grounding & earthing.pdf

Earthing System

By: A.K.Agarwal

PMI, Noida

ContentsIntroduction:

Equipment earthing and neutral point earthing.

Methods & Importance of neutral earthing

Concepts, Objectives & Classification of Earthing

General Considerations

Electric Shock, Touch & Step Potential

Soil Resistivity, Conduction

Fault Levels And Max. Earth Fault Current

Earth Potential Rise (E.P.R) And Interference

With Telecommunication Circuits.

References

INTRODUCTION

Earth - The conductive mass of the earth, whose electric potential at any point is conventionally taken as zero.

Earth Electrode -A conductor or group of conductors in intimate contact with and providing an electrical connection to earth.

Earth Electrode Resistance - The resistance of an earth electrode to earth.

Earth Leakage Current - A current which flows to earth or to extraneous conductive parts in a circuit which is electrically sound.

Earthing Conductor - A protective conductor connecting the main earthing terminal to an earth electrode or to other means of earthing.

Main Earthing Terminal - The terminal or bar (which is the equipotential bonding conductor) provided for the connection of protective conductors and the conductors of functional earthing.

Neutral Conductor - A conductor connected to the neutral point of a system and capable of contributing to the transmission of electrical energy.

Potential Gradient ( At a Point ) The potential difference per unit length measured in the direction in which it is maximum.

Touch Voltage - The potential difference

between a grounded metallic structure and a point on the earths surface separated by a distance equal to the normal maximum horizontal reach, approximately one metre.

Step Voltage - The potential difference between two points on the earths surface, separated

by distance of one pace, that will be assumed

to be one metre in the direction of maximum

potential gradient.

Earthing shall generally be carried out in accordance with the requirements of Indian Electricity Rules 1956, as amended from time to time and the relevant regulations of the Electricity Supply Authority concerned.

All medium voltage equipment are earthed by two separate and distinct connections with earth.

IEEE:80 The IEEE guide for safety in AC substationgrounding

IEEE:142 - Grounding of Industrial & commercialpower systems

IS:3043 - Code of practice for Earthing

Indian Electricity Act.

Indian Electricity Rules.

50 % Failure of equipments attributed to Earthing.

40,000 Lightening storms/day or

100 Lightening storms/second

98 % 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 Earthingin Power System

Purpose To minimize potential transient over voltages, to comply

with personnel safety requirements and to assist in rapid detection & isolation of fault areas

IMPARTS ON SHORT AND LONG TERM LIFE OF ELECTRICAL EQUIPMENTS

AT THE LOW COST OF IMPLEMENTATION THERE IS NO MEASURE THAT IS MORE COST EFFECTIVE

Importance E/F protection is based on method of neutral grounding System voltage during E/F depends on neutral grounding Provided basically for discrimination of protection, against

arcing grounds, unbalanced voltage w.r.t. earth, protection from lightning etc.

Importance of System Earthing

1127 August 2012

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

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

Earthing means an electrical connection done througha metal link between body of any electrical appliance,or neutral point, as the case may be, to general massof earth (deeper ground soil) to provide safe passageto fault current to enable to operate protective devicesand provide safety to personnel and equipments

The metal link is normally of MS flat, CI flat, GI wirewhich should be penetrated to the ground earth grid

What is Earthing?

1427 August 2012

Objectives of Earthing :-

Avoid potential rise of parts of equipments otherthan the live parts.

Safe passage to earth for the fault current.

Suppress dangerous potential gradients on the

earth surface.

To retain system voltages within permissible

limits under fault conditions.

To facilitate using of Graded insulation in power

transformers

..Objectives of Earthing

For safety of equipments

Safety of Operating personnel

Avoid Fire Hazards

Safety of telecommunication equipments

System or neutral earthing to ensure system security andprotection, it is a connection to ground from one of the current-carrying conductors of an electrical power system (connectionbetween LV neutral of a power Transformer winding and earth)

Equipment earthing (Safety grounding) deals withearthing of non-current carrying parts of equipment to ensuresafety to personnel and protection against lightning, it is aconnection to ground from one or more of the noncurrent-carrying metal parts of a wiring system or equipment connectedto the system (connecting body of equipments like motor body,Transformer tank, Switch gear box, operating rods of air breakswitches, LV breaker body, HV breaker body, Feeder breakerbodies etc. to earth)

Classification of Earthing

1727 August 2012

Types of Grounding

A system of conductors in which there is nointentional connection to ground

Early electrical systems were almost universallyoperated ungrounded

On small systems an insulation failure on onephase did not cause an outage

The failure could probably be found andrepaired at a convenient time without a forcedoutage

Un-grounded System

1927 August 2012

29 March 2013 PMI Revision 00 20

Ungrounded neutral system: Normal

Condition


Ungrounded neutral system: Fault

Condition



Condition


It can be seen from the analysis that: 1) In an ungrounded neutral system, under a single line to ground

fault the voltage to earth of the two healthy phases rises from their normal phase to neutral voltage to full line voltage. This may result in insulation breakdown.

2) The capacitive current through the two healthy phases increases to 5 times the normal value. Capacitive fault current flows to earth in excess of 4 A will cause arcing ground

3) A capacitive fault current Ir- flows to the earth.


Advantages of Neutral grounding Persistent arcing grounds are eliminated.

System can be protected against E/F.

Methods of Neutral grounding Solid grounding

Resistance grounding

Reactance grounding

Resonant grounding

SOLID EARTHING

Here neutral is directly connected to earth electrode/mat.

GENERALLY FOR VOLTAGES BELOW 2.2 KV AND ABOVE 33 KV, SOLID EARTHING IS USED. BELOW 2.2 KV, CIRCUIT IMPEDANCE IS SUFFICIENTLY HIGH LIMITTING THE FAULT CURRENT. ABOVE 33 KV, COST OF INSULATION IS VERY HIGH. THEREFORE, GRADED INSULATION IS USED.

RESISTANCE 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.

FOR SYSTEMS OF 2.2 KV TO 33 KV, EARTHING THROUGH RESISTANCE OR REACTANCE IS USED AS INSULATION MATERIAL COST TO TOTAL EQUIPMENT COST IS NOT MUCH.

This is generally applicable to Synchronous generator earthing.

REACTANCE EARTHING

WHILE IN RESISTANCE EARTHING, THE EARTH FAULT CURRENT IS LIMITED TO FULL LOAD CURRENT OF THE LARGEST GENERATOR OR TRANSFORMER, IN REACTANCE EARTHING IT WILL BE ANYWHERE BETWEEN 25% TO 100& OF 3 PHASE FAULT CURRENT.

DIFFERENT METHODS OF EARTHING

Effective Earthing

A system is called effectively earthed if XO/X1 < 3 is true

& R0/R1 < 1 is true

X0 : Zero sequence reactanceX1: Positive sequence reactanceR0 : Zero sequence resistance

Under a phase fault condition the voltage of healthy phase should not rise more than 80% of healthy Line to line voltage.

Magnitude of earth fault current is more than 3phase fault current.


Solid grounding


Salient Features of Solid grounding

a) When a fault to earth occurs on any phase of the system, the

voltage to earth of the faulty phase become zero, but the

healthy phase in general, remain at their normal value. As

such lightning arresters rated for phase voltage can be

insulated for phase voltage. Thus saving in cost.

b) The flow of heavy fault current. Ir will completely nullify the

effect of the capacitive current ICF and so no arcing ground

phenomena will occur.

c) The flow of heavy fault current permits the use of

discriminative protection gear.

d) Used for low voltages up to 600 V and high voltages above 33

kV

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 toequipments.


Resistance Grounding


salient features of the resistance grounding

1. It minimizes the hazard of arcing grounds.

2. It permits to use discriminative protective gear.

3. To limit E/F current, a resistance or reactance is introduced

between neutral and earth. A resistance grounded system will

have low E/F current when compared to solid grounding

system and hence will have less influence on neighboring

communication circuits.

4. By reducing the value of R, possible to eliminate arcing grounds and if value of R is high, system approaches to ungrounded neutral system

5. Resistance grounding normally adopted for system having system voltage between 3.3 kV to 33 kV

6. This system is costlier than solid grounded system.

To limit E/F current, a resistance or reactance is introduced between neutral and earth

Provide additional reactance to system reactance, thereby neutralizes the capacitive currents, hence where high charging currents are involved reactance grounding is preferred

Synchronous motors and synchronous capacitors are provided with reactance grounding

A system is having reactance grounding if XO/X1 > 3

R

Y

B

Reactance Earthing System

3427 August 2012


Reactance grounding

An arc-suppression coil is an iron cored reactor mounted in the neutral earthing circuit and capable of being tuned to resonate with the capacitance of the system when a line becomes earthed, it makes arcing earth fault self-extinguishing

Also referred as Peterson coil or ground fault neutralizer

For balanced condition L= 1/(w2C)

R

Y

B

Resonant Grounding System

3627 August 2012

The fault current and fault voltage at different parts of the network will be affected by the following

Type of fault

Position of the fault

Configuration of the network

Neutral earthing

Fault Analysis

27 August 2012 37

The most dangerous phenomena is normally the high current that occurs at a short circuit

Open circuit faults not cause high Overcurrent or high overvoltages and therefore normally not dangerous to network, but cause heating in rotating machines, due to the negative sequence current that will flow in the system. Machines equipped with negative sequence current protection, needs no fault calculation

The magnitude of the fault current is dependent on type of fault that occurs. At earth faults the size of the fault current is depending on the earthing resistance or reactance (if applicable) and on the resistance in fault. The fault resistance for a phase fault is much smaller than that for an earth fault

Three phase faults normally gives the highest short circuit currents

Fault Analysis

27 August 2012 38

Introduced by Fortescue in 1916

Developed in a book by Wagner and Evans

Very efficient for hand-calculations

Forms the base for computer programs

An unbalanced system of n related phases could be replaced by a system of n balanced phases which were named the symmetrical components of the original phases

Symmetrical Components

27 August 2012 39


27 August 2012 40

Positive-sequence components, consist of three phasors of equal magnitude, spaced 120 apart, and rotating in the same direction as the phasors in the power system under consideration, i.e. the positive direction

Negative-sequence components, consist of three phasors of equal magnitude, spaced 120 apart, rotating in the same direction as the positive-sequence phasors but in the reverse sequence

Zero-sequence components, which consist of three phasors equal in magnitude and in phase with each other, rotating in the same direction as the positive sequence phasors


27 August 2012 41


27 August 2012 42


27 August 2012 43

Computation of Fault Current

27 August 2012 44


27 August 2012 45


27 August 2012 46

MAIN INTENTION OF EARTHING IS TO LIMIT THE TRANSIENT OVER VOLTAGE CAUSED BY RESTRICTING GROUND FAULTS, TO THE LEVEL THAT THE EQUIPMENT CAN BE DESIGNED TO WITHSTAND ABOUT 250 % OF THE RATED VOLTAGE. FOR SAFETY TO MAINTENANCE PERSONNEL AND TO LIMIT THE DAMAGE OF THE EQUIPMENT, IT IS ABSOLUTELY MUST OF FAST CLEARING THE GROUND FAULT.

PROPER SYSTEM EARTHING WILL GIVE A HIGH DEGREE OF PROTECTION AGAINST STEEP WAVE FRONT SURGES ENTERING THE SUB STATION AND PASSING TO EARTH THROUGH ITS GROUNDING SYSTEM.

UNDER FAULT CONDITIONS, THE FLOW OF CURRENT TO EARTH WILL RESULT IN GRADIENTS WITHIN AND AROUND THE STATION. UNLESS THE EARTHING SYSTEM IS DESIGNED CAREFULLY, THE MAXIMUM GRADIENT ALONG THE SURFACE MAY BE GREAT ENOUGH TO ENDANGER A MAN WALKING IN THE VICINITY.

SUBSTATION EARTHING

A CONTINUOUS EARTHING CONDUCTOR IS PLACED AROUND THE PERIMETER OF THE SUB STATION TO ENCLOSE AS MUCH GROUND AS POSSIBLE TO AVOID CURRENT CONCENTRATION AND HENCE HIGH GRADIENTS AT GROUND CONDUCTOR ENDS. WITHIN THE GRID, CONDUCTORS ARE LAID IN PARALLEL LINES AND AT UNIFORM SPACING.

THE MATERIAL OF THE GROUND ELECTRODES SHOULD HAVE HIGH CONDUCTIVITY AND LOW UNDERGROUND CORROSION. STEEL IS USED NORMALLY IN INDIA FOR EARTHING.

ALUMINIUM IS NOT MUCH IN USE AS CORRODED ALUMINIUM IS ALMOST NON- CONDUCTIVE. COPPER IS COSTLY. HENCE MILD STEEL ELECTRODES WITH ADEQUATE CROSS SECTION ARE PREFERABLE.

IT IS A GOOD PRACTICE TO HAVE AN OVER DESIGNED EARTHING SYSTEM AS THERE ARE A NUMBER OF UNKNOWN FACTORS AND THE SAFETY OF THE OPERATING PERSONNEL IS ALWAYS INVOLVED.

PRELIMINARY DESIGN OF GROUNDING SYSTEM

EARTHMAT IS USUALLY DESIGNED WITH THE FOLLOWING SIZES OF MS RODS. 400 KV SUB STATIONS 40 MM DIA. 220 KV SUB STATIONS 40 MM / 32 MM DIA. 110 KV SUBSTATIONS 32 MM / 25 MM DIA.

CONDUCTOR ABOVE GROUND LEVEL FOR EARTHING EQUIPMENT, STRUCTURES, COLUMNS AND OTHER AUXILIARY STRUCTURES SHALL BE GALVANISED FLATS. ROD ELECTRODES SHALL BE OF MILD STEEL OF SAME DIAMETER AS EARTH CONDUCTORS AND OF LENGTH AS REQUIRED IN THE DESIGN.

NEUTRAL POINTS OF SYSTEMS OF DIFFERENT VOLTAGES, METALLIC ENCLOSURES, FRAMES OF ALL CURRENT CARRYING EQUIPMENTS AND ALL METAL WORKS ASSOCIATED WITH THE CURRENT CARRYING SYSTEM SHALL BE CONNECTED TO THE SINGLE EARTHING SYSTEM. STEEL STRUCTURES, COLLUMNS ETC. SHALL BE CONNECTED TO THE NEAREST EARTHING GRID BY TWO EARTHING LEADS.

EARTHING INSTALLATION

Earthmat Layout

EARTHING SHALL BE CARRIED OUT AS PER INDIAN ELECTRICITY RULES AND AS PER IS 3043- 1987

ALL MEDIUM VOLTAGE EQUIPMENT SHALL BE EARTHED BY TWO SEPARATE AND DISTINCT CONNECTIONS TO EARTH.

TO THE EXTENT POSSIBLE, ALL EARTH CONNECTIONS SHALL BE VISIBLE FOR INSPECTION.

THE VALUE OF ANY EARTH SYSTEM RESISTANCE SHALL BE SUCH AS TO CONFORM WITH THE DEGREE OF EARTH PROTECTION DESIRED.

PREFERABLY, NO CUT OUT, LINK OR SWITCH SHALL BE PROVIDED IN THE EARTHING SYSTEM. HOWEVER, THIS DOES NOT INCLUDE THE CASE OF A SWITCH FOR USE IN CONTROLLING A GENERATOR, A TRANSFORMER OR A LINK FOR TEST PURPOSES.

ONLY GOOD QUALITY MATERIALS SHALL BE USED IN THE EARTHING SYSTEM.

STATUTORY PROVISION OF EARTHING

WHILE EARTH GRIDS ARE USED IN MAJOR SUB STATIONS, DIFFERENT TYPES OF EARTH ELECTRODES ARE USED FOR EARTHING HV AND LV INSTALLATIONS. IN 11 KV AND 33 KV SUB STATIONS, PLATE EARTHING PROVES SUFFICIENT.

PLATE ELECTRODES

PLATE ELECTRODES ARE OF MAXIMUM SIZE 1.2 M X 1.2 M. IF MORE AREA IS REQUIRED, INSTEAD OF INCREASING THE SIZE, TWO NUMBER OF PLATES ARE USED IN PARALLEL. CAST IRON PLATES OF 12 MM THICK ARE MOST SUITABLE.

PIPE OR ROD ELECTRODES

PIPES MAY BE OF CAST IRON NOT LESS THAN 100 MM DIAMETER, 2.5 M TO 3 M LONG AND 13 MM THICK.

STRIP OR CONDUCTOR ELECTRODES

STRIP ELECTRODES ARE USED IN HIGH RESISTIVITY SOIL. WHERE ROUND CONDUCTORS ARE USED AS EARTH ELECTRODES, THEIR AREA OF CROSS SECTION SHALL NOT BE LESS THAN THE SIZES RECOMMENDED FOR STRIP ELECTRODES.

C0MMON TYPES OF EARTH ELECTRODES

THE NORMAL MOISTURE CONTENT OF SOIL RANGES FROM 10 PERCENT IN DRY SEASON TO 35 PERCENT IN WET SEASON, AVERAGE BEING 16 TO 18 PERCENT. IF THE MOISTURE CONTENT IS 20 PERCENT OR ABOVE, THE RESISTIVITY IS NOT AFFECTED. BUT WHEN THIS MOISTURE IS REDUCED, SOIL RESISTIVITY ABRUPTLY INCREASES.

ABUNDANCE OF PURE WATER WILL NOT REDUCE THE SOIL RESISTIVITY. NATURAL ELEMENTS AND SOLUBLE INGREDIENTS INCREASES THE CONDUCTIVITY.

ARTIFICIAL TREATMENT OF SOIL

THE RESISTIVITY OF THE SOIL IMMEDIATELY SURROUNDING THE EARTH ELECTRODE CAN BE REDUCED BY

EFFECT OF MOISTURE ON EARTH RESISTIVITY

ADDING SUBSTANCES LIKE SODIUM CHLORIDE, CALCIUM CHLORIDE,SODIUM CARBONATE, COPPER SULPHATE, SALT AND SOFT COKE AND SALT AND CHARCOAL IN SUITABLE PROPORTIONS

DURING DRY SEASONS, EARTH PITS MAY BE REGULARLY WATERED AND KEPT WET TO KEEP THE EARTH RESISTIVITY LOW. GRAVEL OR CRUSHED ROCK COVERING IS ALSO HELPFUL TO RETARD THE EVAPORATION OF MOISTURE FROM EARTH.

EFFECT OF MOISTURE ON EARTH RESISTIVITY

COMPONENTS OF EARTH PIT

1 Conducting Electrode

2 Contact Point of the electrode and Soil

3 Soil 1

2

3

TYPES OF ELECTRODE

1. Plate electrode

2. Mesh electrode

3. Cast Iron Pipe electrode

4. G.I. Pipe electrode

5. Rod electrode

6. Strip electrode

7. Chemical electrode

DISPERSION FROM ELECTRODE

As electrode offers less resistance to

The flow of current compared to soil,

It is better to have one of the dimensions

Stretched in a given area of dispersion.

Hence rod/ pipe grounding may be preferred compared to plate

Grounding

PLATE ELECTRODE

Sizes of Plate electrode are 1.2m X 1.2m, 0.9m X 0.9m

and 0.6m X 0.6 m. Minimum size of plate should be

0.6m X 0.6m

The Resistance practically achieved is proportional to the

Linear dimension to the electrode.

Resistance achievable by different electrode sizes are,

1.2m X 1.2m = soil resistivity / 2.75



Careful

Plate electrode corrodes fast hence recommended thickness

Cast Iron = 12.00 mm

G.I = 06.30 mm

Copper = 03.15 mm

INSTALLATION OF PLATE ELECTRODE

600 mm

1500mm

min

Solid Stratum

1500mm

min

600 mm

Rod or Pipe electrode

The size of cast iron pipe used is 100mm dia, with a 13mm thickness and 3m length.

The size of GI pipe used is 38/50mm dia and 3m length

The size of solid rod used is 13, 16, 19mm having length of 1.2m, in set of 2 or 3

The choice to the electrode is based on Economy of driving it in & Space available

Rod or Pipe Electrode

Reduction of soil resistivity

13mm 16mm 25mm 100mm

1.0 m

1.2 m

2.0 m

2.4 m

3.0 m

3.6 m

0.91 0.590.810.88

0.78 0.76 0.70 0.51

0.51 0.49 0.46 0.35

0.44 0.42 0.39 0.30

0.36 0.35 0.33 0.25

0.31 0.30 0.25 0.22

Comparative Analysis

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.0m 1.2m 2.0m 2.4m 3.0m 3.6m

13mm

16mm

25mm

100mm


Parallel Electrodes

When number of rods are connected in parallel, the resultant is the reciprocal of the rods connected.

Parallel electrodes should be outside the resistance area of each other.

Mutual separation shall more than the depth of the driven electrode

1.2 m1.2 m min

Soil resistivity of 35 ohms

13mm

rod1 rod R= 27.0

2 rod R = 13.5

3 rod R = 09.0

Care taken during parallel electrode installation

Strip Earthing

The sizes of strip electrode generally used are 25 X 3, 50 X 6, 75 X 6 flats and 70 sqmm round bare cables.

Where the sub stratum is very hard and going deep does not help in lowering of resistance, strip electrode is an effective solution

A ready guide for using a strip electrode is presented to you.

The maximum drop in resistance is in the first 50m.

HARD ROCK

600mm

Comparative Analysis

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

3m 6m 10m 20m 50m

25 X 3

50 X 6

75 X 6

70 sq mm


Material of Electrode

Most corrosive but accepted electrode material are cast iron, wrought iron, mild steel etc.

Z-90 grade GI has much better life compared to bare material

Most preferred material of electrode is copper

In many cases molecularly bonded copper over steel is being effectively used.In case of molecularly bonded copper 250 micron thickness of copper is needed over steel.

In case the installation is protected by cathodic protection, the material used for grounding should have the same galvanic voltage as that of the cathodically protected installation. Such material may be selected referring the galvanic series. Please take care that Copper may not be suitable in this case

Contact between the Earth and Electrode

The earth electrode should be thrusted into the ground and not loosely driven.

Compaction of earth is essential for pre-bored pits

In case of a heavy short circuit, the moisture from the neighboring soil may evaporate due to heat. Resulting in infinite resistance and eventual failure.

1

2

3

Use of conductive cement

100mm around the electrode

can be very useful

Resistivity Values

Change in Resistance with Salt and Moisture

What we really need is about 20% moisture by weight

The maximum amount of ionic substance needed is about

5% by weight

Change in resistance with Temperature

This table clearly shows that we need to be careful of

as the water in the soil freezes and introduces very high

Temperature coefficient.

Hence the Electrode needs to be buried 2m below the

surface

Why is artificial Treatment needed

To maintain the ionic level of the soil for long

To maintain the moisture content

To have inner compaction

To constantly diffuse into neighboring soil and increase the resistance area of the electrode

TEREC +

Miracle Compound for

maintenance free Earthing

Horizontal Setting up

Vertical setting up with tubular electrode

Vertical setting with copper / steel rods

More than 1000,000 million installations worldwide

Repeatedly used by Defense, Airports, Research Organizations, Space centers and petrochemical refineries and Power houses

It is the latest patent in the world of Earthing

DRDO, BEL, RBI, BPCL, NPCL, HCL, Hi-tech IT units are already in our list of Indian Clients

AVAILABLE IN INDIA

FOR HT LINES, 25 MM GI PIPES OF LENGTH 1.8 TO 3 M SHALL BE USED AS EARTH ELECTRODES.

COIL EARTHING FOR LT LINE MAY BE PROVIDED WITH NO. 6 OR NO. 8 GI WIRE OF LENGTH 10 TO 25 M CLOSELY WOUND INTO A COIL OF DIAMETER 5 CM TO 10 CM, AT A DEPTH OF 1.5 M FROM GROUND LEVEL.

RUNNING A SEPARATE EARTH WIRE IS NOT IN VOGUE FOR LT LINES .HT LINES ARE NOT PROVIDED WITH EITHER NEUTRAL WIRE OR EARTH WIRE.

EARTHED NEUTRAL IS USED IN LT LINES. NEUTRAL IS EARTHED WITH TWO SEPARATE AND DISTINCT EARTH ELECTRODES ( PIPES ) AT THE TRANSFORMER POINT, BOTH IN HT AND LT.

ALL SPECIAL STRUCTURES OR POLES CARRYING TRANSFORMERS, SWITCHES, FUSES ETC. SHALL BE EARTHED. ALL SUPPORTS CARRYING GUARD WIRES SHALL BE EARTHED.

THE OHMIC RESISTANCE OF THE EARTHING OF HT AND LT SHALL BE BELOW 10 OHMS.

EARTHING OF 11 KV AND LT LINES

CORROSION RESULTS IN EARTHING SYSTEM DUE TO MECHANICAL, CHEMICAL OR ELECTROCHEMICAL CAUSES. EARTHING SYSTEM DEGRADES IN THE FOLLOWING WAYS.

THE CONTACT RESISTANCE OF EARTHING MATERIAL WITH GROUND INCREASES DUE TO A FILM OF CORROSION PRODUCT.

THE SURFACE AREA IS REDUCED DUE TO LOSS OF METAL.

THE CONTACT POINTS DEGRADE LEADING TO LESS EFFECTIVE EARTHING.

CORROSION IN EARTHING SYSTEM

R2

R8

R1

R3R4

R5

R6

R7

MEASUREMENTS ARE MADE ALONG A NO OF RADIALS AT DIFFERENT LOCATIONS IN THE STATION SUCH THAT THE WHOLE AREA IN WHICH

EARTHING ELECTRODES / MAT IS LAID IS COVERED

METHODOLGY ADOPTED

TYPICALLY IF THE STATION IS 100 TO 150 MTRS THE SOIL RESISTIVITY READINGS MAY BE TAKEN FOR A PROBE SPACINGS

OF 1 , 2, 5, 10 , 15, 25 AND 50 MTRS

SPACING BETWEEN THE PROBES WHICH ARE HAMMERED INTO THE SOIL BE VARIED RADIALLY FOR TAKING DIFFERENT READINGS

A FEW DROPS OF WATER MAY BE POURED IN THE NEIGHBOURHOOD OF PROBES TO GET GOOD CONDUCTIVE

CONNECTION BETWEEN PROBE AND THE SOIL SURROUND IT.

MEASUREMENT OF SOIL RESISTIVITY

THE BURIED METALLIC PIPES IN THE NEIGHBOURHOOD

AND RECENTLY FILLED UP SOIL WILL AFFECT THE SOIL

RESISTIVITY READINGS


AS MAGNITUDE OF SPACING B/W PROBES IS INCREASED FROM

SMALL VALUE TO HIGHER VALUE THE MEASURED SOIL RESISTIVITY

REFLECTS THE EFFECT OF SOIL AT DIFFERENT DEPTHS

TWO COMMONLY USED SOIL MODELS ARE

UNIFORM SOIL AND TWO LAYER SOIL MODEL

UNIFORM MODEL IS CHOOSEN IF THE MEASURED SOIL RESISTIVITY

VALUES VARY WITHIN 30 % OF AVERAGE VALUE

CASE-1 RESISTIVITY OF UPPER LAYER MORE THAN LOWER LAYERS

Rg ( Uniform layer value ) < Rg obtained

( Etouch & Estep) ( Uniform layer value ) < (Etouch & Estep) obtained

CASE-2 RESISTIVITY OF UPPER LAYER LESS THAN LOWER LAYERS

Rg ( Uniform layer value ) > Rg obtained

( Etouch & Estep) ( Uniform layer value ) > (Etouch & Estep) obtained

MOISTURE

FACTORS DETERMINING SOIL RESISTIVITY

DISSOLVED SALTS

GRAIN SIZE AND DISTRIBUTION

TEMPERATURE

SEASONAL VARIATION


CURRENT MAGNITUDE

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 TO MOISTURE,TEMPERATURE,SALT

Effect of Salt Moisture and Temperature on Soil Resistivity

88

Safety to operating personnel

by limiting step & touch potential

To provide a sufficiently low-resistance path to ground to minimize rise in ground potential with respect to remote ground for proper functioning of the protective devices of the substation

Healthiness of the power equipments by providing ground connection for transformers, reactors and capacitors

To provide path for lightning rods, arresters and similar devices

To provide a means of discharging and de-energizing equipment in order to proceed with maintenance on the equipment

The objectives of earthing system :-The objectives of earthing system :-

89

Magnitude and Time of fault current Earthing Conductor Material Earth Electrode Resistivity of Soil Resistivity of Surface Insulation Material (gravel) Design Methodology

The Parameters which influence the earthing

system :-

The objectives of earthing system :-

90

Earthmat of substation shall be suitable for the expected maximum current (including expected increase in future expansion)

Time of current clearing shall be such that it covers the time of back-up protection

Shock duration (0.5 sec) shall be such that the human body can tolerate the intended current passing through the body.

Maximum fault current can be obtained from Power System Studies. Future expansion shall also be considered.

The worst fault current is the equipment short Ckt current rating which is normally higher than power system fault current

Magnitude & Time of Fault Current:-The objectives of earthing system :-

91

Size of earthing conductor shall be suitable for the worst fault current with 1 sec as fault clearing time

Normally for conductor sizing, the equipment short Ckt current rating is considered.

Earthing Conductor:-

92

For MS Rod conductor & corrosion allowance of 0.12mm /yr for 40 years

Earthing Conductor size:-

1.0 Magnitude of Fault Current 31.5kA 40kA 50kA 63kA

2.0 Duration of fault Current (Sec) 1 1 1 1

3.0 Minimum Area of the earth

conductor (sq.mm)

406 515.5 664.4 811.9

4.0 Minimum Diameter of the

earthing conductor (mm)

22.7 25.6 28.6 32.2

5.0 Diameter of the conductor with

corrosion allowance (mm)

32.3 35.2 38.2 41.8

93

Safe Current for human body

Current Range Effects on Humans

1 mA Threshold of perception 1-6 mA Let go currents 9-25 mA Pain full, hard to let go 25-60 mA Muscular contractions 60-100 mA Ventricular fibrillation

Maximum Body Current: Ik = for t = .03s to 3s0,116t

Potential Rises during fault

94

95

Touch Potential & Step Potential Tolerable

0,11

6t

Touch potential:- The potential difference between a point on the ground and a point on an object likely to carry fault current (e.g., frame of equipment) which can be touched by a person

Step potential: The potential difference shunted by a human body between two accessible points on the ground separated by a distance of one pace assumed to be equal to one meter

Touch voltage circuit

Mesh potential: The maximum touch potential within a mesh of the grid.

Transferred potential: A special case of touch potential where a potential is transferred into or out of the sub-station

98

For Safe Design,

(i) Attainable Touch Potential shall be less than Tolerable Touch Potential

(ii) Attainable Step Potential shall be less than Tolerable Step Potential

(iii) Earth Potential Rise (EPR) shall remain within permissible limit

(iv) For most transmission and other large substations, the ground resistance is usually about 1 or less. In smaller distribution substations, the usually acceptable range is from 1 to 5 , depending on the local conditions.

99

For Safe Design,

i. In any switch yard, chances of exposure to Touch potential is higher than that to step potential.

ii. Resistance offered by the feet of a person against Touch potential is much less compared to that against Step potential.

iii. Hence Touch potential is more critical for design while Step potential is usually academic.

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

v. For 400% increase in diameter, reduction in Touch potential is only 35%.

vi. Thus cross- section has minor influence on Touch and Step potentials.

vii. Length of earthing conductor has significant effect on Touch and Step potentials.

100

For Safe Design,

(i) Attainable Touch Potential shall be less than Tolerable Touch Potential

(ii) Attainable Step Potential shall be less than Tolerable Step Potential

(iii) Earth Potential Rise (EPR) shall remain within permissible limit

(iv) For most transmission and other large substations, the ground resistance is usually about 1 or less. In smaller distribution substations, the usually acceptable range is from 1 to 5 , depending on the local conditions.

101

For Safe Design,

i. In any switch yard, chances of exposure to Touch potential is higher than that to step potential.

ii. Resistance offered by the feet of a person against Touch potential is much less compared to that against Step potential.

iii. Hence Touch potential is more critical for design while Step potential is usually academic.

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

v. For 400% increase in diameter, reduction in Touch potential is only 35%.

vi. Thus cross- section has minor influence on Touch and Step potentials.

vii. Length of earthing conductor has significant effect on Touch and Step potentials.

102

Soil Resistivity varies with type of soil, temperature, moisture contents and climatic condition

Measurement of soil resistivity shall preferably be done in dry season

Maintaining accuracy in soil resistivity measurement is difficult

Analysis of soil resistivity with the type of soil may be necessary

In case of +30% variation, two layer soil modeling helps in correct modeling and optimal design

Measurement of soil resistivity in eight directions with Wenners four electrode method is better

A non-accurate soil resistivity will lead to unsafe earthing

Soil Resistivity

103

Gravel resistivity is generally considered as 3000 ohm.m in the design though the range of gravel resistivity is 1000 10000 ohm.m

Considering of higher gravel resistivity (>3000 ohm.m) means withstanding of higher touch & step voltages

As the gravel resistivity also changes with environmental condition, a lower value of gravel will lead to risk of limit of step and touch potentials

Hence, measures such as integration of gravel with a P.C.C layer under the gravel may be applied.

Requirement of gravel for future equipment area shall examined w.r.t the requirement of step voltage.

More shock duration means less the withstanding voltage

Tolerable Touch & Step Potentials

104

Equipment Area has to be graveled as the design is done for the same.

Requirement of gravel in future area of the substation with no equipment shall be seen from possible rise in step potential. The step voltages (tolerable & attainable) shall be calculated (which is different with consideration of gravel) without gravel, gravel resistivity is equal to soil resistivity if empirical formula are applied.

Gravel shall also to be laid 2m away from fencing to ensure that if a person touches the fence, he should stand on the gravel.

Even spreading of gravel may be reviewed if the design is safe and have enough confidence.

Area of Gravel spreading

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 withsuitable chemicals.

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

Satellite Earthmat

The diversion of fault current through the main earth mat.

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

References1. IEEE guide for AC Substation Grounding ( IEEE 80)

2. IEEE guide for Measuring earth Resistivity, Ground

impedance, and earth surface potentials for a ground

system(IEEE 81)

3. IEE recommended practice for grounding industrial and

commercial power systems ( IEEE 142)

4. IEEE Guide for generating station grounding(IEEE 665)

5. Indian standard specifications ( 3043 Earthing)

Thank You

grounding & earthing.pdf

Documents