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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
Effective Resistance
Areas (Overlapping)
R e s i s t a n c e
X Y1 Y Y11 Z
Reading Variation
X-Y Distance
Effective Resistance
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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4 of 5
Reliance Energy Center, Santacruz, Mumbai – 400 055, Tel – (022) 3009 9999
Fig-1
Fig-2
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