2 earthing1 3hrs

8/3/2019 2 earthing1 3hrs

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EARTHING

EHV Sub-Stations

T.R.SATHAYNARAYANA RAO

consultant

POWER ENGINEERING TRAINING AND

CONSULTANCY SERVICES


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Agenda

Introduction Concepts

Electric Shock

Objectives of Earthing

Classification of Earthing Basic Design

General Considerations

Earth Potential Rise ( E.P.R)

Case studies References


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INTRODUCTION


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IMPORTANCE OF EARTHING,EARTH CONDUCTION,VOLTAGE

GRADIENTS

ELECTRIC SHOCK,TOUCH AND STEP POTENTIALS

SOIL RESISTIVITY,CONDUCTION

FAULT LEVELS AND MAX. EARTH FAULT CURRENT,

DESIGN AND LAYING OF EARTHMAT,

DESIGN OF EARTHMAT UNDER DIFFICULT CONDITIONS,

EARTH POTENTIAL RISE (E.P.R) AND INTERFERENCE WITH

TELECOMMUNICATION CIRCUITS.

Introduction


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50 % Failure of equipments attributed to Earthing.

40,000 Lightening storms/day or

100 Lightening storms/second98 % of the faults in the system are due to SLG Faults

1.5 % of the faults are due to Line to Line Faults

0.5 % of the faults are due to 3 Phase Faults

Importance of Earthing,in Power System


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PLAY A FUNDAMENTAL ROLE IN PREVENTING OVER VOLTAGE

AND CURRENT CONDITIONS

IMPARTS ON SHORT AND LONG TERM LIFEOF

ELECTRICAL EQUIPMENTS

AT THE LOW COST OF IMPLEMENTATION THERE IS NO

MEASURE THAT IS MORE COST EFFECTIVEEARHING IS THE ONE COMMONfactor WHICH CONTROLOVER VOLTAGES AND CURRENTS DURING ABNORMAL

CONDITIONS

Importance of Earthing,in Power System


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EARTH IS A GOOD CONDUCTOR

GROUND POTENTIAL IS ALWAYS ZERO

PROTO TYPE EARTHING DESIGN IS SUFFICIENT

EARTHING IS JUST BURYING CONDUCTOR

EARTHING IS ONLY FOR ACHIVEING LOW RESISTANCE VALUE

USE OF COPPER FOR EARTHING WILL GIVE LOW RESISTANCE

POPULAR ( MIS ) CONCEPTS ABOUT EARTHING


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EARTH IS A POOR CONDUCTOR

NON HOMOGENEOUS

CONDUCTORS BURIED IN SOIL HAVE COMPLICATED SHAPE

ACTIVE ONLY DURING FAULT CONDITIONS

MOST OF THE ANALYSIS OF EARTHING IS BY EMPIRICAL FORMULAE

REASONS WHY EARTHING PROBLEMS ARE COMPLEX


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What is Earthing ?

Earthing means an electrical connection

to the general mass of earth to provide

safe passage to fault current to enable to

operate protective devices and provide

safety to personnel and Equipments.


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Objectives of Earthing :-

Avoid potential rise of parts of equipments other

than the live parts.

Safe passage to earth for the fault current.

Suppress dangerous potential gradients on theearth surface.

To retain system voltages within permissible

limits underfault conditions.

To facilitate using of Graded insulation in power

transformers


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ELECTRIC SHOCK


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The effect of electric currentpassing through vital organs of the

body depends on:-

magnitude,duration and

frequency of current.

The most dangerous consequence is a

heart condition known as ventricular

fibrillation, which results in stoppage

of blood circulation.

ELECTRIC SHOCK


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Fibrillation current verses Body weight for various

Animals based on a three second shock


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ELECRIC SHOCK IS DUE TO ENERGYABSORBED BY BODY

( IB)2 t = SB

Where

IB = magnitude of current through the body in amps

t = duration of the current exposure in seconds

SB= Empirical constant related to the shock energy

absorbed by a certain percent of a given population

ELECTRIC SHOCK


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Effect of magnitude of current

Thethreshold of perception is a current of 1 m.A.

Currents in the range of 1-6 m.A are known as let go current becausethese currents, though unpleasant, do not impair the ability of a person,holding an energized object to release it.

Currents in the 9-25 mA range may be painful and impair the ability torelease energized object.

Still higher currents make breathing difficult.

If the current is less than about 60 mA, the effects are not permanent and

disappear when current is interrupted.

Currents higher than 60 mA may lead to ventricular fibrillation, injury and

death.

ELECTRIC SHOCK


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Effect of frequency

The tolerable currents mentioned above are for

50 - 60 Hz. It has been found that human body

can tolerate about 5 times the direct current, at

high frequencies (300010000 Hz).

ELECTRIC SHOCK


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Effect of duration of current

The magnitude of 50 HZ tolerable current is related to

duration. According to tests reported by Dalziel, 99.5% of

persons of 50 Kg weight can withstand the current given by

equation.

IB = 0.116 /t

Where IB is the rms value of body current in amperes and t

is the time in seconds. If the weight of body is 70 Kg., the

equation for tolerable current is

IB = 0.157 /t

These equations are valid for 0.03 < t < 3 seconds.

ELECTRIC SHOCK


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Objectives of Earthing

For safety of equipments

Safety of Operating personnel

Safety of telecommunication equipments


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Types of Earthing

Neutral Earthing : deals with the earthing of

system neutral to ensure system security and

protection.

Equipment Earthing : deals with earthing of non-

current carrying parts of equipment to ensuresafety to personnel and protection against

lightning.


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1. Solidly Earthed

Here neutral is directly connected to earth

electrode/mat

2. Resistance/Impedance Earthing

Here a resistance or an impedance, in

general a potential transformer or a single

phase distribution transformer is connected

between the neutral and the earth

electrode/mat.

This is generally applicable to Synchronous

generator earthing.

Neutral Earthing


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Disadvantages of solidly

grounded systems

High fault currents interfere with communicationcircuit.

Danger to personnel in the vicinity of fault is high.

Heavy fault currents may cause considerable damage to

equipments.


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BASIC DESIGN


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Earthing of H.V. Sub-stations

Factors to be considered :

Soil Resistivity :

Wenners four electrode method of measurement.

Earth Fault Current :

Determine the max. fault current for the location.

Safe Body Current :

Effect of magnitude.

Effect of duration.

Effect of frequency.


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IV. NATURE OF AN EARTH ELECTRODE:Resistance to current through an electrode actually has three components

Components of earth resistance in an earth electrode

1. Resistance of the electrode itself and connections to it

2. Contact resistance between the electrode and the soil adjacent to it.

3. Resistance of the surrounding earth.

2 1

3 ?R=

A

I I

GL


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DESIGN OF EARTHMAT FOR HVSTATIONS

As the earthing system has to carry the earthcurrents, the maximum earth fault current likely toflow in the system which is generally S.L.G fault isconsidered for designing the earthing .A good

earthing system for H.V. station can be designedusing an earthmat which is formed by a grid ofhorizontally burried conductors which serves todissipate the earth fault currents to earth, also asan equipotential bonding conductor system, alongwith the required number of vertical earthelectrodes which are connected to the points ofearthing of various equipments and structures andalso interconnected with the horizontal earthmat.


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Before designing earthmat, it is necessary to determinethe soil resistivity of the area in which H.V. sub-station is

to be located. The resistivity of the earth varies

considerably from 10 to 10,000 mtr. depending uponthe types of soil.

Further, the resistivity may also vary at different depthdepending upon the type of soil, moisture content and

temperature etc., at various depths which affects the flow

of current due to the fact that the earth fault current is

likely to take its path through various layers.

SOIL RESISTIVITY

T i l l f i ti it f i


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Typical values of resistivity for various

types of soils are as follows :-1 Red loamy soil 40-200 -m2 Red sandy soil 200-2000 -m3 Laterite soil 300-2600 -m4 Shallow black soil 20-100 -m5 Medium black soil 50-300 -m6 Deep black soil 50-250 -m7 Mixed red & black soil 50-250 -m8 Coastal alluvium 300-1300 -m9 Laterite gravelly 200-1000 -m


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SEASONAL VARIATIONS-RESISTIVITY

To account for the seasonal variations , the average Soil

resistivity is multiplied by the factor as shown below,

which is termed as the apparent resistivity.Season of measurementMultiplication factor

Summer 1.0Winter 1.15Rainy 1.3

VARIATIONS IN RESISTIVITY DUE TO


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SEASONAL VARIATIONS

To account for the seasonal variations , the average Soilresistivity is multiplied by the factor as shown below, which istermed as the apparent resistivity.

Season of measurement Multiplication factor

Summer 1.0

Winter 1.15

Rainy 1.3

VARIATIONS IN RESISTIVITY DUE TOMOISTURE,TEMPERATURE,SALT


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Effect of Salt Moisture and Temperatureon Soil Resistivity

Choice of materials and size of earthmat conductor


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Choice of materials and size of earthmat conductor

Cross-section of the M.S. conductor in Sq mm is given by the formula.= I f * 12.5 * tc Sq mm. for welded joints

= I f* 15.8 * tc Sq. mm. for bolted joints.

Where If= Fault current in K.Amps.tc = fault clearing time in seconds.

Suitable correction shall be made to this cross sectional area by

providing an allowance for corrosion as below:a) If>100 mtr : no corrosion allowance be made.b) If>25


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Touch potential:- The potentialdifference between a point on theground and a point on an objectlikely to carry fault current (e.g.,frame of equipment) which can betouched by a person

Step potential: The potentialdifference shunted by a humanbody between two accessiblepoints on the ground separated bya distance of one pace assumed tobe equal to one meter

Touch voltage circuit


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Mesh potential: The maximum touch potential within a mesh ofthe grid.

Transferred potential: A special case of touch potential where a

potential is transferred into or out of the sub-station


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Tolerable values of Touch and Step Potential

Etouch = [ 1000 + 1.5 Css ] (0.116 /ts ) Volts.Estep = [ 1000 + 6 Css ] (0.116 /ts ) Volts.Where ts = Fault duration in secs.s = Surface layer resistivity in mtr.

= 3000 mtr. for crushed stone layer.Cs = 1-a [(1- /s) / (2 hs + a)]Cs = 1 when no protective surface layer or crushed stone is used.Where a = 0.106 mt

hs= Height of surface layer i.e., thickness of the

crushed

stone layer which is normally 0.1 mt.


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Calculation of grid resistance Rg

Rg = /4(( /A)+ /L

Ground potential rise:GPR = Ig * Rg Volts.

DESIGN POTENTIALS SHOULD BE LESS THAN TOLERABLE


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DESIGN POTENTIALS SHOULD BE LESS THAN TOLERABLE

LIMITS

DESIGN STEP AND TOUCH POTENTIALS DEPENDS ON:-

MAX. FAULT CURRENT

AVG. & SURFACE SOIL RESISTIVITY

TOTAL LENGTH OF BURIED CONDUCTOR-WHICH

INTURN DESIDES THE SPACING B/W PARALLEL

EARTHMAT CONDUCTORS

DEPTH OF BURIAL OF EARTHMAT

NO. AND LENGTH OF VERTICAL ELECTRODES

POTENTIAL DISTRIBUTION FOR A GROUND MAT WITH VARIOUS MESH SIZES


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POTENTIAL DISTRIBUTION FOR A GROUND MAT WITH VARIOUS MESH SIZES

( GROUND MAT POTENTIAL = 100 PERCENT )

36

100

80

60

40

20

0


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Typical Earthmat


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GENERAL

CONSIDERATIONS



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Guide lines for laying Earthmat

Earthing of Power Cables

Providing crushed rock surface layer

Separation between CI Pipe Electrodes

Precautions

Enhancing Sub-Station Earthing

Maintenance Of Earthing System

Test Configuration for earthing

Earthing in difficult situations


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Guide lines for laying Earthmat

Earth Connections.

Power Transformer Neutral.

Lightning Arrestor and Mast Earthing.

Switchgear & Control Room Earthing.

Noncurrent carrying metal parts.


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Power Cables

Earthing of Three Core Cable Sheath.

Earthing of Single Core Cable Sheath.

Clearance between Power and Control

Cables.


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Earthing of

Out going 11 KV feeders within the

Sub-station area.

Out going 11 KV feeders outside the

Sub-station area.

Sub-station fencing.

P d h d k f


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Providing crushed rock surfacelayer

Controls step and touch voltages.

Avoids weed growth.

Retains moisture in the soil.

Avoids movement of reptiles in the yard.


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Minimum Distance for C:

Earth electrode P C

Resistance in

Distance P from earth electrode

(a)

P C

62%

Resistance

in

Distance P from earth electrode

(b)

Effect of C location on the earth resistance curve.

Earth electrode resistance

C Electrode resistance

Separation between CI Pipe


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Separation between CI PipeElectrodes

Minimum distance between two

electrodes - twice the length.

The lead connecting the equipment to

the earthmat should have least length.

Ensure that the CI Pipe is uncoated.


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General

Provide dedicated Auxiliary Transformer.

Labeling of Electrodes.

Earthing of Control Panel.

Earthing of Switchgear Panel.

Auxiliary TransformerNeutral Earth.


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Precautions

Provide dedicated Auxiliary Transformer.

Labeling of electrodes.

Earthing of Control Panel.

Earthing of Switchgear Panel.

Auxiliary TransformerNeutral Earth.


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Enhancing Sub- StationEarthing

SIZE CONDUCTORS FOR ANTICIPATED FAULTS

SELECT THE RIGHT CONDUCTOR

PAY ATTENTION TO EARTH RODLENGTH,NUMBER,PLACEMENT AND SPACING

PREPARE THE SOIL

ELIMINATE STEP AND TOUCH POTENTIAL

EARTH THE FENCE

EARTH ALL SWITCH HANDLES

EARTH ALL SURGE ARRESTORS

EARTH ALL CABLE TRAYS


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The resistance to earth of an electrode is directly

proportional to soil resistivity and inversely proportional to

the total area of contact established with the soil.

For fixed land areas, additional vertical rods or

horizontal cables produce diminishing returns because of

increased mutual coupling effects.

The most straight forward enhancement method is to

reduce soil resistivity. The parameters which strongly affect

soil resistivity are moisture content, ionizable salt content, and

porosity; the latter determining the moisture retention

properties of the soil.

Thus two recommended techniques for reducing earth

resistivity are water retention and chemical salting.


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Water Retention.

Overdrainage of soil leaches away salts that arenecessary for high conductivity and dries out the

deeper layers, thereby increasing their rcsistivity.

Surface drainage should be channeled so as to

keep the earth electrode subsystem moist.

Maintaining moist earth over the extent of the earth

electrode subsystem will keep soil salt in solution as

conductive ions.


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A porous clay, bentonite (also known as well

drillers mud) can absorb water from surrounding soil

and has hydration as well as water retentionproperties.

When placed around ground rods, it greatly

increases the effective area of the rod and cable which

in turn reduces the resistance of the earth electrode

subsystem to earth.

Bentonite is generally available in dry (powder)

form, must be saturated with water after initial

installation as it expands.


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This can also prove to be a disadvantage of

bentonite since it expands and contracts so much with

moisture content, it can pull away from the groundrod and surrounding soil when moisture is lost.

A much better backfill around ground rods is

a mixture of 75 percent gypsum, 20 percent bentoniteclay, and 5 percent sodium sulfate. The gypsum,

which is calcium sulfate, absorbs and retains moisture

and adds reactivity and conductivity to the mixture.

Since it contracts very little when moisture is lost, itwill not pull away from the ground rod or

surrounding earth.


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The bentonite insures good contact between

ground rod and earth by its expansion, while the

sodium sulfate prevents polarization of the rod by

removing the gases formed by current entering the

earth through the rod.

This mixture is available from cathodic

protection distributors as standard galvanic anode

backfill and is relatively inexpensive. The backfill

mixture should be covered with 12 inches ofexcavated soil. This mixture is superior to chemical

salts since it is much more enduring.


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Chemical Salting.

Reduction of the resistance of an electrode may also

be accomplished by the addition of ion-producing

chemicals to the soil immediately surrounding the

electrode. The better known chemicals in the order of

preference are:

a. Magnesium sulphate (MgS04) - epsom salts.

b. Copper sulphate (CuS04) - blue vitriol.

c. Calcium chloride (CaC12).

d. Sodium chloride (NaCl) - common salt.

e. Potassium nitrate (KNO3) - saltpeter.


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Magnesium sulphate (epsom salts), which is

the most common material used, combines low cost

with high electrical conductivity and low corrosiveeffects on a ground electrode or plate.

The use of common salt or saltpeter is not

recommended as either will require that greatercare be given to the protection against corrosion.

Additionally, metal objects nearby but not

related to grounding will also have to be treated toprevent damage by corrosion. Therefore, salt or

saltpeter should only be used where absolutely

necessary.


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Electrode Encasement.

The calculations of resistance of earth electrodes

invariably assume zero contact resistance between the

electrode elements and the earth. In reality, however, the

interface between the surface of the rod and the earth is

far from uniform except when the earth is tamped clay or

its equivalent.

Encasing the electrode in conductive mastic or

conductive concrete is one approach to improving the

contact between the electrode and the earth. Effects of

local variations or moisture content will also be reducedand stabilized, if the encasement material absorbs and

holds moisture.


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Salting Methods.

The trench method for

treating the earth around a

driven electrode is illustrated

A circular trench is dug

about one foot deep around

the electrode. This trench is

filled with the soil treating

material and then covered

with earth. The material

should not actually touch therod in order to provide the

best distribution of the

treating material with the least

corrosive effect.


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Another method for treating

the earth around a driven

electrode, using magnesium

sulphate and water, isIllustrated.

A 2-foot length 8-inch

diameter tile pipe is buried in

the ground surrounding theground electrode.

This pipe is then filled with

magnesium sulphate to within

one foot of gradelevel andwatered thoroughly. The 8-inch

tile pipe should have a wooden

cover with holes and be located

at ground level.


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None of the aforementioned chemical

treatments permanently improve earth electrode

resistance. The chemicals are gradually washed awayby rainfall and through natural drainage.

Depending upon the porosity of the soil and

the amount of rainfall, the period for replacementvaries. Forty to ninety pounds of chemical will

initially be required to maintain effectiveness for two

or three years.

Each replenishment of chemical will extend the

effectiveness for a longer period so that the future

treatments have to be done less and less frequently.


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Another method of soil treatment or electrode

enhancement involves the use of hollow made electrodes

which are filled with materials/salts which absorb external

atmospheric moisture.

These electrodes (generally 8-feet long) must be

placed in holes drilled by an earth auger making sure the

breather holes at the top are above grade level. Moisture

from the atmosphere is converted to an electrolyte which in

turn seeps through holes in the electrode into the

surrounding soil.

This keeps the soil moist and thereby reduces theresistance of the electrode to earth. These electrodes should

be checked annually to ensure sufficient quantities of

materials/salts are available and that good continuity exists

between the rod and interconnecting cable.

CATHODIC PROTECTION


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CATHODIC PROTECTION.

When two metals of different types are

immersed in wet or damp soil, a basic electrolytic

cell is formed. A voltage equal to the difference of

the oxidation potentials of the metals will be

developed between the two electrodes of the cell.

If these electrodes are connected together

through a low resistance path, current will flowthrough the electrolyte with resultant erosion of the

anodic member of the pair.


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Unfortunately, those factors that aid in the

establishment of low resistance to earth also foster

corrosion. Low resistance soils with a high moisture

level and a high mineral salt content provide an

efficient electrolytic cell with low internal resistance.

Relatively large currents can flow between

short-circuited electrodes (such as copper ground

rods connected to steel footings or reinforcing rods in

buildings) and quickly erode away the more activemetal of the cell. In high-resistance cells, the current

flow is less and the erosion is less severe than in low-

resistance cells.

P i T h i


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Protection Techniques.

Three basic techniques can be used to

lessen the corrosion rate of buried metals. Theobvious method is to Insulate the metals from the

soil by the use of protective coatings. This

interrupts the current path through the electrolyte

and stops the erosion of the anode. Insulation,however, is not an acceptable corrosion preventive

for earth electrodes.

The second technique for reducinggalvanic corrosion is avoiding the use of

dissimilar metals at a site.


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Copper is a desirable material for the earth

electrode subsystem; apart from its high

conductivity, the oxidation potential of copper issuch that it is relatively corrosion resistant.

Since copper is cathodic relative to the more

common structural metals, its corrosion resistance isat the expense of other metals.

Because of the greater conductivity and

corrosion resistance of copper, it is normally used for

the grounding of buildings, substations, and other

facilities where large fault or lightning currents may

occur and where voltage gradients must be

minimized to ensure personnel protection.


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The third technique for combating the corrosion

caused by stray direct currents and dissimilar-metal

unions is commonly called cathodic protection.Cathodic protection may be implemented through the

use of sacrificial anodes or the use an an external

current supply to counteract the voltage developed by

oxidation.

Sacrificial anodes containing magnesium,

aluminum, manganese, or other highly active metal can

be buried in the earth nearby and connected to an ironpiling, steel conduit, or lead cable shield. The active

anodes will oxidize more readily than the iron or lead

and will supply the ions required for current flow.


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The iron and lead are cathodic relative to the

sacrificial anodes and thus current is supplied to

counteract the corrosion of the iron or lead.

The dc current is normally derived from rectified

alternating current, but occasionally from photovoltaic

cells, storage batteries, thermoelectric generators, or

other dc sources. Since the output voltage is adjustable,any metal can be used as the anode, but graphite and

high silicon iron are most often used because of their low

corrosion rate and economical cost.

Cathodic protection is effective on either bare orcoated structures. If the sacrifical anodes are

replenished at appropriate intervals, the life of the

protected elements is significantly prolonged.


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Sacrificial Anodes.

Sacrificial anodes provide protection over

limited areas. Impressed current cathodic

protection systems use long lasting anodes of

graphite, high silicon cast iron or, to a lesser extent,

platinum coated mobium or titanium.

This dc current is normally derived from

rectified ac and occasionally from photovoltiac

cells, storage batteries, thermoelectric generators,

or other dc sources.

Maintenance of Earthing System


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Maintenance of Earthing SystemKEEP TOP PORTION OF ELECTRODES FOR INSPECTION

CHECK FOR ARCING FAULTSPERIODICAL INTEGRITY CHECK OF EARTHING

ARREST WEED GROWTH

REPLACE ALL DETERIORATED ELECTRODES AND EARTH

CONNECTION

AUXILLARY SUPPLY TO THE STATION FROM DEDICATEDTRANSFORMER ONLY

DO NOT RUN METALLIC WATER PIPE OUTSIDE STATION

SWITCH GEAR AND CONTROL PANEL SHOULD BE MADEVERMIN FREE

E thi i diffi lt it ti


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Earthing in difficult situationsThe earthing resistance can be improve by any one

or more of the following methods.1. Increase the area of the earth mat.

2. Provide deep earth electrodes.

3. Provide auxiliary earth mat in a near by place wherethe resistivity is low and connect it to the main earthmat.

4. Treating the earthmat and the electrode with

suitable chemicals.

Depending upon the situation any one or more of theabove methods can be used to reduce the earthresistance.

EVALUATION OF GRID RESISTANCE AND G.P.R


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Eq-1

Eq-2

Eq-3

A Minimum value of the Sub-Station grounding resistance inuniform soil can be estimated by

Upper value of the Sub-Station grounding resistance in uniform soil can be

estimated by

Where L is the total buried length of

conductor in mtr.

Where h is the depth of Grid in mtr.

For grid depths between 0.25 to 2.5 mtr. Is given by incorporating correction factorfor grid depth

Ground potential rise: (GPR) = Ig * Rg Volts

Satellite Earthmat


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The diversion of fault current through the main earthmat.

Selection of site and interconnection.

SATELLITE EARTHMAT

MAIN EARTHMAT

TRANS.

GROUND WIRE

FAULT-1

FAULT-2

Ig

Igw

IsIn

Ig5 Ig4 Ig3 Ig2

Is

If

Im

If = Fault current In = Neutral current

If = InIm = Total current flowing in the Earthmat

Ig = Part of the fault current Entering the

Main Earthmat through the Earth.

Igw = Part of the fault current Entering the

Is = Part of the fault current Entering the

EARTH POTENTIAL RISE ( E.P.R )

Ig = Ig1+Ig2+Ig3.........+Ign

In = Im = If = Ig+Igr+Is

ONLY THE CURRENT Ig CONTRIBUTES TO THE E.P.R AND

NOT THE TOTAL CURRENT FLOWING IN THE EARTHMAT (Im) OR NEUTRAL (In)

Ig1

Main Earthmat through the Overhead Ground wirw.

Main Earthmat through the Satellite Earthmat.

Ig = Ig1+Ig2+Ig3.........+Ign


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EARTH POTENTIAL

RISE

EARTH POTENTIAL RISE ( E.P.R )If = Fault current In = Neutral current

If = In


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Path of the Earth fault current

Earth Potential Rise (E.P.R)

In Solid Earthing systems large fault current flows from the fault point to the

neutral via earth

NOT THE TOTAL CURRENT FLOWING IN THE EARTHMAT (Im) OR NEUTRAL (In)

ONLY THE CURRENT Ig CONTRIBUTES TO THE E.P.R AND

CASE-1 Fault outside the substation

In = Im and E.P.R is Zero w.r.t remote point

Ig = Ig1+Ig2+Ig3.........+Ign

CASE-2 Fault within the sub-station

In = Im = If = Ig+Igr+Is

Ig = Igr = 0

Ig5 Ig4

In

MAIN EARTHMAT

FAULT-2

Im

Ig

TRANS.

Igw

Ig3 Ig2 Ig1

Main Earthmat through the Overhead Ground wirw.

Im = Total current flowing in the Earthmat

Ig = Part of the fault current Entering the

Main Earthmat through the Earth.

Igw = Part of the fault current Entering the

Ig = Ig1+Ig2+Ig3.........+Ign

FAULT-1If

GROUND WIRE

If In

is directly proportional to IgE.P.R of Earthmat w.r.t remote point

HAZARD ZONE

STATION EARTHMAT


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Magnitude and Zone of Influence of E.P.R.

Limits of E.P.R in our country :-

Telephone Exchange / Terminal apparatus ......... 430 Volts.

Tecommunication Lines .........650 Volts

Factors affecting Voltage gradients

An Earth mat designed to limit Voltage gradients

E.P.R. HAZARD ZONE AND INFLUENCE

CONSTANT VOLTAGE

COUNTOUR

VOLT

AGERISE

DISTANCE FROM EARTHMAT

ZONE OF INFLUENCE


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REMOTE EARTH

E.P.R. WITH REFERENCE TO REMOTE EARTH AND HAZARD ZONE

EpVe = E.P.R AT THE STATION EARTHMAT

Ep = E.P.R AT THE TELEPHONE EXCHANGE

Vf = E.P.R AT THE FAULT POINT

t

RNEThe ill-effects of E.P.R reported in our system are :

Burning of Telephone cards and Mother boards at Tel.Exchange,

Charred out Tag blocks and cables at Pillar boxes ,

Melting of telephone Sets,modems and service main at consumer premises,

Frequent failure of telephone cables,

Transfer potential between the E.P.R areas and outside places


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Minimum separation for Telecom cables in the soil


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Earth Resistivityin Ohm Metres

Power network system with

LocationIsolated Neutralor Arc separation

coil

Directly earthed

neutral


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Theoretical formula for EPR Zone

Applicable for

single electrode earthing

Applicable for large earthmats


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References1. IEEE guide for AC Substation Grounding (IEEE80)

2. IEEE guide for Measuring earth Resistivity, Ground

impedance, and earth surface potentials for a ground

system(IEEE81)

3. IEE recommended practice for grounding industrial and

commercial power systems ( IEEE142)

4. IEEE Guide for generating station grounding(IEEE665)

5. Indian standard specifications (3043 Earthing)

Th k Y


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Thank You

T.R.SATHAYNARAYANA RAO

consultant

POWER ENGINEERING TRAINING AND

CONSULTANCY SERVICES# 57 3rd Block, 3rd Stage, 2nd Main. Behind

Kashyap Lodge, Basaweswara nagar

Bangalore560079.

Ph: 23390918,23313633.

Cell: 94496 52508

[email protected]

PRINCIPAL OF EARTH TESTING

A
mailto:[email protected]:[email protected]


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(a)Rod 3 Rod 2(c)

Rod 1 (p)

EARTHElectrode

Being tested

D

62 feet

50

(b)

40Principles of an

30 earth resistance test

20

10

62 % line

0

0 20 40 60 80 100Distance (from Rod1 to Rod3), Feet

V


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MEASUREMENT OF EARTH RESISTANCE R


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MEASUREMENT OF EARTH RESISTANCE R

V

I

E V--I

R

--

P C


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TEST CONFIGURATION FOR SUB-STATIONS:

Earth tester

Test electrode

0.62 D

D

C1 P1 P2 C2

o o o o

2 earthing1 3hrs

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