Earthing of Ehv Stns

7/26/2019 Earthing of Ehv Stns

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Earthing for EHV Sub-stations

Electricity being the driving force of our very civilisation in thelast hundred years, the extent of its usge is considered as an index of

development. Like all good things it brings ith it a degree of danger to

life and property due to defects in the electrical apparatus or their usage.

!aking into account the number of lives or property lost"damaged, due to

electrical shocks and fire accidents caused by electricity, it is necessary

and inevitable to think of measures of safety. #roviding suitable

protective e$uipment can ensure safety from electrical shocks and fires,

apart from proper design of the electrical apparatus.

%ne of the important aspects in the operation of the protective

e$uipment is proper earthing. &y earthing, it means making a connectionto the general mass of the earth. Earthing also increases the reliability of

the supply service as it helps to provide stability of voltage conditions,

prevent excessive voltage peaks during disturbances and also as a means

of providing a measure of protection against lightning.

Earthing means making an electrical connection to the general

mass of earth. 'ts use is ide spread in the supply netork right from the

generation to the apparatus on the consumers premises. !he


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re$uirements of earthing varies at different points based on the fault

level, soil resistivity and the safety considerations

Objectives of earthing :-

() !o ensure that no part of e$uipments, other than live parts, should

assume a potential hich is dangerously different from that of

surroundings.

*) !o allo sufficient current to flo safely for proper operation of

protective devices.

+) !o suppress dangerous potential gradients on the earth surface hich

may cause incorrect operation of control and protective devices and

also may cause shock or inury to personnel.

) 't plays a very important part in increasing the reliability of the supply

service and it helps to provide stability of voltage conditions,

preventing excessive voltage peaks during disturbances and also in

providing protection against lightning surges.

Types of Earthing :-

Earthing can be divided into neutral earthing and e$uipmentearthing. eutral earthing deals ith the earthing of system neutral to

ensure system security and protection ,here as e$uipment earthing


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deals ith earthing of non- current carrying parts of e$uipment to ensure

safety to personnel and protection against lightning.

/epending on the type of installation i.e, generating station, H.V.

substation, transformer centre, pole"toer and consumer installations,

suitable earthing system has to be designed duly taking into

consideration, the various re$uirements such as fault current, limiting of

earth potential rise, safety of nearby communication circuits and safe

body currents etc.

0or a H.V. station earthing the to important factors to be

considered are Earth potential rise and safe touch and step potentials.

't is to be noted that limiting the step and touch potentials to safe

value is more important than attaining a lo value of the resistance.

Hoever the earth resistance of the sub-station has to be brought don

to the loest possible level. !he safe value of earth resistance for any

sub-station depends upon not only the level of fault current and the

resistance but also on the vicinity to communication stations.


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Factors to be considered for design of earthmat for a H.V. Sub-

station are:

(. Soil resistivity.

*. !olerable limits of body current.

+. 0ault current.

Earthing system of a H.V. Sub-station plays a maor role in the

maintenance of the e$uipments of the sub-station. 1 good design of the

earthing system not only helps in the proper operation of the protective

e$uipment and also provides safety to e$uipments and personnel. !o

achieve the above obectives, the earthing system is designed to

discharge the fault current safely into the earth and also to limit the

touch and step potentials ithin the area of the sub-station.

efinitions:-

a) Earth Electrode is a rod, pipe, plate or an array of conductors,

embedded in earth hori2ontally or vertically. 'n distribution system

the earth electrode may consist of a rod, of about * m long, driven

vertically into ground. 0or sub-stations an elaborate earthing system

knon as earthmat is used.b) Earth current 3 !he current dissipated by earth electrode into the

ground.


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c) 4esistance of earth electrode3 - is the resistance offered by the earth

electrode to the flo of current into the ground. !his resistance is

not the ohmic resistance of the electrode but represents the resistance

of the mass of earth surrounding the earth electrode. umerically it

is e$ual to the ratio of the potential of earth electrode ith respect to

a remote point, to the current dissipated by it.

d) Step potential3 !he potential difference shunted by a human body

beteen to accessible points on the ground separated by a distance

of one pace assumed to be e$ual to one meter. 5 see fig.( in

1nnexure()

e) !ouch potential3- !he potential difference beteen a point on the

ground and a point on an obect likely to carry fault current 5e.g.,

frame of e$uipment) hich can be touched by a person. 5see fig * in

1nnexure ()

f) 6esh potential3 !he maximum touch potential ithin a mesh of the

grid.

g) !ransferred potential3 1 special case of touch potential here apotential is transferred into or out of the sub-station.


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/esign of earthmat

&efore designing earthmat, it is necessary to determine the soil

resistivity of the area in hich H.V. sub-station is to be located. !he

resistivity of the earth varies considerably from (7 to (7,777 mtr.

depending upon the types of soil.

0urther, the resistivity may also vary at different depth depending

upon the type of soil, moisture content and temperature etc., at various

depths hich affects the flo of current due to the fact that the earth

fault current is likely to take its path through various layers.

!ypical values of resistivity for various types of soils are as follos 3-

!able o (

Sl.

No

.

Nature of soil Range of Resistivity

( 4ed loamy soil 7-*77 -m

* 4ed sandy soil *77-*777 -m+ Laterite soil +77-*877 -m Shallo black soil *7-(77 -m9 6edium black soil 97-+77 -m8 /eep black soil 97-*97 -m


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: 6ixed red ; black

soil97-*97 -m

< =oastal alluvium +77-(+77 -m> Laterite gravelly *77-(777 -m

!ethod of measuring the soi" #esistivity

!he most common method employed in the measurement of soil

4esistivity is ?enner@s four Electrode method. 'n this method, four

probes are driven into earth along a straight line at e$ual intervals, say

AS@ mtr. apart. !he current terminals =( and =* of an earth tester are

connected to outer electrodes and the potential terminals #( and #* are

connected to inner electrodes as shon in the figure in 1nnexure +

!he readings of the earth tester A4@ are recorded hile turning the

crank at about (+9 rpm. !he resistivity is calculated from the folloing

formula.

S 4 B

*S C *S

( D S*De* S*De*

?here e B depth of burial of the probe in mtr. and is S"*7


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'f Ae@ is FF than S the above formula reduces to B* S4 -mtr..

!o account for the seasonal variations , the average Soil resistivity

is multiplied by the factor as shon belo, hich is termed as the

apparent resistivity.

Sl. No.Season of

measurement

Multiplication

factor

( Summer (

*. ?inter (.(9+. 4ainy (.+

4esistivity for design purpose is arrived by comparing the apparent

soil resistivity ith the range of the resistivity of the soils as given in

!able o. (

'f the apparent soil resistivity is more than the minimum of the

range of the resistivity of the soil, the apparent soil resistivity is taken

for design if not the minimum of the range of the resistivity of the soil is

taken for design purpose.

Tolerable limits of body current.

!he effect of electric current passing through vital organs of the

body depends on magnitude, duration and fre$uency of current. !he


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most dangerous conse$uence is a heart condition knon as ventricular

fibrillation, hich results in stoppage of blood circulation.

a$ Effect of magnitude of current :-!he threshold of perception is a

current of ( m.1. =urrents in the range of (-8 m.1 are knon

as A let go current A because these currents, though unpleasant, do not

impair the ability of a person, holding an energised obect to release

it. =urrents in the >-*9 m1 range may be painful and impair the

ability to release energised obect. Still higher currents make

breathing difficult. Hoever, if the current is less than about 87 m1,

the effects are not permanent and disappear hen current is

interrupted. =urrents higher than 87 m1 may lead to ventricular

fibrillation, inury and death.

b$ Effect of duration of current:!he magnitude of 97 HG tolerable

current is related to duration. 1ccording to tests reported by /al2iel,

>>.9 of persons of 97 Ig eight can ithstand the current given by

e$uation.

'&B 7.((8 " t

?here '&is the rms value of body current in amperes and At@ is

the time in seconds. 'f the eight of body is :7 Ig., the e$uation for

tolerable current is

'&B 7.(9: "t

!hese e$uations are valid for 7.7+ F t F + seconds.


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c$ Effect of fre%uency :-!he tolerable currents mentioned above are

for 97 - 87 H2. 't has been found that human body can tolerate about

9 times higher direct current. 1t high fre$uencies 5+777 J (7777 H2)

still higher currents can be tolerated.

Fault current to be handled: -

1s the earthing system has to carry the earth currents, the

maximum earth fault current likely to flo in the system hich is

generally S.L.K fault is considered for designing the earthing .1 good

earthing system for H.V. station can be designed using an earthmat

hich is formed by a grid of hori2ontally burried conductors hich

serves to dissipate the earth fault currents to earth, also as an

e$uipotential bonding conductor system, along ith the re$uired number

of vertical earth electrodes hich are connected to the points of earthing

of various e$uipments and structures and also interconnected ith the

hori2ontal earthmat.

Choice of materials and sie of earthmat conductor :


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'n olden days, copper as used as the conductor for earthmat,

though it is an ideally suited material, from the economic point of vie

and due to the fact that it is highly prone to thefts, no mild steel

conductors are used.

=ross-section of the 6.S. conductor in S$ mm is given by the

formula.

B ' f (*.9 tc S$ mm. for elded oints

B ' f (9.< tc S$. mm. for bolted oints.

?here 'fB 0ault current in I.1mps.

tcB fault clearing time in seconds.

Suitable correction shall be made to this cross sectional area by

providing an alloance for corrosion as belo3

a) &f (77 mtr 3 no corrosion alloance be made.

b) &f '() *+,, mtr 3 an alloance of (9 is to be made.

c) &f *() mtr 3 an alloance of +7 is to be made.

To"erab"e va"ues of Touch Step otentia"

EtouchBM(777 D (.9 =ssN/7.((80 ts ) Volts.


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?here tsB 0ault duration in secs.

s1 Surface layer resistivity in mtr.

1 +777 mtr. for crushed stone layer.

=sB (-a M5(- "s) " 5* hsD a)N

2s1 ( hen no protective surface layer or crushed stone is used.

?here a B 7.(78 mt

hsB Height of surface layer i.e., thickness of the crushed stonelayer hich is normally 7.( mt.

?here s 1 !he resistivity of surface layer B +777 mtr if

crushed stone is used, otherise s1 1 Soil resistivity of the area

etermination of spacing bet3een para""e" conductors of the

earthmat:-

!he minimum length of the total earthmat conductor re$uired to

keep the mesh voltages ithin safe limits is obtained by e$uating actual

Emeshto tolerable Etouch. !he result is

L m B I m I i ' gt c

Estep B M(777 D 8 =ssN7.((8" ts


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5(777 D(.9 s) 57.((8)

&ased on the area available for earthmat, the mesh voltage is

calculated assuming the spacing beteen conductors as

(7,>,


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?here n B 5na n b)

?here na B number of parallel conductors of rectangular grid length

ise.

nb B number of parallel conductors of rectangular grid

breadthise.

Ii B corrected factor for grid geometry.

B 7.898 D 7.(:* n

IhB (Dh"ho?here hoB ( mt, reference depth of grid.

'g B 6ax. Krid current for design purpose.

B ' f /fSK0 =0K

here 'f B 6aximum fault current .in I 1mps.

/fB /ecrement factor

B ( for fault duration of more than 7.9 seconds.

=0K B =urrent floing in the grid B7.9

SK0 B Station groth factor B (.9 5=onstant)

Hence 'gB 'f 7.:9

L B !otal length of the burried grid conductors and the total

length of vertical electrodes in mts.

=ompare Emeshith Etouch tolerable.

'f EmeshFEtouch, then calculate

Estepusing the formula EstepB IsIi'g

L


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?here ,

I sB Spacing factor for step voltage

B ( ( D ( D (5(-7.9)n max-*

*h / D h /

?here n maxB 6aximum of n aand n b

'f Estepis found to be less than the tolerable E stepalready calculated,

the assumed spacing beteen earthmat conductor be adopted. Hoever

check hether the total length of the grid conductor as per design is

more than the minimum length i.e., Lm. %therise decrease the spacing

and recalculate untill the above condition is satisfied.

'nvariably, hen EmeshF Etouch5tolerable),Estepis also found to be ithin

the tolerable value. 'f Emesh Etouch,repeat the procedure using the next

loer spacing till Emeshand Estepare found to be ithin tolerable limits.

2a"cu"ation of grid resistance #g

4g B "55"1)D "L

4round potentia" rise:

K#4 B 'g 4g Volts.

!he e$uipment earth point or the neutrals of transformers are

connected to vertical earth electrodes hich in turn are connected to the


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earth mat. !hese earth electrodes must be capable of handling the

current passing through them under fault conditions and discharge into

the earth safely ithout giving rise to heating. !hese electrodes also

contribute to reduction in the earth resistance because of the fact that

they connect layers of the earth up to the depth to hich they are driven.

!hese electrodes can be of different materials, si2e and shapes. 0or H.V.

Sub-stations, it is found that =' pipes of *.:9 mts long (+ mm thick (77

mm dia are very much suited. !he e$uipment earth points are connected

to the electrode using 6S flat of the si2e of the earth mat conductor by

elding. !his ensures pucca contact to enable to discharge the fault

current to the earth through the electrode as ell as the earth mat.

Earthing in difficu"t situations:-

!he earthing resistance can be improve by any one or more of the

folloing methods.

(. 'ncrease the area of the earth mat.

*. #rovide deep earth electrodes.

+. #rovide auxiliary earth mat in a near by place here the resistivity is lo

and connect it to the main earth mat.

. !reating the earthmat and the electrode ith suitable chemicals.


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/epending upon the situation any one or more of the above

methods can be used to reduce the earth resistance.

0rom experiments it is found that bentonite clay hich is anaturally occurring material is the best suited for the purpose. &entonite

has the important properties vi2., high conductivity, high sell index and

non-corrosive and retains moisture by absorbing ater from neighboring

soil. Onlike salt bed, this electrolyte ill not gradually leach out and

remains as a part of the clay itself and ill adhere to nearly any surface

it touches. Even hen exposed to sunlight, it tends to seal itself off

preventing the drying process from penetrating deeper.

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Earthing of Ehv Stns