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7/30/2019 Neutral Earthing Final
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SYSTEM NEUTRAL EARTHING
THE PURPOSE OF EARTHING
A number of points from Generators to consumers installations are earthed (orGrounded).
In our country we have an effectively grounded system.
The purpose ofearthing is:
To provide a low impedance path to facilitate satisfactory operation ofprotective schemes during fault conditions.
To ensure that living beings in the vicinity of sub stations are notexposed to unsafe potentials under steady state or fault conditions.
To retain system voltages within reasonable limits under faultconditions and ensure that insulation breakdown voltages are notexceeded.
To provide an alternative path for induced current and there byminimize electrical noise in communication circuits.
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SYSTEM NEUTRAL EARTHING
UNGROUNDED SYSTEM
A system, circuit, or apparatus without an intentional connection toground.
Though called ungrounded, this type of system is in reality coupled to
ground through the distributed capacitance of its phase windings and
conductors.
In the absence of a ground fault, the neutral of an ungrounded systemunder reasonably balanced load conditions will usually be close to earth
potential, being held there by the balanced electrostatic capacitance
between each phase conductor and earth.
GROUNDED SYSTEM
A system of conductors in which at least one conductor or point(neutralpoint of transformer or generator windings) is intentionally grounded,
either solidly or through an impedance.
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SYSTEM NEUTRAL EARTHING EFFECTIVELY GROUNDED SYSTEM
Grounded through a sufficiently low impedance suchthat for all system conditions the ratio of zero sequencereactance to positive sequence reactance (X0 / X1) is
positive and less than 3, and the ratio of zero sequenceresistance to positive sequence reactance (R0 / X1) ispositive and less than 1.
The effectively grounded system permits the applicationof surge arresters with less than line- to- line voltageratings. Ground fault currents will be approximately ofthe same magnitude as threephase fault currents.
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SYSTEM NEUTRAL EARTHING Advantage of neutral grounding
The system is not subjected to over voltage surges due toarcing grounds during earth faults as the current flowingthrough the neutral to ground connection will be almostequal and opposite to the capacitive current from healthy
lines to ground.
During a line to ground fault the voltage of healthy line donot increase beyond 3 times thenormal value.
Earthing the neutral point secures maximum effectivenessof automatic protective gear immediately after an earthfault occurs.
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SYSTEM NEUTRAL EARTHING
Advantage of neutral grounding
Earth fault relaying is simple and isolation of faulty circuit is quickand fast.
Earth fault can be located easily as compared with ungroundedsystem.
Improved service reliability.
Due to stable neutral point, the voltages are held nearer thedeclared voltages.
The over voltage due to lightning is discharged to earth as an easyearth path is provided.
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SYSTEM NEUTRAL EARTHING
Advantage of neutral grounding When solidly earthed, the voltage of any live conductorcannot exceed the voltage from line to neutral and neutral is
held at earth potential(almost zero).
It is possible to effect appreciable reductions in the insulation
to earth of cables and over head lines and hence saving in
cost.
Similar insulation reductions can be made in major insulation
of transformers by grading down from line terminal to the
neutral point.
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SYSTEM NEUTRAL EARTHING
Advantage of neutral grounding In EHV systems, on account of their electrostatic capacitance,the voltage of the two sound lines may, at the first instant of
the fault, reach a value approaching twice the normal line
voltage.
If the neutral point is earthed, the induced static charges on
lines are conducted to earth as they appear, and all danger to
the insulation of the line and apparatus is removed.
Therefore, no part of the system can reach more than line to
neutral voltage. The insulation of line can be limited to the
line to neutral voltage.
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SYSTEM NEUTRAL EARTHING
EQUIPMENT GROUNDING
Equipment grounding is different from system neutral grounding.
Equipment grounding is to ground the non-current carrying parts withground connection.
The objective of equipment grounding is ;
1. To ensure freedom from dangerous electric shock voltage exposureto persons in the area.
2. To provide current carrying capability both in magnitude andduration, adequate to carry ground fault current permitted by overcurrent protective system without creating a fire or explosive hazard.
3. To contribute to better performance of the electrical system.
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SYSTEM NEUTRAL EARTHING
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SUB STATION EARTH
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SYSTEM NEUTRAL EARTHING
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SUB STATION EARTH
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SYSTEM NEUTRAL EARTHING
Causes of over voltages
Neutral displacement
When a line to ground fault occurs, even on a solidlygrounded system, the line to ground voltage of the healthy
phases increases because of the neutral displacement.
For effectively grounded systems, during single line to groundfault, the voltage rise of the healthy phase (phase to neutral)
is up to 80 % of the line to line voltage.
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SYSTEM NEUTRAL EARTHING
Causes of over voltages
Operation of switching and over current protective
equipment.
The operation of switches and over current protective devices
produces a short time transient over voltage or switching
surge.
The normal operation of these devices will not produce
voltages exceeding twice normal voltage.
During switching of capacitors the voltage may rise to 2.5
times the normal due to re-striking phenomenon
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SYSTEM NEUTRAL EARTHING
Causes of over voltages Resonance effects
Ferro resonance is another potential source of
transient over voltages. In three phase circuits, singlephase switching can result in over voltages when
ferroresonance occurs between the magnetising
impedance of a transformer and the system
capacitance of isolated phase or phases. The term ferro is used because of the steel core of
the transformer.
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SYSTEM NEUTRAL EARTHING
Causes of over voltages There is an external cause for over voltages due to
the atmospheric conditions of lightning discharges-
inter clouds, intra clouds and cloud to ground or line.
These may be in the form of direct lightning strokes,and indirect lightning, i.e., induced strokes.
Basically, lightning is a gigantic spark resulting from
the development of millions of volts between cloudsor between a cloud and the earth. It is akin to the
dielectric breakdown of a huge capacitor.
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A: Coupling driven effects B : Direct effectsC: Increases in site potential D : Direct conducted effects
Effects of Lightning
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SYSTEM NEUTRAL EARTHING INSULATION CO-ORDINATION
Transformers are subjected to over voltages.
The insulation level of a transformer should co-ordinate with theprotective level of the surge arrester.
Neutral grounding at each voltage level from generation to distributionis necessary from the view point of over voltages.
The transformers experience the over voltage at power frequency andalso impulse voltage due to lightning and switching surges.
Switching transients and travelling waves are the major cause for overvoltages.
The surge arresters (lightning arresters) divert the transient overvoltages to earth and protect the transformer insulation.
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SYSTEM NEUTRAL EARTHING
INSULATION LEVEL OF A TRANSFORMER
Basic insulation level is a term which includes the
following characteristics of a transformer;
1. Power frequency voltage withstand level
2. Lightning impulse voltage withstand level
3. Switching impulse voltage withstand level
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SYSTEM NEUTRAL EARTHING
Insulation co-ordination is correlation ofinsulation of equipment and circuit with thecharacteristics of protective devices such that theinsulation is protected from over voltages.
The surge arrester provided for transformerprotection should spark over at a voltage lessthan the insulation withstand voltage of the
transformer. In other words , the protectivedevice must have a lower break down voltagethan insulation to be protected.
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INSULATION COORDINATION
Insulation coordination is the process ofbringing the insulation strengths of electrical
equipment into the proper relationship with
expected over voltages and with the
characteristics of surge protective devices.
The insulation withstand level of equipment is
usually defined at three points through the
use of the standard switching surge and the
full wave(basic impulse insulation level BIL -)
and the chopped wave tests.
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Intrument Transformers
Lightning impulse test - Full wave (1.2/50 ms)
100 %
T
90
30
T1
T1=1,67 x ( t90 - t30 )
50
T2
U
t
FrontQueue
Simulates a lightning stroke
overvoltage travelling along a HV
transmission line
Lighting impulse
BIL
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Intrument TransformersSwitching impulse test (250/2500 ms)
Tcr= 250 ms
T2 = 2500 ms
U
t
100 %
Tcr
Tc
50
T2
90
Simulates
overvoltages resulting
from line switching
operations
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Intrument TransformersLightning impulse (chopped wave)
U
t
100 %90
30
Tc
a
Imulse wave is
chopped byusing sparking
gaps after 2
5 ms
Simulates a flash over on an
insulator resulting from a
lightning stroke
f
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Instrument transformers
Lightning impulse - chopped on the front
U
t
100 %90
30
Tc
Tc < 1,2 ms
Simulates a direct lightning on the
transformer. The quick voltage
increase provoke a floshover.
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INSULATION COORDINATION
The insulation withstand of electrical
equipment is generally quantified in terms of
its basic impulse insulation level (BIL) which is
based on tests made with a voltage surgerising to crest in 1.2 micro seconds and falling
to half value in 50 micro seconds, which is
denoted as a 1.2/50 wave. The three point method is usually applied for
insulation coordination.
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INSULATION COORDINATION
The minimum protective ratios recommended
by ANSI and IEEE standards for satisfactory
coordination are
Switching-surge withstand
------------------------------------------------ >= 1.15
Switching surge protection level
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INSULATION COORDINATION
BIL
------------------------------------- >= 1.2
Lightning protection level
Chopped wave withstand------------------------------------------ >=1.2
Front-of-wave protective level
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SYSTEM NEUTRAL EARTHING STANDARD VALUES OF WITHSTAND VOLTAGES
Normal voltage kV Highest system
voltage kV
Impulse withstand
standard wave kV
(peak)
One minute power
frequency
withstand kV (rms)
11 12 75 35
33 36 170 75
66 72.5 325 140
110 123 550 230
132 145 650 275
220 245 1050 450
400 420 1550 680
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THANK YOU