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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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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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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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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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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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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SUB STATION EARTH


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SUB STATION EARTH


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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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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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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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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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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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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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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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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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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

Neutral Earthing Final

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