earth grd note

8/18/2019 Earth Grd Note

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Reliance Energy Limited(A Dhirubhai Ambani Enterprise)

Course

on

E RTHING

SYSTEM

By

Dr. K. Rajamani

On 4th and 11th February ’ 2005

Reliance Energy Center, Santacruz, Mumbai – 400 055, Tel – (022) 3009 9999


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E RTHING


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EARTHING OF ELECTRICAL SYSTEM

Introduction

Definition of Earthing and Grounding:

• Grounding implies connection of power system neutral to ground (earth). e.g.

neutral grounding / system grounding. In grounding current carrying parts are

connected to ground.

• Earthing implies the connection of non current carrying parts to ground e.g

metallic enclosures. Another term for earthing is equipment grounding.

Earthing is done for human and equipment safety.

Human Element

• Electric 'shock' is possible only when the human body bridges two objects ofunequal potential. Current flows when potential difference exists between

hand and feet (touch potential), or between feet (step potential).

TRANSFORMER GENERATOR

NG NG

EARTHING


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• Maximum tolerable current for a human body is approximately 160 mA for

one second duration (i.e. if current through body exceeds about 160 mA, for

more than a second, almost certain death, due to ventricular fibrillation or

heart attack).

• Allowable body current ΙB (Ampere) as per IEEE Standard 80 is as given

below:

ΙB = 0.116 / √TS, for a body weight of 50 Kg

ΙB = 0.157 / √TS, for a body weight of 70 Kg

where TS is the duration of current exposure (fault clearance time).

• For various exposure times, the withstand currents of human body are as

follows:

TS B (50 Kg) B (70 Kg)

0.2 sec 259 mA 351 mA

0.5 sec 164 mA 222 mA

1 sec 116 mA 157 mA

Thus human body can withstand higher current for shorter time duration. The

advantage high-speed protection is evident from human safety point of view.

• Average value of human body resistance RB is approximately 8000 Ω to 9000

Ω (under dry conditions). For standards purposes, RB is taken as 1000Ω as

per IEEE Std 80. Use of lower RB value results in conservative values for

allowable touch and step potentials.


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Soil Resistivity

Resistivity ( ρ) of element:

• R = ρ L / A ⇒ ρ = R A / L ⇒ Ω M2 / M = Ω M ; where ρ - Resistivity of

element, R – Resistance of element, L – Length, A – Area,

• Earth is a not a good conductor. In fact it is one of the worst equipment-

grounding conductors. The comparison of resistivity is given below:

Material ResistivityEarth 100 ΩM

GI 10-7

ΩM

Copper 1.7 x 10-8

ΩM

• Soil resistivity ( ρ in Ω M ) is value of resistance in Ω of 1M cube. Soil

resistivity for different types of soil is given below:

Type of Earth Wet Soil Moist Soil Dry Soil Bed Rock

Resistivity ( - M ) 10 100 1,000 10,000

Effect of moisture on soil resistivity:

• Soil resistivity ρ rapidly increases for moisture content of less than 10 % of

soil weight. There is marginal decrease in soil resistivity for moisture content

exceeding 25% of soil weight.


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Effect of salt on soil resistivity:

• Minute amount of salt causes sharp decrease in soil resistivity provided there

is moisture content of say 10%. Salt when added to dry soil gives hardly any

improvement in the resistivity value.

• Substances used for improving soil resistivity are sodium chloride (common

salt), copper sulphate, calcium chloride and magnesium sulphate. To account

for corrosion, electrode size is increased from calculated value.

Corrosion intensity:

• Soil classification based on corrosion intensity is given below:


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ρ of virgin soil M 100

Corrosion Intensity Severe Moderate Mild Very Mild

• Alternative substances used for earthing are coke or wood charcoal and

Bentonite. Coke is less corrosive compared to salt. Bentonite is a natural clay

and non-corrosive and needs watering periodically.

• Performance over time for earth resistivity is shown below:

After treatment there is initial decrease in ρ. However there is gradual

increase in soil resistivity ρ with time as the salt is washed away by continual

water seepage. Hence, re-treatment is typically required to be carried out

once in 3 years.

Effect of Temperature on Soil Resistivity:

• Soil resistivity decreases with increase in temperature. In summer ρ is less

and in winter it is more. However, effect of temperature on ρ is not serious

until freezing point is approached. Near 0°C, ρ abruptly rises to a very high


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value. It is preferable to place electrodes well below ground level. Surface

temperature may be freezing but below 1M, soil temperature will be higher.

Soil resistivity measurement:

• Soil resistivity measurement setup is as shown below. AC supply source is

preferred for the set up as compared to the DC supply source.

• Resistivity measured for spacing ‘A’ represents apparent soil resistivity todepth of 'A'. Measurements are made with different spacings. Rapidly

increasing value of ρ with spacing 'A' indicates underlying stratum is rock and

it is difficult to install earth electrodes to great depths.


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Resistance to earth

• Resistance between metal of electrode in question and general mass of earth

is known as earth resistance. It is resistance between specific electrode and

imaginary electrode of zero resistance placed at infinity. 90% of resistance is

contributed by earth within 5 meters distance.

Resistance vs Distance:

• Resistance to earth of hemi-spherical electrode is shown below:

dX

A

X


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R = ρ L / A; RX = ∫ ρ dX / 2 Π X2

Resistance area of tube or rod electrode:

• Resistance area is the region of earth that immediately surrounds the

electrode and contributes to practical value of resistance.

• Current flows away from electrode in all directions and through a series of

shells of earth of continuously increasing cross section. At sufficient distance

from earth electrode, shells approach hemi-spherical shape. Hence resultsgiven earlier for hemi-spherical electrode are also nearly valid here.

Resistance of electrode to earth here also is predominantly influenced by

earth with in the vicinity of electrode. This justifies artificial treatment of soil in

the immediate neighborhood of soil to achieve low resistance between

electrode and earth.

Resistance of Driven Rod or Pipe Electrode:

L : depth of Driven Rod in met

Φ : diameter of Driven Rod in met

R = (ρ / 2 Π L ) [ LN (8L /{Φ x 2.7183 })]

R ≅ (ρ / 2 Π L ) [ LN (4L / Φ )]L


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Length / diameter of rod vs Resistance is given below:

The diameter of the rod has minor influence on the resistance. Length of the rod has

major impact. For lengths more than 3M, resistance is almost the same even if

diameter is increased by four times.

Resistance of Horizontal Wire (Strip) Electrode:

If excavation is difficult beyond a meter due to underlying rock, strip electrode is the

alternative. The earth electrode is as shown below:

RESISTANCE OF ROD ELECTRODE

0

50

100

150

200

250

300

0 1 2 3 4 5 6 7 8 9 10

LENGTH OF ROD, MET

R E S I S T A N C E , O H M S

2.5 CM

10 CM

DIAMETER : 2.5, 5, 7.5, 10 CM

RHO - 100 OHM-MET

ROD LENGTH: 6M

R10 = 15.3

R2.5 = 16.4 φ ⇑ 300%

R⇓ 7%

RESISTANCE OF ROD ELECTRODE

0

50

100

150

200

250

300

0 1 2 3 4 5 6 7 8 9 10

LENGTH OF ROD, MET

R E S I S T A N C E , O H M S

2.5 CM

10 CM

DIAMETER : 2.5, 5, 7.5, 10 CM

RHO - 100 OHM-MET

ROD LENGTH: 6M

R10 = 15.3

R2.5 = 16.4 φ ⇑ 300%

R⇓ 7%


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Exact Formula (RYDER)

R = (ρ / 2 Π L) [LN(8L/T) + LN(L/h) – 2 + (2h/L) – (h2 / L2)]

Where, h is depth in Met, L is length in Met.

T : width in Met (for strip)

: 2 x diameter in Met (for wire)

Approximate Formula:

R = (ρ / 2 Π L) [LN( 2L2 / hT )] { IS 3043 }

• Length / diameter of wire Vs Resistance variation is shown below,

• The diameter of the rod (width in case of strip electrode) has minor influence

on the resistance. Length of the rod has major impact on resistance value.

For length more than 50M, resistance is almost the same even if diameter

is increased by four times.

• Resistance of electrode to earth is only influenced by maximum dimension of

electrode, i.e. depth in case of rod electrode and length in case of wire

electrode. It is not much influenced by minor dimensions like diameter or

width. It is not dependent on material of electrode. It is the function of physical

dimensions of the electrode and not its physical properties.


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• The cross section of strip with width 'W' is approximately equivalent to round

conductor with diameter of 'W / 2'.

W

W/2

Plate electrode:

• In early days only plate electrode were used. It was assumed that to get low

resistance, surface area of electrode be increased. The fallacy of increased

electrode surface area persisted for a long time. But as can be seen from

following figures, plate electrode is very inefficient. It is rarely used in modern

times.


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Two electrodes in parallel

• To obtain low effective earth grid resistance, earth rods are connected in

parallel. For achieving minimum resistance, resistance area of each electrodemust be clear of one another. Theoretically, the effective resistance shall be

half of one electrode. If electrodes are well separated, this can be achieved.

The figure below shows the relation between percentage effective resistance

and separation between electrodes in meters.

• If rod length is 'L' meters, spacing between electrodes shall be greater than

2L meters, as shown below.

L

2L

Three electrodes in parallel

Theoretically, the effective resistance shall be 33% of one electrode. If electrodes

are well separated, this can be achieved. The figure below shows the relation

between effective resistance and separation between electrodes in meters.


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• If rod length is 'L' meters, spacing between electrodes shall be greater than

2L meters.

2

L

1 3

1 2 3 > 2L

Grounding Grid Resistance

• Previous discussions centered around resistance to earth from individual

electrodes. Present discussion is on resistance to earth from entire grounding

grid.

Sverak formula

C1 = 1 / L ; C2 = 1/√(20A); C3 = 1 + h √(20A)

RG = ρ [ C1 + C2 {1 + (1 / C3)} ]

Where,

RG = Grid resistance to earth in Ω

h = Depth of grid in m

ρ = Average earth resistivity in Ω Μ

A = Area of grounding grid M2

L = Total length of buried conductor, including rod electrodesin meters


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• Example of rectangular grounding grid is shown below:

Area of Grounding grid = A = 80 x 50 = 4000 M2

Horizontal electrodes length = LH = (9 x 50 ) + (6x 80) = 930 M

Vertical rods length = LV = 18x 6 = 108 M

L = LH + LV = 930 + 108 = 1038 M

H = depth of grid = 0.5 M

ρ = soil resistivity = 100 Ω - M

Applying Sverak Formula, RG = 0.79 Ω Caution in using formula for individual electrodes is illustrated below :

• For Vertical rod electrodes:

Soil resistivity ρ = 100 Ω M ; L = 6 M; Φ = 0.05 M ( ≅2")

Formula for individual vertical rod:

R = (ρ / 2 Π L ) [ LN (8L /{Φ x 2.7183 })] = 15.5625 Ω

For 18 rods in parallel, RV = 15.5625 / 18 = 0.8646 Ω

• For Horizontal Electrodes:

Soil resistivity = ρ = 100 Ω M; L = LH = (9 x 50 + 6 x 80) = 930 M

T = 0.1M ; h = 0.5 M

Applying Ryder's formula for horizontal electrodes

R = (ρ / 2 Π L) [LN(8L/T) + LN(L/h) – 2 + (2h/L) – (h2 / L2)]

= 0.2866 Ω


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• Thus effective Grid Resistance = R'G

= RV in parallel with RH

R'G

= ( RV R

H ) / ( R

V + R

H ) = 0.2152 Ω

But as per Sverak formula for entire grid, equivalent resistance = RG

= 0.79 Ω .

• Thus RG is very much greater than R'G. This is due to the fact that resistance

areas of electrodes are not independent and partially overlap.

• Sverak formula for grounding grid resistance does not involve conductor size

or material at all. It involves only linear dimension (length of horizontal or

vertical electrodes) that makes it very special.

Measurement Of Earth Electrode Resistance by “Fall Of Potential” method.

It is also called “Two-current and one potential electrode” method.

In the above figure,

• C_E_T implies current electrode under test. It can be a single electrode or

earthing grid whose resistance to earth is to be measured. Test current ('Ι')enters C_E_T.

• C_E_R implies reference electrode placed at sufficient distance ('L') from test

electrode. Test current ('Ι') leaves C_E_R.

• V_E implies Voltage electrode.


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Voltage ('V') between C_E_T & V_E measured

Electrode (grid) resistance is given by R = V / Ι Ω

Take three measurements with,

L_P = L / 2

L_P = L / 2 + D

L_P = L / 2 – D

L = 100 to 500 M, D = 5 to 10 M

If three readings agree within tolerable accuracy, electrode (grid) resistance is the

mean of the three readings. If the three readings are not sufficiently close, increase

spacing 'L’ between test electrode and reference electrode and repeat the test.

Overlapping resistance areas

Non-overlapping resistance areas

X-Y Distance

R e s i s t a n

c e

Reading Variation

X Y1 Y Y11 Z

Effective Resistance

Areas (No Overlap)

X Y1 Y Y11 Z

Reading Variation

X-Y Distance


Areas (Overlapping)

R e s i s t a n c e

X Y1 Y Y11 Z

Reading Variation

X-Y Distance


Areas (Overlapping)

R e s i s t a n c e


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Electrode Sizing

• Materials generally used for earth electrodes are Galvanized Iron, Copper

and Aluminum. Current ratings for above materials are given below in A /

mm2 (as per IS 3043):

Material GI Copper Aluminum0.5 sec Rating 113 290 178

1 sec Rating 80 205 126

3 sec Rating 46 118 73

For EHV switchyards, electrodes are designed for 0.5 sec duty. Primary

protection clears fault within 0.1 sec. Back up protection operating time isabout 0.5 sec. For electrodes other than those in EHV switchyards, design

duty is for 1 sec. Rating of 3 sec is rarely used.

Example

The fault current magnitude is 40 kA. The duration of fault is 0.5 sec. The electrode

material is GI.

Minimum cross section:

113 A - 1 mm2

40 KA - 40,000 / 113 = 353 mm2

Taking corrosion allowance as 10%,

Desired cross section = 353 x 1.1 = 388 mm2

Chosen size: 50 x 8 mm

General Formula for Electrode rating in Amps / mm2 is K / √TWhere,

K implies constant defined for 1 sec duty (e.g. 80 for GΙ )

T implies time considered for grid design (e.g. 0.5, 0.7, 1, 3 Sec )

Considering mechanical strength and ruggedness requirements, minimum electrode

size shall be greater than 50 mm2 for GΙ and 25 mm2 for Copper.

Resistance of electrode to earth (REL) is independent of electrode material (GΙ, CU,

AL). It is hardly influenced by cross section (e.g. REL not much different if cross


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section is 400mm2 or 600 mm

2 ). In fact, Sverak’s formula for ground grid resistance

does not even involve diameter or cross section!.

Earthing in LV & MV Systems

Following three cases have been considered for illustrating the concepts of earthing

in LV and MV systems.

Case –1: Source grounded – Equipment ungrounded

For easy conceptualization, single-phase network is shown below.

In the above figure, AB indicates source of supply.

C indicates equipment.

Point B is earthed trough earth electrode 'E'

RC indicates equipment load resistance (e.g. 2302 / 1000 = 53)

RΙ indicates equipment insulation resistance (MΩ)

RH indicates resistance of a person (e.g. 2000 ohms)

Under normal conditions, RΙ

is very high. Even if a person touches the body, very

little current flows through him. Under the condition of insulation failure of equipment,

RΙ reduces to 0. Current through body is given by,

ΙH = V / (RH + RE1) = 240 / (2000+1) ≅ 120 mA


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This current is small and not sufficient to blow the fuse, but it can be dangerous to a

person (IEEE Std- 116 mA for 1 sec).

Case –2: Source grounded – Equipment grounded

In the above figure B & C are earthed trough earth electrodes E1 & E2.

In the event of insulation failure of equipment RΙ reduces to 0.

Equivalent resistance REQ = RE2 RH = 1Ω 2000Ω ≅ 1Ω

Fault current, ΙF = V / (REQ + RE1) = 240 / (1+1) = 120 A

Current through body, ΙH = {1/(1+2000)} x 120 ≅ 60 mA

ΙF is significant but not very high. Fuse may or may not blow. But current ΙH, through

body, though low, is not insignificant.

Case –3: Source grounded – Equipment grounded – With Bonding


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Bonding conductor, which is the physical connection between equipment casing and

source, is also called ‘earth continuity conductor’.

Bonding conductor resistance is indicated as RB, which is very small.

Even assuming RB = 0.01 Ω,

Fault current, ΙF = 240 / 0.01 = 24 kA

ΙF is high enough to cause instantaneous fuse blowing. Thus the human safety is

inherently achieved. Hence, it is not reliable to depend solely on earth for return of

fault current. Physical earth continuity conductor (bonding) that runs from equipment

to source is the most reliable conductor for return of earth fault current.

Low Voltage System (415V)

Generally Low Voltage System is solidly grounded as per IE rules. Best earth

electrode resistance is approximately 1Ω. on 415 V system. If only earth is used for

return of fault current,

ΙMAX ≤ (415/√3) / 1 ≤ 240 A

If fault current is limited to 240A, neither over current relay nor fuse will ever operate.

Hence, earth shall not be treated as sole equipment grounding conductor for return

of fault current. We can make many supplementary connections to earth from

equipment. But metallic connection (bonding conductor) must exist between

equipment and source neutral. Majority of fault currents shall be carried by

grounding grid conductor and very little by earth.


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EARTHING IN LV SYSTEM CORRECT METHOD

EARTHING IN LV SYSTEM WRONG METHOD


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Unsafe earthing is shown below where no earth continuity conductor is provided.

Safe Earthing is shown below where earth continuity conductor is provided.

MV (Resistance Grounded System) is shown below:

Even here, earth conductor must run all over the plant and must carry return current

back to NGR.

MV (Ungrounded System) is shown below:


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Even for ungrounded system earth continuity conductor is recommended to be

provided.

`Clean’ Earth:

Correct method of electronic equipment earthing is shown below:


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Wrong method of electronic equipment earthing is indicated below:

Monitoring currents through neutral and ground is recommended:

High neutral current indicates unbalance load. High current through ground

conductor indicates earth fault.


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Two cardinal principles of electronic earthing are as follows:

• Neutral conductor and ground conductor are connected only at the service

entrance. From this point on, neutral and ground conductors should not be

bonded together. i.e. neutral and ground conductors should never touch each

other after leaving service entrance panel board.

• Unbalanced load currents shall return only through the neutral back to the

service entrance. Ground shall carry only fault current and not unbalanced

load currents.

References

• IEEE std – 80 : Guide for safety in AC substation grounding

• IS – 3043 : Code of practice for earthing

• Earthing principles and practices: R W RYDER

• Electrical earthing and accident prevention: Edited by M G SAY


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GROUN ING


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Conversion of 11kV System Grounding

1.0 Reasons for various grounding practices at different voltage levels

Since the ground fault current magnitude is high, the core damage at the

point of fault in rotating machines like generator and motor will be high.

To limit the damage to the core, manufacturers allow only a limited ground

fault current. This information is usually provided in ‘core damage curves’

supplied by manufacturer. A typical core damage curve is shown in Fig 1.

For example, ground fault current upto 25A is tolerated for 1 sec. This

curve is used as a guide when selecting NGR and setting stator earth fault

relays in generator protection.

Winding damages in rotating machines are not of serious concern. The

repairs can be done by local rewinding agency. However in case of

damage to core, repairs can not be carried out at site. The machine has to

be sent back to manufacturer’s works for repair resulting in prolonged loss

of production.

Since rotating machines are not present in voltage levels from 22 kV and

above, usually these systems are solidly grounded. At EHV level solid

grounding is universally adopted for two reasons: (a) cost of insulation at

EHV level is high (b) primary protections clear the fault within 5 cycles and

backup protections clear the fault within a second.

If rotating machines are present at 3.3 kV, 6.6 kV and 11 kV levels, the

systems are grounded through resistor or reactor to limit the ground fault

current. If rotating machines are not present at 3.3 kV, 6.6 kV and 11 kV

levels, the systems are solidly grounded.

In case of LT (415V) system, though rotating machines are present, the

system is solidly grounded to conform to IE rules. Since LT system is also


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handled by ‘general public’, for safety reasons solid grounding is

mandated. Sufficient ground fault current is allowed to flow so that

protective devices can operate and clear the faults at the earliest. Of

course, core damage at the point of fault in rotating machines will be high.

2.0 Present grounding practices adopted in Mumbai distribution

The majority of 20 MVA, 33 kV / 11 kV and 10 MVA, 22 kV / 11 kV

transformers have vector group of Dzn10. The ground fault current of

11kV system is limited to full load current of the transformer (1050A for

20MVA transformer and 525A for 10MVA transformer). Considering the

magnitude of ground fault current, it is classified as Low Resistance

Grounded system. Since rotating machines are not present at 11 kV urban

distribution system, it can be converted to solidly grounded system.

3.0 Disadvantage of Resistance Grounded System

One of the major concerns with regard to non-solidly grounded systems is

over voltages during line to ground faults. In a distribution system, majority

of faults are single line to ground faults (more than 70%). Every time a

ground fault occurs on a particular cable, not only the cable under fault butall cables emanating from that switchgear and associated distribution

transformers suffer the voltage rise. Hence cumulatively all the cables

experience insulation degradation and ultimately failure. Also it shall be

emphasized that as soon as a small resistance is connected from neutral

to ground, the system behaves almost like a ungrounded system from

over voltage point of view. In Fig 2, open delta voltage and phase to

ground voltage are plotted. It can be seen that until the ground fault

current reaches a value close to that obtainable from solidly grounded

system, the over voltage magnitudes are high. The over voltages come

down only when solid grounding is established. Thus conversion to solidly

grounded system will mitigate insulation failure problems.


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We understand that resistance grounded system has been adopted to limit

the ‘through-fault’ current of the transformer in case of an earth fault in

downstream system. This ‘advantage’ of reduced through-fault current of

transformer gets neutralized by the disadvantage of subjecting the

insulation of entire system to repeated over voltages.

Also it may be emphasized that a well coordinated protection system can

clear any internal fault within the transformer within 100 milliseconds and

external ground faults within a maximum of 1 second. These times are

well within the over-current withstand capability of transformers as

specified in standards. Refer Fig 3.Since ground fault current magnitudes are high, selective isolation is more

probable as sensing quantity (current) is substantial. REL has recently

procured state of the art numerical protection relays. If proper relay setting

is adopted, the faults can be cleared at the minimum permissible time.

REL is in the process of installing FPIs (Fault Passage Indicators) on all

11 kV RMUs. If the system is solidly grounded, FPI operation becomes

more definitive.

4.0 Conclusion and recommendation

1. New 33/11 kV substations shall be solidly grounded on 11 kV side.

2. The existing NGRs on 11 kV side shall be bypassed.

3. Before (2) can be implemented following shall be checked:

• Review and revise relay settings for ground relays considering fault

current corresponding to solidly grounded system.

• Review of CT specification for Standby Earth fault protection.

• Test the complete scheme including instrument transformers, over

current relays and unit protection schemes (differential and REF) for

stability and sensitivity verification.

4. Provide Standby Earth fault protection wherever not presently available.


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Fig-1

Fig-2


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earth grd note

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