20869333-Earthing-Concepts.ppt

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Earthing in a EHV Substation

Providing adequate ‘Earthing’ in a

substation is an important safety measure.

Earthing means connecting the electrical

equipment to the general mass of earth of

low resistance.

Objective is to provide under and around

the substation a surface of uniform

potential

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Earthing in a EHV Substation

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 property against dangerous potentials under fault conditions

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Earthing in a EHV Substation

I.E.Rules 1956

 Rule 92

Every substation /generating station exposed to

lightning shall adopt efficient means for diverting the electrical surges due to lightning to earth

Earth lead of any lightning arrestor shall not pass

through any iron or steel pipe.

It shall be taken directly, as far as possible, to a

separate earth electrode and/or junction of the earth mat.

Bends Shall be avoided where ever practicable

Earth screen if provided for lightning protection

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Earthing in a EHV Substation

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 thermal

or mechanical damage to the insulation of connected plant / equipment

Every exposed conductor part and extraneous conductive

part 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 transfer

potentials) shall occur under normal or abnormal operating conditions

To minimize electromagnetic interference between power

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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 turn

interconnected with the station grounding mat.

 Equipment frame work and other non-current parts (two connections)

 All extraneous metallic frame works not

associated with equipment ( two connections)

 Lightning arrestors should have independent earths, in turn connected to the station

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Earthing System

Points to be earthed-cont’d

 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 connected directly to the earth electrode by two

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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 the

core damage in stator of rotating machines.

 In solidly grounded system substantial ground fault current flows enabling fault detection and faster clearance.

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Earthing and grounding -distinction

Earthing:- connection of non current

carrying parts to ground. Ex : Metallic

enclosure.

This is for human safety.

Earthing system plays no role under

balanced power system conditions.

Under ground fault conditions, enables

ground fault current to return back to

source 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-spherical

shells of earth of continuously increasing cross sections.

Almost 95% of final resistance is contributed by soil

within 5mts of the electrode.

If current is discharged from a grid towards another

grid 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

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Electric field – Earth resistance

Earth with its huge mass offers equi-potential everywhere

A very large charge is required to change earth potential everywhere

Disturbance due to current injection at a point 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 can

safely carry

Permissible earth potential raise that may take

place due to the fault conditions

Shock duration

Material of Earth- mat conductor.

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Substation earthing

Design of Earth mat

Parameters for the calculation of Maximum

permissible 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

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Earth mat parameters

Let go current

Maximum safe current a person can

tolerate and still release grip of an

energised object, using muscles affected

by the current

The magnitude of let go current adopted in

calculating maximum permissible step and

touch potentials (As per IEEE – 80 – 1976)

for man – 9 milli amps

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Substation Earthing

Non-fibrillation current

Developed by Dalziel and approved by AIEE80-1963

Magnitude of power frequency alternating current (mA) that a human body of average weight( 50kgs to 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 9 K-ohms

Adopted value for designing Earthing system– 1Kohms

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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 current that a human body of average weight( 50kgs to 70 kgs) can with stand without ventricular fibrillation, I =0.165

√t

I = rms current through human body in amps

t =durtation of shock in seconds

Assumption /considerations in deriving the above

equation

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

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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

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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 are considered

 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

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Substation Earthing

Soil resistivity

 To design most economically and technically sound earthing system accurate data of soil

resistivity and its variation with in substation soil is essential.

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

 Variation in soil Resistivity with depth is more predominant as compared to variation in

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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 moisture

content earth resistivity readings to be obtained in summer or dry season.

Weiner's 4 electrode method is generally adopted for

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Substation Earthing-

Soil resistivity

Weiner's 4 electrode method

 Earth resistivity tests shall be carried out at least in 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

ρ=

4

s

ΠR

- __s__

1

+

2s

___

√s²+e²

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Substation Earthing

Soil resistivity

 Where

ρ= Resistivity of soil in ohm-meter

s= Distance between two successive electrodes in meter

R= Ratio of voltage to current or electrode resistances in ohm

e= depth of burial of electrodes in meters

 In case depth of burial of the electrodes in the ground (e) is negligible compared to electrodes spacing. This formula is the adjusted ρ=2ΠsR (This formula is normally adopted in AP Transco Ltd.)

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Substation Earthing

Measurement of

Soil resistivity

There point method

Two temporary electrodes spikes are driven in to the

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

Former is for current and the later is for voltage.

Ohmic values of earth electrode resistances are

obtained using earth meager R = ρ log 10 (4L/P) where

2 Π

R = Electrode resistance in ohm

L = Length in cms of the rod driven under ground D = Dia in cms of the rod

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Resistance of the earthing system

R = ρ + ρ 4r L

ρ = 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 the impendence 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

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Substation Earthing

Step and touch potential

Step potential - Difference in surface

potentials experienced by a man bridging a distance of 1 mt with his feet, with out

contracting any other grounded object.

Touch potential- potential difference between the earth potential raise and the surface

potential at the point where a person is standing touching an earthed structure.

Tolerable touch potential of human body is less than tolerable step potential.

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Substation Earthing

Step and touch potential-contd

In any switch yard, chances of exposure to

‘Touch potential’ is higher than that to ‘step potential’.

Resistance offered by the feet of a person against ‘Touch potential’ is much less

compared to that against ‘Step potential’. Hence ‘Touch potential ’ is more critical for

design while Step potential is usually academic.

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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 Touch

potential is only 35%.

Thus cross- section has minor influence on Touch

and Step potentials.

Length of earthing conductor has significant effect

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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.)ρs) (volts)

√t

E touch (LMT) = 0.116 (1000+ 6Cs.(hs.K.)ρs) (volts) √t

Where Cs= Reduction factor for de-rating normal value of surface layer resisvity, a function of K.

K= ρ-- ρs ρ+ ρs

ρ, ρs are 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.ρs) (volts)---(1)

E touch (LMT) = IB (RG + 6Cs.ρs) (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- ρ/ρs)

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

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Substation Earthing

Step and touch potential-contd.

 Sample calculation for E step (LMT) and E touch (LMT)

Data

Weight of the man =70kgs Fault duration =0.5 sec

Resistivity Soil = ρ=100 ohm-mt, Surface layer =ρs=2000 ohm-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= 222mA

From (1) E step (LMT)= 691V

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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 section

I = 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

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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 (K o + Tm) tc x iØr ρr (Ko + Ta) Where In case of steel

A = I x 12.3 √tc mm² for welded joints = I x 15.13 √tc mm² for bolted joints In case tc = Duration of current =1sec A = 12.3 x I mm² for welded joints = 15.3 x I mm² 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 (As adopted 5to 6mm)

Minimum dia for steel rod = 5mm

 Standard sizes of conductor as, As per IS 1730 – 1989

(I)10 x 6mm² (II)20x6mm² (II)30 x 6mm² (IV)40 x 6mm² (IV)50 x 6mm² (VI)60 x 6mm² (VI)50 x 8mm² (VIII)65 x 8mm² (IX)75 x 12mm² (X)100 x 16mm²

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Earthing materials

Up 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 than

64.5 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 apart c) Raisers : 50x8m MS flat

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

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Pipe earthing

a)

EHT Substations :

(i) Cast iron pipes 125

mm in diameter 2.75 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 minimum number

2. All joints and connections in earth grid are to be

brazed, riveted, sweated, bolted or welded.

3. For rust protection welds shall be treated with

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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 impedance

In places where soil resistivity is high steps to be taken to reduce earth impedance by one or combination of following:-

a. Connection of substation grid with a remote ground grid and

adjacent grounding facilities.

b. Use of deep driven ground rods or longer ground rods or maximum

number of ground rods along the perimeter of the earth grid.

c. Use of foundation rods as auxiliary grids where feasible

d. Formation of auxiliary grids if soil of low earth resistivity is available close by

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

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

f. If equipment is to be kept at corners of the mesh. Auxiliary grids are

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Earthing System

Earthing of switch yard fencing

Two methods of fence earthing

a) 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 connecting to the main earthing grid.

In the former case substantial reduction in the effective substation earthing resistance is possible but at additional cost.

In the later case any inadvertent connection could give rise to dangerous potential under fault condition unless special care is taken.

Electrical isolation of fence into short section with individual earthing is required where fence is closer to a single phase reactor or an electrical plant generating large electromagnetic fields.

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Earthing System

Earthing of switch yard fencing- con…

Methods of earthing of fencing – As per CBIP report A.

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 connecting

the fencing at frequent intervals to earth mat

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Earthing System

Earthing of switch yard fencing- con…

C.

Design permits extension of earth mat up to

fencing

Calculated touch potential beyond the fence

above the permissible limit for touch potential

Termination of earth mat within 1.5 mt of

fencing

Fence electrically isolated and independently

earthed by running an earthed conductor underneath the fence connecting it to the fence at frequent intervals

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Earthing of gas insulated

substation

 In GIS multi-components like buses, switch gear associated equipment are present in an earthed metallic housing

 They are subjected to same magnitude of fault current and require low impendence earthing

 Compared to a conventional substation, as GIS requires only 25% of land area design of earth mat is comparatively difficult.

 Metallic enclosures of GIS have induced currents, specially during internal earth faults.

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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 difference

on and between GIS enclosures or

between GIS enclosures and supporting

structures

ET (max) = maximum permissible touch

voltage

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.

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Substation Earthing

Case studies

Karimnagar132kV ss

Kamalapuram 132kV ss –fencing giving

shock

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Figure

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References

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