Ear Thing Concepts

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EARTHING CONCEPTS


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Earthing in a EHV Substation

Providing adequate Earthing in asubstation is an important safety measure.

Earthing means connecting the electrical

equipment to the general mass of earth oflow resistance.

Objective is to provide under and around

the substation a surface of uniformpotential

-- At near zero or absolute earth potential


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1. Objective:

The touch and step potential shall be within

limits under all conditions including fault

condition

Grounding resistance shall be lower.

Effective earthing system shall aim at

providing protection to life and propertyagainst dangerous potentials under fault

conditions


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I.E.Rules 1956

Rule 92 Every substation /generating station exposed to

lightning shall adopt efficient means for divertingthe electrical surges due to lightning to earth

Earth lead of any lightning arrestor shall not passthrough any iron or steel pipe.

It shall be taken directly, as far as possible, to aseparate earth electrode and/or junction of the

earth mat. Bends Shall be avoided where ever practicable

Earth screen if provided for lightning protectionshall be connected to main earth grid.


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I.E.Rules 1956 Functioning of earthing in a substation

It shall be capable of passing maximum earth fault current

The passage of fault current does not result in any thermalor mechanical damage to the insulation of connected plant /equipment

Every exposed conductor part and extraneous conductivepart may be connected to the earth.

There is no danger to the personnel

Ensure equi-potential bonding within the power system

No dangerous potential gradients (step or touch or transferpotentials) shall occur under normal or abnormal operatingconditions

To minimize electromagnetic interference between powerand control/ communication system


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Earthing System

Points to be earthed in a substation The neutral point of each separate system

should have an independent earth, in turninterconnected with the station grounding mat.

Equipment frame work and other non-currentparts (two connections)

All extraneous metallic frame works notassociated with equipment ( two connections)

Lightning arrestors should have independentearths, in turn connected to the stationgrounding grid.


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Earthing SystemPoints to be earthed-contd

Over head lightning screen shall also be

connected to main ground mat.

Operating handles of Isolators with a auxiliary

earth mat underneath, if necessary. Peripheral fencing

Buildings inside the switch yard.

Transformer Neutrals shall be connecteddirectly to the earth electrode by twoindependent MS strips


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Earthing and grounding -distinction

Grounding:- connection of current carrying parts

to ground. Ex :Generator or transformer neutral.

This is for equipment safety.

In a resistance grounded system it limits thecore damage in stator of rotating machines.

In solidly grounded system substantial ground

fault current flows enabling fault detection andfaster clearance.


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Earthing and grounding -distinction

Earthing:- connection of non currentcarrying parts to ground. Ex : Metallicenclosure.

This is for human safety. Earthing system plays no role under

balanced power system conditions.

Under ground fault conditions, enablesground fault current to return back tosource without endangering human safety.


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Basics of Earthing

Resistivity of earth

Resistivity of earth:-

Mother earth is a bad conductor.

Resistivity is normally around 100 ohm mt.

GI of 65x10mm section will have same resistance

as copper of 25x4mm section.

Corresponding figure for earth is 800x800mt

(158acres) Metallic conductor is a preferred alternative to

earth to bring the fault current back to source.


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Electric field Earth resistance

Current flows through a series of hemi-sphericalshells of earth of continuously increasing crosssections.

Almost 95% of final resistance is contributed by soilwithin 5mts of the electrode.

If current is discharged from a grid towards anothergrid at B100 km away, only soil with in 5 to10 mts of

the electrode contributes maximum resistance.

Earth beyond, offers very minimum resistance.

This is the concept of treating the soil around

electrode of an earth pit.


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Electric field Earth resistance

Earth with its huge mass offers equi-potential everywhere

A very large charge is required to changeearth potential everywhere

Disturbance due to current injection at apoint is felt, only locally.


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Substation earthing

Design of Earth mat

Design depends upon the following parameters

Durational and magnitude of the fault current Resistivity of the surface layer of the soil

Resistivity of the soil Magnitude of current that the human body cansafely carry

Permissible earth potential raise that may takeplace due to the fault conditions

Shock duration Material of Earth- mat conductor. Earth- mat geometry


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Substation earthing

Design of Earth mat

Parameters for the calculation of Maximumpermissible step and touch potential Fault duration :- Fault clearing time of back up

protection is adopted Modern protection systems provides for fast

acting back up protection Considerable saving can be made by optimizing

the size of the conductor of earthing grid by

considering lesser fault duration. These will change the earth potential raise due to

which Step and Touch potentials arise.


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Earth mat parameters

Let go current

Maximum safe current a person cantolerate and still release grip of anenergised object, using muscles affected

by the current The magnitude of let go current adopted in

calculating maximum permissible step andtouch potentials (As per IEEE 80 1976)

for man 9 milli ampsfor woman 6milli amps


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Substation Earthing

Non-fibrillation current

Developed by Dalziel and approved by AIEE80-1963Magnitude of power frequency alternating current

(mA) that a human body of average weight( 50kgsto 70 kgs) can with stand without ventricular

fibrillation,

I =0.116 for a body of 50kgs wt.

t

I =0.157 for a body of 70kgs wt.

t

Av. Value of human body resistance (dry) 8 to 9K-ohms

Adopted value for designing Earthing system1Kohms


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Substation Earthing

Non fibrillation current contd Non fibrillating current adopted for earth grid design

in India.Magnitude of power frequency alternating currentthat a human body of average weight( 50kgs to 70kgs) can with stand without ventricular fibrillation,

I =0.165t

I = rms current through human body in amps t =durtation of shock in seconds

Assumption /considerations in deriving the aboveequation

--The duration of shock is from 8 milli-seconds to 3seconds


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Substation Earthing

Fault duration and magnitude

During a line to earth or double line earth fault

current through earthing system causes

a) Heating of earthing conductor

b) Potential gradients in the soil

For earthing design single line to ground fault

is considered as

Most of the faults are of this type Current through earth in case of single line

to earth fault is higher that in the later case.


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Substation Earthing

Fault duration and magnitude-contd.

For determining maximum permissible step and

touch potentials

Fault duration corresponding to maximum fault

clearing time of back up protection relays areconsidered

Normally in modern sub station clearance time

of primary protection is 0.2 sec, ie., 200 milli sec

and clearance time for back up protection is 0.5

sec, ie., 500 milli sec

A fault duration time of 0.5 sec (500 mill sec) is

adopted for design


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Earthing conductor once placed under

earth may not be inspected normally.

Prudent to make it capable of carrying

maximum possible current for maximum

time.

If felt necessary and if it is economical,

fault duration of 1 sec can be adopted for

design.

Substation Earthing

Fault duration and magnitude-contd.


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Substation Earthing

Soil resistivity

To design most economically and technicallysound earthing system accurate data of soilresistivity and its variation with in substation soilis essential.

Resistivity of soil in many substations has beenfound varying -at times between 1 and 10,000ohm meters.

Variation in soil Resistivity with depth is morepredominant as compared to variation inhorizontal distances.


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Substation Earthing

Soil resistivity Large variations in stratification of earth layers will

result in large variations in earth resistivity.

Highly refined techniques for the determination of

resistivity of homogeneous soil( non

uniform soil)is available.

As resistivity of soil varies widely based on moisturecontent earth resistivity readings to be obtained in

summer or dry season.

Weiner's 4 electrode method is generally adopted fortesting.


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Substation Earthing- Soil resistivity

Weiner's 4 electrode method

Earth resistivity tests shall be carried out at leastin 8 directions

If results obtained indicate wide variation, test

shall be conducted in more number directions. Four electrodes are driven into earth along a

straight line at equal intervals.

Current is passed through two outer electrodes

and earth. Voltage difference is measured between two

inner electrodes.


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Substation Earthing

Soil resistivity

Current flowing through the earth

produces are electric field proportional to

current density and resistivity of soil.

Voltage measured is proportional to the

ratio of voltage to the current i.e R

= 4sR - __s__

1+ 2s___ s+e

s+4e


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Substation Earthing

Soil resistivity Where

= Resistivity of soil in ohm-meters= Distance between two successive electrodesin meter

R= Ratio of voltage to current or electroderesistances in ohm

e= depth of burial of electrodes in meters

In case depth of burial of the electrodes in theground (e) is negligible compared to electrodesspacing. This formula is the adjusted =2sR(This formula is normally adopted in AP Transco Ltd.)


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Substation Earthing

Measurement of Soil resistivityThere point method Two temporary electrodes spikes are driven in to the

earth at 150ft and 75ft respectively from earthelectrode under test.

Former is for current and the later is for voltage. Ohmic values of earth electrode resistances are

obtained using earth meagerR = log 10 (4L/P) where

2

R = Electrode resistance in ohmL = Length in cms of the rod driven under groundD = Dia in cms of the rod

= Earth resistivity in ohm-meter


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Resistance of the earthing system

R = +

4r L

= Soil resistivity in ohm meter

L = Length of conductor buried in meters

r = radius in meters of circle having the same

area as that occupied by the earth mat.

The value of the R should be less than theimpendence to ground values stated below


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Earthing System

Permissible resistance of earthing system

Primary requirements : Impendence to ground

(resistance of earthing system)

Small substations 2 Ohms

EHV substations up to 220 kV 1 Ohm

Power stations and 400 kV substations 0.5 Ohms

Distribution transformer - 5 Ohms.

In order to avoid abnormal shift of the neutral

potential, earth resistance of the station earthing

system shall be normally less than or equal to 1ohm.


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Substation Earthing

Step and touch potential

Step potential - Difference in surfacepotentials experienced by a man bridging adistance of 1 mt with his feet, with outcontracting any other grounded object.

Touch potential- potential difference betweenthe earth potential raise and the surfacepotential at the point where a person isstanding touching an earthed structure.

Tolerable touch potential of human body isless than tolerable step potential.


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Substation Earthing

Step and touch potential-contd

In any switch yard, chances of exposure toTouch potential is higher than that to steppotential.

Resistance offered by the feet of a personagainst Touch potential is much lesscompared to that against Step potential.

Hence Touch potential is more critical fordesign while Step potential is usuallyacademic.


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Substation Earthing

Step and touch potential- contd.

Step potential is independent of the diameter ( cross-section) of the earthing conductor.

For 400% increase in diameter, reduction in Touchpotential is only 35%.

Thus cross- section has minor influence on Touchand Step potentials.

Length of earthing conductor has significant effecton Touch and Step potentials.


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Substation Earthing

Step and touch potential Tolerable Step and touch potentials (CBIP Publication no.

223)

E step (LMT) = 0.116 (1000+1.5Cs(hs.K.)s) (volts)t

E touch (LMT) = 0.116 (1000+ 6Cs.(hs.K.)s) (volts)

tWhere Cs= Reduction factor for de-rating normalvalue of surface layer resisvity, a function of K.

K= -- s+ s

, sare resistivities of soil and surface layer respectively.cs =1 when crushed rock has resistivity equal to that of soil.

Otherwise it is derived from reference graphs ( Cs. vs hs.)

hs =thickness of surface layer in meter.t = Duration of shock current flow in secs.


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Substation Earthing

Step and touch potential-contd.

Tolerable Step and touch potentials as adopted by certain

utilities.

E step (LMT) = IB(RG +1.5Cs.s) (volts)------(1)

E touch (LMT) = IB (RG + 6Cs.s) (volts) ------(2)RG= body resistance in Ohms= 1000

IB= Permissible body current of human beings.

Cs=Reduction factor(0 to 1)=1-(k / (2h+0.09) ------(3)

k=0.09x(1- /s)

s= surface layer resistivity ( taken as 2000 ohm- mt.) h= Thickness of gravel in cm.

= Soil resistivity ( taken as 100 ohm- mt.)


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Substation Earthing

Step and touch potential-contd. Sample calculation forE step (LMT) and E touch (LMT) Data

Weight of the man =70kgs

Fault duration =0.5 sec

Resistivity Soil = =100 ohm-mt, Surface layer =s=2000ohm-mt,h= Thickness of gravel in cm.=10cm

From (3), Cs=0.705

From table in slide 24 for a 70 kgs man and for a shock duration of

0.5 sec IB= 222mAFrom (1) E step (LMT)= 691V

From (2) E touch (LMT) =2100V

E thi S t


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Methodology of design as adopted in APTransco

Size of earth mat conductor (steel strip ) Shall be :

A (Steel) = 0.0013 x I t sq. mm for bolted joints

= 0.011 x I t sq. mm for welded joints

Where A = Area of Cross sectionI = Fault current in Amps. at the station

= Fault MVA x 1000

3 x system kV

and t = Time in seconds during which current is

applied

Earthing System

Size of earth mat conductor


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Earthing materials

Determination of size of conductor for earth mat.- Based on thermal stability determined by an approximate

formula of IEEE - 80-1986

A = I/ ( TCAP x10 4) I n (Ko + Tm)

tc x irr (Ko + Ta)Where

In case of steel

A = I x 12.3 tc mm for welded joints= I x 15.13 tc mm for bolted joints

In case tc = Duration of current =1secA = 12.3 x I mm for welded joints

= 15.3 x I mm for bolted joints


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Earthing materials

Based on Mechanical ruggedness of conductor and for easy installation.

Ratio of max width to thickness =7.5

Thickness for flat shall not be less than = 3mm (Asadopted 5to 6mm)

Minimum dia for steel rod = 5mm

Standard sizes of conductor as, As per IS 1730 1989(I)10 x 6mm (II)20x6mm

(II)30 x 6mm (IV)40 x 6mm

(IV)50 x 6mm (VI)60 x 6mm

(VI)50 x 8mm (VIII)65 x 8mm(IX)75 x 12mm (X)100 x 16mm

- For 33kV Substations 75x8mm and 50x6mm


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Earthing materialsUp to 220 kV substation

Earth mat

a) Peripheral or main earth mat : 100x 16m MS flat

b) Internal earth mat : 50x8m MS flat placed at 5 m apart

c) Branch connections : cross section not less than64.5 sq.m

d) Raisers : 50x8m MS flat

For 400 kV substation

Earth mat

a) Peripheral or main earth mat :40mm dia MS rod of 3mt. length

b) Internal earth mat 50x8mm MS flat placed at 5m apartc) Raisers : 50x8m MS flat

Where necessary, 40mm rods will be driven in to earth verticallyalong the periphery of the earth mat.


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Pipe earthing

a) EHT Substations :(i) Cast iron pipes 125mm in diameter2.75 m long and not less than 9.5 mm

thick.

(ii) Pipes 50.8 mm in dia and

3.05 m long

1. Joints are to be kept down to the minimumnumber

2. All joints and connections in earth grid are to bebrazed, riveted, sweated, bolted or welded.

3. For rust protection welds shall be treated withBarium chromate.


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Earthing

2. Welded surfaces to be painted with red lead

and aluminium paint and then with bitumen.

3. Joints to be broken periodically shall be bolted

and joint faces tinned.4. All exposed steel earthing conductors should

be protected with bituminous paint

5. All joints in steel earthing system shall be

welded except joints to be removed for testing

shall be bolted.


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Earthing system

Lowering of earth impedance

2) Lowering of earth impedanceIn places where soil resistivity is high steps to be taken to reduceearth impedance by one or combination of following:-

a. Connection of substation grid with a remote ground grid andadjacent grounding facilities.

b. Use of deep driven ground rods or longer ground rods or maximumnumber of ground rods along the perimeter of the earth grid.c. Use of foundation rods as auxiliary grids where feasibled. Formation of auxiliary grids if soil of low earth resistivity is available

close bye. Max. touch potential occurs in the corner of mesh of the grid. No

equipment are to be kept in such areas. higher values of touchpotential than the tolerable limit can be accepted if step potentialare within permissible limits

f. If equipment is to be kept at corners of the mesh. Auxiliary grids areto be created at those corner to limit touch potential.


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Earthing System

Earthing of switch yard fencing

Two methods of fence earthinga) Extension of substation earth grid up to 0.5 to 1.5 m beyond the

fence, bonding the fence to the grid at regular intervals.b) Keeping the fence beyond the perimeter of the switch yard

earthing grid, providing its own earthing system not connectingto the main earthing grid.

In the former case substantial reduction in the effectivesubstation earthing resistance is possible but at additionalcost.

In the later case any inadvertent connection could give rise todangerous potential under fault condition unless special care istaken.

Electrical isolation of fence into short section with individualearthing is required where fence is closer to a single phasereactor or an electrical plant generating large electromagneticfields.


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Earthing System

Earthing of switch yard fencing- con

Methods of earthing of fencing As per CBIP reportA. Design permits extension of earth mat within 1.5mt inside

perimeter fencing

Electrical isolation of fencing can be ensured Isolate fencing for earth mat Running of independent earth conductor underneath boundary

and connecting it to fencing at frequent intervals.

B. Design permits extension of earth mat up to fencing Calculated touch potential within safe limit

Extending the earth mat up to perimeter fencing and connectingthe fencing at frequent intervals to earth mat

Spreading crushed metal 1.5mt beyond fencing


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Earthing System

Earthing of switch yard fencing- con

C.

Design permits extension of earth mat up tofencing

Calculated touch potential beyond the fenceabove the permissible limit for touch potential

Termination of earth mat within 1.5 mt offencing

Fence electrically isolated and independentlyearthed by running an earthed conductorunderneath the fence connecting it to the fenceat frequent intervals

f


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Earthing of gas insulated

substation

In GIS multi-components like buses, switch gearassociated equipment are present in an earthedmetallic housing

They are subjected to same magnitude of faultcurrent and require low impendence earthing

Compared to a conventional substation, as GISrequires only 25% of land area design of earthmat is comparatively difficult.

Metallic enclosures of GIS have inducedcurrents, specially during internal earth faults.

E hi f i l d


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Earthing of gas insulated

substation Inductive voltage drop occurring with GIS assembly shall be

taken into account for the design of earth mat

Touch voltage criteria = (FA)2+(EG)2 < ET (max)Where FA = Actually calculated touch voltage

EG= Max value of metal to metal voltage differenceon and between GIS enclosures or

between GIS enclosures and supportingstructures

ET (max) = maximum permissible touchvoltage

Metallic enclosures of GIS may be continuous or not In either case provision of earth bond frequently is essential to

minimize hazards of touch potential In addition, earthing of GIS structures and service platforms at

frequent intervals are to be done.

S b t ti E thi


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Substation Earthing

Case studies

Karimnagar132kV ss

Kamalapuram 132kV ssfencing givingshock

Auxiliary Earth grid at RTPP


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