 probe method with a probe spacing of 10 m is sufficient for earthing system design but in cases where a multilayer earth is clearly indicated, two layer model system can be resorted to. The resiSUlnce of such a model can be evaluated as explained below. However, for potential (step & touch) calculations, solution can be obtained by solving Laplace's equations for a point curret source. Since this involves infinite series of , computer usage is inevitable. Alternatively the earthing grid (from potentials point of view) can be designed based on the resistivity of the upper layer where the grid is laid. Inaccuracies can sometime creep up with this assumption but the same can be verified by making a few measurements of gradients after installation of the grid. In case of unsatisfactory results, special meshes etc. can be buried, around structures/equipment normally accessible to persons standing on the ground or by providing adequate layer thickness of crushed rock/g-ravel. Evaluation of Resistance

Earth grid of area tA) & dimension axb......

//~/~

/~~~~

Pt

Figure 5

The resistance for a non-uniform soil strata (two layer) can be evaluated as under: RI R"

RI

RI

+

-

R"

2 RI2

-

2R

12

where RI

( :\)

1

~_l

P,

[In (21/h') + kl (1/$)

-KJ

11

R2

Pia

(

2n 7t 12

)

[In (81!d)

-1 + 2 kl (JI/JA

R12 (~)

[In (21/1) + KI (I/[A)

P. = 12(PI, p)

(P2. H + PI ( 12 - H))

7t II

P.

= 12(PI, P2) (P2 (H-h)

The various parameters PI

P. H

+ PI (12 + h - H) where Rod top is in the same depth as the grid.

arc as given below:

apparent soil resistivityas seen by a ground rod in n-m

=

thickness of the upper layer soil in m soil resistivity from depth H downward Q-m

12

==

h ==

total length of grid conductors in m average length of a ground rod in m depth of grid burial in m

~ h

A

=

n

= = = = =

b

where Rod's top is flushed with earth surface.

= =

d2 a

-K2 + 1]

soil resistivity encountered by grid conductors buried at depth h in n-m

11

KI' K2 dl

1)2]

==

P2

hI

(-.;;:-.

==

for conductors buried at depth h, or 0.5 dl earth's surface)

for conductors at

= a (on

area covered by a grid of dimensions a.b in m2 number of ground rods placed in area A constants related to the geometry of the system (Figures 6 (a) and (b)) diameter of grid conductor in m diameter of ground rods in m short-side grid length in m long-side length in m

(xiv) Lowering of Earthing Impedance The solution to the other problem of achieving low impedance earthing in smaller area can be achieved by employing anyone or combination of the following methods. These methods can also be employed to coventional substations where soil resistivity is high 1. Connection of substation grid with remote ground grids and adjacent grounding facilities if available. 2. Use of deep driven ground rods or use of longer ground rods and more number of such rods along the perimeter of the grid. 3. Use of foundation rods where feasible as auxiliary grids. 4. Wherever practical, a nearby deposit of low resistivity material of sufficient value is available, it is ideal to form an extra grid at such locations and connect it to the substation grid. such extra grids are also known as satellite grids. . 5. The maximum touch (mesh) potential occurs in the comer mesh of the grid. Normally no equipment are placed in this area. In such cases, the touch (mesh) potential even if greater than permissible values can be accepted, if s.tep potential in the corner mesh are within permissible limits. When equipment are placed near the comer mesh, it may not be necessary to change the e~tire grid design to reduce the corner mesh potentials within the maximum permissible values. Instead it would be econ6mical to form auxiliary grids in the corner mesh to reduce the touch (mesh) potentials. Figure-7 shows a grid without any conductors in the comer mesh. Figure-8 shows grid with auxiliary grid in comer mesh. It has been observed

12

-"_.~----

~-'~.,-.:..w,~~

~.

, ..

Iflit

~

'<,I/'.-;-"-'"N,,1I1

c_"''''''\::(~;'~M'>,

---::..

ID

\

~:5

E

'-0

....

u.. .1/1 N

-0

~\

0

:;: ..

'"

J\

"0 ~":; I

r-:

....I

N

0

11 .&. .&.

....

t0

II "' .&. ~.&. ....

i

!

-

-0

0

'" ''' 1!

a.o

-~-~-~ I ~I X I

~-k +

0 vi

'" vi

.... ~~c ., 'w :;::

"'C c: IV

W ~°

1/1

a

c: QI

-~aI

.

u ;0::

-

QI 0 W

\oJ

CD

0 ..Q

('f

N

),£

.&. ....

"0 ~II!~,~

.c'-:CD~WI ~0 0

u VI

., II .. 11 II > > > 5 ~5 cD!s~ w)c)w)()u

\ ..,; '"

0 .... 1: .... a> c..

",

1\

0

...

.&. ....

'

0... .......

IV ~'-

=..t::.

0 .;

.; '"

..0

0 rri

rri '"

QI

'-::J

Z)I ~uap!Hao)

.£1 u.. ID

/

l

....

J

-0

/

"'

/ v

/

/ I 0

-k ~

vi /

.

,., '" ~

~~

Vv

ul

CD

'/

N '" ~

0

~

¥! ~

S! .,,;:

1/

",

.&.

"" "0 ':; I

0 .... I .&. ....

GI c

/

N

Q/ ....J

0... '"...

0 11 s:: .&. ....

.. .&. .&. ....

~..:

g

~

..z

0

13

~ 0

'" -0

-0

'"...

81 .&. .&. ....

:..::

....

t~

~...i"o .., "0 .-:...:.-: "0 N ... + .... + .... 0>(0)(0)(

.3 ;,;::

-

CII 0 W

--...,.---",--U'"\ I~

0

I 0 0

I~

0

~..n CD. II C\/ ..

«

I

>> 5~5~5~ u \oJ

')I ~U~J7)O)'

,

-k

1/

.S! ~~.... rJ ...

I W

I

> u u

that addition of auxiliary grid as shown in Figure 2 reduces the comer mesh potentials to about 2/3 of the value of comer mesh potential without auxiliary grid.

1.31.3 1.0 1.0 0.9~.9 to 1.31.3to to 0.910.9 to

,-

to ~.9Io.9 h.o to t3 t3 to 0.910.9to to 1.3t3

to to - 1.0 - to to t1 1.0

t1

1.1

1.1

0.90.9 ~~t1 ~.90.9

1.1

to

1.0

t1

1.1

to

to

to

to

t1

to to to to to to D.9 0.9

1.9 1.6 1.4 1.2 t2

1.4 t6

1.6 1.2 1.1

t1

1.1

t2

1.6

to to

to

1.1 t1

1.1.

to to t1

to

to

to

to

1.1 to 1,0

0.9 0.9 1,1 0.9 0.9

-

1,1

1.0 to

",.

1,1

~to

t1 .11

1.1

1.1. t1

1..1 1.0

1.2 1.1

to

1.0 1.0 to

t9

1.1 t2

t2 1.' 1.0 1.0 to to t1 1.2 1.1. t1 1.1 to to 1.1 t1 1.1. t6

1.2 t1

1.9 U

tit

1.1 t1 12

t2

to -1.0 to to

1.1 1.2 1.6 tit

1.1

t3 t3 to to ~.9Io.9to 1.3t3 h.o to ~.910J to

1.6 1.9

t1

1.0 to

0.9 0.9 0) 0.9

1.0 to to 1,0- -

t1

to to to 0.910.9to to 1.3t3 to 10.91°.9to to t3 t3

Figure 7 Figure 8 (xv) Check up for Transferred Potential Where a possibility of the places outside the eant.ing mat area being subjected to the earthing mat potential exists, the communication and signal circuits, low voltage wiring, conduits, pipes, rails, metallic fences etc. should be checked for transferred potential and adequate protection against transferred potential should be provided where necessary. If this is not conveniently possible, the resistance of the earthing system should be further lowered by increasing the earthing conductor lengths or by increasing the substation area under the earthing mat till the desired voltage is attained. For further information Transferred Potentials and Solutions, CBI&P Technical Report No. 49 on "Earthing Parameters ~f HV, EHV and UHV Sub-Stations" may be referred. (xvi) Earthing of Gas Insulated Substations GIS is a compact, multi-component, assembly enclosed in a earthed metallic housing in which the primary insulating medium is a compressed gas and it normally consists of buses, switchgear and associated ec:uipment. GIS are subjected to same magnitude of fault current and require low impedance earthing as in case of coventional substation. But GIS installation require only about 25% of land area of conventional S/S,thus making design of system more difficult. Another area required allention in GIS stations is earthing of metallic -enclosures. The metallic enclosure of GIS have induced currents and specially during an internal earths fault the inductive voltage drop occurring with the GIS assembly must be taken into for design to touch potential in GIS station. The touch voltages criteria of GIS station is

/ Where

FA Eo

= The

actual calculated touch voltage (Calculated in a manner similar to conventional 5/S).

=Maximum

and the supponing

S

(FA)2 + (Eo)2 < Er (max.)

(max)

value of metal to metal voltage difference on and between GIS enclosures or between GIS enclosure structures. Refer Annexure for sample calculation.

=

Maximum permissible touch voltage.

The mel
-'_._'-'

'

~

--=-""",-_"""",~,,,,"~CJ:",,~~:...,

,..::'_'_'.'-:",:-:,:.:--:;:.~;:--.""

6.

Construction and Installation of Earthing Mat All ts in the steel earthing system should be made by welding except those where earthing mat may have to be separated from equipment, cable sheath etc., for testing. These ts should be accessible and frequently supervised. All exposed steel conductors should be protected with bituminous paint. For protection against rusting, the welds should be treated with Berium chromate. Welded surfaces should then be painted with red load and aluminium paint in turn and afterwards coated with bituminous paint. The ts in the earthing conductor between the switchgear units and cable sheaths and such other points which may require to be opened subsequently for testing should be bolted. 7. Earthing Mat and Perimeter Fence Connection Whether the earthing mat and perimeter fence should be connc:ctedor not should be decided on the basis of study/ analysis of individual cases as indicated below: (a) If the design of earthing mat permits termination of the mat more than 1.5 metres inside the perimeter fence and electrical isolation between the fence and earthing mat can be ensured, the unc1imbable fence should be kept isolated ftom the earthing mat and the fence should be independently and effectively earthed by running on earthing conductor ''''rIer the boundary and connecting it to the fence at frequent intervals. If the design of earthing mat requires extension of the mat upto the perimeter fence or where electrical isolation between \

/

the earthing mat and fence cannot be ensured, but the design calculations reveal that the values of touch potentials both within and outside the fenced - in area arc within safe limits, the fence should be connected to the earthing mat at frequent intervals. (c) If the design of earthing mat requires extension of the mat upto the fence and calculation reveal that touch potential at the fence exceeds the tolerable limit, the earthing mat should be tenninated about 1.5 metre or more within the boundary line of the fence and the fence electrically isolated and independently earthed by running earthing conductor under the fence and bounded with the fence at frequent intervals or by means of adequate number of earthing electrodes. Example for the design of Earthing Mat for a substation with high resistivity Let a 132 kV line AB feed a substation B at a distance of 32 km. Let the fault level at Bus A be 975 MV A and the resistivity of the soil of the switch yard at B be 250 ohm metre. At substation B, a 15 MV A, 132/66 /ykY,transformers Y/y steps down the received power to 66 kY level from where it is further stepped down by 3x2.5 MY A, 66133 K~ti to 33 kY as shown in Figure 2. Calculation of fault current Fault level at 132 kY Bus A = 975 MV A 100 Xpu at Bus A on 100 MVA base 975 0.1026 pu T ~ngth of 132 kV line AB 32 km

= = =

Xl

= X2 for

the 132 kV line

Xo for the 132 kY line

Xpu at bus B

= = = = =

0.422x32 174.24 0.0775 pu 1.47x32 174.24 0.26997 pu 0.1026 + 0.0775 + 0.0775 + 0.26997

Phase to ground fault current at 132 kV Bus B

=

Impedance of 15 MVA, 132/66 KV transformer Xl.

= X2 for

transformer

=

= 0.5276

3xl00xl000 .J3.'(132xO.5276

=

= 2487.11 Amp.

7.5%

7.5xl00 l00xl5

= 0.5pu 15

= 0.8xO.5 As this transformer

= 0.4 pu is Y/Y connected X at 66 kV bus at station B 0.5276+0.5+0.5+0.4 Xo

=

Phase to ground fault current at 66 kVbus at station B

= -

1.9276 pu 3.x1oox1000

= 1361 Amp

../3.;: 66 x 1.9276 Impedance of 3x2.5 MV A, 66/33 kV transformer @8% per transformer

= X2

Xl

for the transformers

As these transformers bus at station B

8x100 = 3x2.5x1oo

=

1.067 pu are MY connected, zero sequence reactance will not come in the circuit. Therefore X at 33 kV . = 1.9276+1.067+1.067 \ 4.0616 pu

=

Phase to ground

fault currenl at 33 kV bus

=~

3.x1oox1ooo 33.x 4.0616

= 1292 Amp. From the above. it is seen that the faull current is the maximum on 132 kV Bus B. However, it is less than the short time current rating of the switchgear. Therefore the earthing mat will be designed on the basis of fault currents of 20 kA. Area of ear/hing mat conductor Area of steel conductor = 12.30I 12.30x20000 1000 = 246 mm2 Resistivity of soil of station B=250 ohm metre. Since the resistivity of soil is higher than 100 ohm metre, no allowance is necessary for corrosion. The nearest standard steel section in the form of mild steel flat that can be used will be 6 mmx50 mm giving on area of 300 mm2. Maximum Grid Current IG

= o DrIg

- As expansion factor has been taken as 1.5.



= 1.0

- For the duration of flow of fault current equal to 1.0 sec. Df= 1.0 I. = Sf (3 IJ 3 10= 20000 Amp. In the absence of full details regarding exact system configuration of which the substation form a card, at the design singe, il will be. fairly accurate 10 adopt a vuluc of 0.5 for Sf to dctcrmine the fault current that flows through the grid . to rcmote carlh.' Thus

IG

= 0.5

x 1.0 x 20000

= 10000

Amps

Surface Layer

In conformity with the normal design practice, it will be assumed that a 10 cm thick layer of stones with an average resistivity of 3000 ohm metre will be provided around all the metallic structures. Tolerable Values of Touch and Step Potentials F touch = [1000 + 1.5 C.. (h, K) p]I 0.116

rt

I

16

'-

- ,---~,--

250 - 3000

K

=

h,

= 20 cm

250 + 3000

= [1000

- 0.8462

= 0.2 metre

C, from figure 1 t, F touch F step

2750 =- 3250

= 0.77 = 0.5 second .

0.116

= [1000 + 1.5 x 0.77 x 3000]

= 732 Volts

r::-::v 0.5

0.116 + 6C "

(h, K) P2,

r::-::0.5

'/

0.116

=2438 Volts = [1000+ 6 x 0.77x 3000] v r::-::0.5 Arrangement of Earthing Mat Let L = Length of Earthing mat conductor in metres

69+36 x 69+ 8 x- 2 + 9 x 22 53.5+74 + 2 x 38.5 + 14 x + 15 x 74 2

.

Frnm Figure 3, L

A

= .

.

=8 x

= 4513

100 + 7 x 76

=7

metre

38+73

104 x 24 + 80 x 18.5 + 73 x15 + 20.5 x - 2

= 2496 +

1480 + 1095+ 1137.75 = 6208 Estimated Values of Mesh and Step Potentials

=

F mesh Ki

Where n K. I

Volts

L 0+ 1.15 L r

= 0.656 + 0.172 n = vn A . n B = ../30x 40 = 34.64 = 0.656

+ 0.172 x 35

=~21t [In ( ~16 hd' +

Km Let

p.km. kj 10

m2

~

35

= 6.676 (d+2h)2 8 Dd

-

h

4d

)

-

Kii

In

Kh

1t

(:n-1)

]

D h

= 2.5 m

Kh

=..J 1 + h

Kii d

= 1.0 (Assuming that the earthing mat will be provided with ground rods along the parimeter). ==Equivalent Diameter of earthing mat conductor, in metres For earthing mat conductor consisting of rectangular flat d = VI/2 = width of flat 0.050 =0.025 metres =~

Where, W

= 0.5

m

-ho

= ...;1+

0.5

- 1.0

= JC5

17

= 1.2247

km

-

21t

F

me'"

= =

4513 m Lr

=

250 x 0.1962 x 6.676 x 10000

Ki = Where n = KI =

.

=

K

(31.25 + 24.5 --5) + 0.81653 In 0.0369054]

0.15915 (3.9269-2.694) = 0.1962

= 136

0.656 + 0.172 x 40

[ - 1 +2 h I

1t

m

Ioucb

of 732 V

= 7.536

I

1

+

D+h

(1--0.5n..2)]

D

1

1 +

2.5+0.5

1 + 0.3334

+ 0.4

(1--0.5(40-.2»)]

2.5 :v; (1)]

0.55176 250 x 10000 x 0.55176 x 7.536

=

'lep

= 136

0.656 + 0.172 n 40

= 1t [ F

x 1 m

4513 + 136 x 1.15 701.3 vo~ which is less than E p. IG. K. . Ki Volts LC + 1.15 Lr

= -1t [-2xO.5 +

=

4xO.025

1t (2x35-1)]

1 '2;l1n

=

llep

1.2247

0.5

8.0

In

=

= E

(2.5+2xO.5)2

+ 16xO. 5xO.025 8x2.5xO.25

1

+-

Lc

(2.5)2

[ In

4513 +136 x1.15

2226.2 volts which is less than the pennissible value of E ItI:p of 2438 V Thus the values of mesh and step potentials likely to be experienced are less than those of the tolerable touch ang =

.

step potentials. Ground Resistance

.

RI R2- RI/

R&

= RI+Rl-2

= RI

=(p/1tI)

RI2

.

dI

=

conductor

h

=

0.5

hi

A

=

21 (In -=..:L + K, hi

dia =

.

l/F

-

k2)

r:;;;;;;o

~ ---n-

= 19.54 mm = 0.01954 m.

metre

-/d.h =0.09885

= 6208 m2 18

-

~-----

for k

1

=

0.1 -78.79

-

gh

105 but h = 0.5 m > 0.0012 ,0.167 1 for k = =

-78.79

5

6

h

= 0.0012

L : W ratio

= 102 : 76 = 1.34 :

1

=0.0021

but h = 0.5 > 0.0021. KI & K2 have to be taken from curves falling below 'c' since such curves are not available using curve c.

= 1.07,

KI

(1n 2 ~ 4513 X 4513 0.09885

250

=

Rl

= 4.5

K2

1t

4513

+ 1.07 x

-fiiii

= 0.017633 (In 91310 + 61.288 - 4.5)

= 0.017633 R2

=

(L2)

81

~

[In --1- -1 +2 K:y-A d2

2n n12

pa

Let

(11.422 + 61.288-4.5) :t 0.0176 x 68.21

= PI = 250 ohm

-4.5

= 1.20

(...r;- -1 )2]

metre

12 = 1 metre

=

d2 0.0254 m n =J36

.

250 2x136 1t xl

=

R

8xl

[1 n

= 0.2925[In 314.96= = Rl R2

-1 + 2 x 1.07 x -L

0.0254

.J 6208

1 + 3.0875]

0.2925 (5.7524 -- 1 + 3.0875) 2.2937

pa

=

1tl1 250

=

-211

[ 1n

12

[In

1tx4513

+ kl

2 x 4513. 1

(II) r:-" - k2 + 1] VA A 4513.

+ 1.07

78.79

(9.1079 + 61.288 - 3.5) = 1.1796 = 0.01763 X 66.896 2 ' Rl R2 - Rl2

= 0.01763 Rg

= RI+R2 = =

-

2RI2

1.2x2.3222 1.2+2.3222

- 1.17962 - 2 X 1.1796

2.70756 - 1.3914

3.5222

-

2.3592

=

= 1.1997

1.39524 1.163 = 1.2 ohms 19

.. 4,5

+ 1]

(J136-1)2]

Rise in ground potential

=

1.2 x 10000 12000 Volts This is very high, obviously on of the high soil resistivity. Addition of more conductor or rods is not helpful in this case. In such cases, chemical treatment of soil is called for.

Station 'A' 132kV bus 'A'

r

--

I

I I I I

15 HVA,132/~V trll'llfor

I

3x2:5HVA, 66/33kV

f

I

- -

-

132kY

c'"" ;. ..

1

. .

,\"

bu'

I I

I I

66IcVbus

:

transtOl"ll8rs

I

33kV bus

I

.

L

;

r

! J

Station '8'

Figure 9: Line Diagr;m

20

'..."."

,,....,".,".'-.-'.

"

--'::'~;;;;4~--'

1__,"","<~ -~

~

---.--..------

~-

,

1

!=

e ~

...

.I~

.,

t

""

E

, " ' ""

oJ N

e 0'" N

E

,.., '" ,..,

.~

)

-.

t-

()

Te N N

"I.'" ,

:"::':! :':~, I

, '...

01) '" <:

t n .9c'

I~ I~ ~

1.0 :;:-1 I :J ~ v I"" I> :: I ~ =

E .., ,..,

I,

,-

1I

f'V

I.c(

NE ~

I ;;

I I

::>I ~~~ "1:) § V

0

-6e ~.9-

~ c.: f'TJ t

I

r..:..:

,.

I > ~i-I:o~

.

-,:,

0"

I~I~~~

I I !

~~:; - -111° ~~~~

to V': It C!J VOl 0'1'- .c -=' -eJEa.nI ::J .,.

"'::> > &..c .~ .2:: OJ

..~

~ :; wI:V1V1

.~';;

I

j ~~ ~

.u 1.:>,.., '0z:

,

~,. i I;

'" f'/"'I

t, , e ~

~ ::s ,1>0 [:

0

0

c .9 I'ien .6 ::s en

-f"

I

I

"

I

.I

;1 E <:> ~

21

1""-

' i

Determination

of Potential rise in CIS enclosures under short circuit current.

r--

5.5..

--i

13.50

ToTR

t /!-,I L --.J I!:

-.k 0

, L.. oJ

GCB

~j P2

I

P,

E

~ I!:

-.k N

Structure

1

Structure

-1

-

-::";"

Insulating point of enclosure

Figure 11: 2ltSkV GIS Grounding

System

The short circuit current flows in GIS enclosure and Structures from a earth fault point of the enclosure to the grounding points, the potential rise at fault point can be calculated from the following formula: V = I Z I . I. (1) I,

IZ I Ze Re Xl"" Zs Rs xl~

= =

Ze + Zs

=

Re + j x

Short circuit current (A)

Where Ze : impedance of enclosure (D)

Z. : impedanceof structure(D) Lo

Resistance of enclosure (D) Inductive reactance of enclosure (D) Rs + j x u Resistance of structure(D) Inductive reactance of slruclurc(D)

Rc

k1 A

Here p

Resistivity (D-m) . 14.SxlO.8 D-m Mild sleel Aluminimum : 2.9x 10-8 D-m Cross sectional area (102)

A

A r1 r2

(2)

=

1t (r22-rI2)

Inner radius in m Outer radius in m

22

------------

I Rs

=

~

Where f L

length in m Depends on the type of H-beam and Characteristics have to be obtained for this purpose. (3) 2 1t f L.l Frequency in HZ Inductance in Him

2 IJ.S

Le

(

1

)

r2 -r2 2

(r/--3rI2

\:

r14

+

4

1

r2

.In 2'"'""2 rI r 2-r 1

)

x 10

Specific permeability 600 I Inductance of struclure in HIm. Depends on the type of II-beam and characteristic have to be obtained for this purpose.

J.lS Mild steel Aluminium: LS, :

Example: Refer Fig A,the 245 kV GIS potential rise for fault at PI and P2 can be calculated as follows: A. 1.

For earth-fault point "PI" GCB Enclosure

Mild steel r1

= 0.30

Structure Four (4) pil1ars I

m, r2

= 0.306

m, I =1.5m, f = 50 Hz

= 1.3 m

From formula (2) and (3),

= 1.9xl0"

(0) 3.69xlO-4(0) XLe (CB) =

Re (CB)

2.

Rs and Ls Rs = (390 J.LOImx 1.3 m)/4 pillars = 1.27x10-40 1.3/4=6.64x10" 0 Ls = 650nH/m, X Ls :;: 21t fx 650 x 1O-9x GIS(expect GCB) Enclosure Aluminium rl = 0.1675 m, r2 = 0.1742m, I = 3.5 m

From formula (2) and (3), Re (GIS)

= 1.4lx 10"(0)

XLe (GIS)--2.83x 10.6(0) Z=Ze (CB)+Zs(CB)+Ze(GIS)

= 1.6xl0

+ j4. 38xl0-4 0

I Z I = 4.66xlO-4 0 EG= V= I Z I .1= 18.6V B. For earth-fault point "P2" 1. GIS Enclosure

Aluminium

r1

= O.l675m,

Structure Two (2) pil1arsI

r2

= O.l742m, I = 5.5 + 0.4=5.9m

= 204m

From formula (2) and (3) Re (GIS) XLe(GIS)

= 2.38xlO" = 4.76xlO.6

(0) (0)

23

Rs

and Ls

Rs

= (3901lOlm x 2.4m)12 pillars = 4.68xlQ4.Q = 650 mnH/m, XLs = 2 f x 650x IO x2.4/2=2.45xIO-4.Q

Ls Z

= Zc(GIS) + Zs(GIS) = 4.92xIO-4+j2.5xIO-4.Q

IZ I

= 5.52xIOo4 .Q

Eo

=V =I Z

I .I

= 22.1V

24

n___-

,,'

,"--.'

.,-.--

-"

ANNEXURE

Estimation

--A

of Mesh and Step Potentials by Graphical Method

The calculated values of mesh and step potentials for the design square, ground mat without ground rods in uniform soils can be given a quick check with the help of graphs developed by the Georgia lnstitute of Technology and included in the EPRI Final Report EL 2682, VoL I. The graphs applicable to square grids without ground rods and with uniform conductor spacings in both directions are incorporated in this chapter. The applicable in the use of these graphs and method of the using the graphs are explained below to facilitate checking of the values of mesh and, step voltages. Corner Mesh Voltage The corner mesh voltages (Em) is calculated by multiplying the ground potential rise (GPR) by the comer Mesh Voltage percentage obtained from Figure 13. Thus, the Comer Mesh Voltage:

Em

= GPR

percentage value of Comer Mesh Voltage as per graph

x

100 Figure 13 gives the Corner Mesh Voltage percentage of GPR for a grid depth of 0.5 metre and conductor diameter of 0.01 metre. The grid depth and conductor diameter have been found to have negligible effect on Em for grid depths from 0.25 metre to 0.5 metre and for diameter from 2.5 mm to 10.0 mm. Corner Step Voltage The Comer Step Voltage (Es) is determined by multiplying the ground potential rise (GPR) by Corner Step Voltage percentage obtained from Figures 14 to 16 which gives the percentage values for three grid depths viz. 0.25 m, 0.5 m and 1.0 m. The conductor diameter has been found to have negligible influence on step voltage for conductor diameters from 2.5 mm to 10.0 mm. percentage value of Comer Step Voll2ge as per graph ES

= GPR

x

100

Grid Resistance The value of grid resistance (Rg) is given by Figure 12 as follows: Rg

=

Soil resistivity (Ohm metre) value as per graph

Ohm 1000 The graph for grid resistance (Figure 12) is also for grid depth of 0.5 m and conductor diameter of 10 mm. It has been found that grid depth between 0.25 metre and 0.5 metre and conductor diameter between 2.5 mm and 10 mm have negligible effects on value of grid resistance. Ground Potential Rise The Ground Potential Rise (GPR) is given by the current 10 injected into the grid and the grid resistance Rg. GPR 10' Rg volts. Application of the Graphical Method For applying the Graphical method, the length of the side of square grid in metres, number of meshes on the side of square, the value; of soil resistivity in ohm metres, and the magnitude fault current injected into grid in Amperes should be known.

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ANNEXURE

-- B

Example showing Division of Fault current Between the Overhead Earthwire and Earthing Grid Let two 132 kV D/C lines feed subst:J.tionB from subst:J.tionA. Let length of the line AB be 77 kIn with an average span between tower of 250 m and tower footing resist:J.nceof 10 ohms. At substation B, there are.four 15 MYA, 132/66 kV transformers to step down the received power which is further stepped down to 11 kV by 6 X 5 MVA, 66/ 11 kV transformers. TI1emaximum S-L-G fault current of 10 kA is at 132 kV bus. The ground resistance of station B is assumed as 0.9 ohm. For a typical 132 kV D/C tower with one ground wire. 1.

Mutual impedance between phase cond.uctors and the ground wire (Zgm)

= 0.294 fjoo ohms/km. 2. Self Impedance of ground wire with ground return (Zg) = 2.30/].0.830 ohms/km. Calculation of Division of Current The fault current supplied by the four circuits of 132 kV line (i.e., 10 KA) will not completely flow into the ground as part of the current will be diverted by ground wires due to induction and conduction. Diversion of current due to induction Fault current flowing in a line conductor (Ir) will induce current in the overhead ground wires (I) of the same line. Ii = /m/If Zgm 0.294 L80 .where,m

=-

=

Zg

2.3

L 20.83

= 0.128

/m/

. That is 12.8% of fault current flowing in the line will be induced in its earth wire. The fault current If supplied by both double circuit line together is 10 KA. .

Therefore current induced in both earth wire (I)

= 0.128

x 10

= 1.28

KA

Diversion of Current due to conduction Overhead ground wires and tower footing resist:J.nceform a ladder network. As the number of towers is more than 20, the length of line can be considered as infinite for purpose of determining the ittance (y) of the ladder network which is given by

y= Z span +..j Z span x R, 2 Where, Z span = Self impedance of one span of ground wire with ground return in ohms/km R, = Average tower footing resisulnce for first 20 towers in ohms. S pall bet ween

= 250

towers

III

.z span = 250x2.3 1000 y

=

= 0.575 ohms

I 0.575

-

+VO.575xlO

=

2.685

= 0.372

mho.

2 28

, ,,-~~.-'--'--""~W_"-'.

"-

"", "',~"-"'~'

- ._-~---~---'

~~

of ladder network = 1/Y = 2.685 ohms = Resultant impedance of ground wires due to two double Ckt lines (Le., Z(2) = 1.34 ohms The current discharged to the ground from the station will be given by

= Impedance

Z Zl

IG =

Where

IG1 I

IGI

I

Zl

.x

= Total = Ir -I

Rg + Zl

fault current minus the current diverted by the ground wires due to induction

t

Zl

Rg IG

= 10-1.28 = 8.72 KA = Resultant impcndancc of ground wires due to two double ckt lines = 1.34 o~ms Groundl resistance = 0.9 ohm

=

= 8.72

1.34

x

= 8.72

1.34 + 0.9 x

1.34 2.24

=8.72 x 0.598

= 5.51 KA . Thus out of a total of 10 KA supplied by the two lines only 5.21 KA flows into the ground. i.e., only 52.1% of the total fault current flows to the ground. References (1) "Guide for Safety in Alternating Current Sub-Station Grounding"

ANSI-IEEE Standard 80-1986.

(2)

Indian Standards: - Dimensions"

(3)

"Review on Corrosion in Earthing Equipment" Review No.1, Central Board of Irrigation and Power, 1973.

(4)

"Steel Grounding Systems - Where Grounding Mat is not Needed' Irrigation and Power, July 1976, Reprinted March, 1985.

(5)

Indian Standards, IS 3043-1987 "Code of Practice for Earthing First Revision. Current for Design of Grounding Systems - B.Thapar& Sunil K. Madan IEEE Trans. on PAS, Vol -103 No.9

(6)

IS : 1730-1989 Steel Plates, Sheets and Flats for Structural and General Engineering Purpqses

September, 1984.

29

.....

- Technical Report

No.5, Central Board of