surge arrester.pptx

7/28/2019 surge arrester.pptx

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Surge Arrester

M. M. Meraat

Spring 1392


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introduction Surge arresters constitute an indispensable aid to insulation

coordination in electrical power supply systems.

The overvoltages that can affect the power system can be classified

into: lightning overvoltages (seconds)

switching overvoltages (mseconds)

temporary overvoltages (seconds)

highest continuous system oper. voltage.


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Overvoltages in the power system


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overvoltage Temporary overvoltage

conversion into standard voltage:

A sinusoidal voltage with frequency between 48 Hz and62 Hz

T1 = 60 s


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overvoltage Transient overvoltage

Short-duration overvoltage of few milliseconds or less,

oscillatory or non-oscillatory, usually highly damped.

May be followed by temporary overvoltages. In this

case, both events are considered as separate events.

Types: Slow front

Fast front

Very fast front


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Slow front

Transient overvoltage, usually unidirectional

5000 s Tp > 20 s

T2 20 ms

Main reasons: line faults, switching

Conversion into standard:

An impulse voltage of

Tf = 250 s

tT = 2500 sCurrent standard impulse:

Tf = 30 s

tT = 70 s


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Fast front

Transient overvoltage, usually unidirectional

20 s T1 > 0.1 s

T2 300 s

Main reasons: lightning strokes


An impulse voltage of

Tf = 1.2s

tT= 50sCurrent standard impulse:

Tf = 8 s

tT = 20 s


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Very Fast front

100 ns Tf > 3 ns

tT 3 ms

superimposed oscillations 30 kHz f 100 MHz

Main reasons: switching of disconnectors in GIS


not standardized


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Temporary OvervoltagesEarth Faults

In case of earth faults the overvoltage amplitudesdepend on

neutral earthing

fault location

Important parameter: Earth fault factor k


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Temporary OvervoltagesEarth Faults

Earth fault factork


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Temporary Overvoltages Load Rejection:

When a transmission line ora large inductive load that is fedfrom a power station is suddenly switched off, the generatorwillspeed up and the bus barvoltage will rise. The amplitude ofthe overvoltage can be evaluated approximately by


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Load Rejection:

Temporary Overvoltages


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Load Rejection:



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Load Rejection:



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Load Rejection:

Voltage increase factors due to load rejection:

moderately extended systems: < 1.2 p.u. for up to severalminutes

widely extended systems: 1.5 p.u. for some seconds close to turbo generator: 1.3 p.u.

close to salient pole generator: 1.5 p.u.



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Resonance:

Temporary overvoltages caused by resonance

phenomena generally arise when circuits with large

capacitive elements, such as:

Lines

Cables series compensated lines

and inductive elements having non-linear

magnetizing characteristics, such as

Transformers

shunt reactors

are energized, or as result of load rejections.



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Resonance:



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Resonance:



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Temporary Overvoltages and Surge Arresters

Surge arresters cannot limit TOV!

Exception: resonance effects may be suppressed or

even avoided by MO arresters. Care has then to be

taken not to thermally overload the arresters!


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Slow front overvoltages

Slow-front overvoltages have front durations of some

tens to some thousands of microseconds and taildurations in the same order of magnitude and are

oscillatory by nature. They generally arise from:

line energization and re-energization;

faults and fault clearing; switching of capacitive or inductive currents;

distant lightning strikes to the conductor of overhead lines.

The representative voltage stress is characterized by:

a representative voltage shape 250/2500 S a representative amplitude which can be either

an assumed maximum overvoltage or

a probability distribution of the overvoltage amplitudes.


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Slow-front overvoltages have front durations of some

tens to some thousands of microseconds and taildurations in the same order of magnitude and are

oscillatory by nature. They generally arise from:

line energization and re-energization;

faults and fault clearing; switching of capacitive or inductive currents;

distant lightning strikes to the conductor of overhead lines.

The representative voltage stress is characterized by:

a representative voltage shape 250/2500 S a representative amplitude which can be either




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Thestatistical BSL is the crest value of a standard

switching impulse for which the insulation exhibits a90% probability of withstand, a 10% probability offailure.

The conventional BSL is the crest value of a

standard switching impulse forwhich the insulationdoes not exhibit disruptive discharge whensubjected to a specific number of applications ofthis impulse.


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SOV for a 500kV line


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Statistical SOV: E2 is the "statistical switching

overvoltage. The probability that the SOV equalsor exceeds E2 is 0.02.


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Fast front overvoltages

Fast-front overvoltages may be:

lightning overvoltages affecting overhead lines;

lightning overvoltages affecting substations;

Reasons for lightning overvoltages affecting OHL:

direct lightning strikes to the phase

lightning strikes to tower/ground wire and subsequent

back flashover;

induced by lightning strikes to ground nearby the

OHL.


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Fast front overvoltages

The representative voltage stress is characterized

by:

a representative voltage shape : 1.2/50s;

a representative amplitude which can be either



Amplitudes and rates of occurrence depend on:

Lightning performance of the OHLs connected to it;

Substation layout, size and in particular number of

OHLs connected to it; instantaneous value of the operating voltage (at the

moment of strike).


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Very Fast Front Overvoltages

originate from disconnector operations or faults within

GIS due to the fast breakdown of the gas gap and thenearly undamped surge propagation within the GIS.

Amplitudes are rapidly damped and front times

increased when leaving the GIS through the bushing.

VFFO are usually not a concern or a dimensioningparameter for the hv insulation. Therefore no

standardized test has yet been defined (and is not under

consideration, either).

Mainly an EMI problem, as external electric fields may

appear between the metal enclosure and ground .

problem for secondary control circuits.

Countermeasures: usual means of EMC.


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overvoltage


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Metal Oxide (MO) Arresters Even though a great number of arresters which are

gapped arresters with resistors made of silicon-carbide(SiC), are still in use, the arresters installed today are almostall metal-oxide (MO) arresters without gaps

The distinctive feature of an MO resistor is its extremelynon-linear voltage-current or U-I characteristic, renderingunnecessary the disconnection of the resistors from the

line through serial spark-gaps, as is found in the arresterswith SiC resistors.

The currents passing through the arrester within the rangeof possibly applied power-frequency voltages are so smallthat the arrester almost behaves like an insulator.

If, however, surge currents in the kiloampere range areinjected into the arrester, such as is the case whenlightning or switching overvoltages occur, then theresulting voltage across its terminals will remain lowenough to protect the insulation of the associated devicefrom the effects of overvoltage.


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Metal Oxide (MO) Arresters


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U-I Characteristic of MO Arrester U-I-characteristic of a typical MO arrester connected between phase

and ground in a solidly earthed neutral 420-kV-system. (voltage peak

value is depicted linearly, while current peak values are given in alogarithmic scale.)


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Arrester parameters The power-frequency voltage, while continuously

applied to the arrester, is the highest phase-to-earthvoltage of the system. In this case the peak value is:

At the same time, the so-called leakage current flows

through the arrester. This consists of a large capacitiveand a considerably smaller, resistive component.


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Arrester parameterscontinuous operating voltage

the power-frequency voltage which the arrestercan be operated at, without any type ofrestrictions

the continuous operating voltage is greater than the

highest continuously occurring phase-to-earth voltage.An allowance of at least 5% (IEC 60099-5, clause 3.2.1)

is recommended. With this, possible harmonics in the

system voltage are taken into account.

rated voltage (Ur):

is somewhat misleading. Instead it characterizes the

capability of the arrester to deal with temporary

overvoltages in the system. It can only be applied

temporarilyfor a time period of 10 seconds.


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Arrester parameterscontinuous operating voltage

the power-frequency voltage which the arrestercan be operated at, without any type ofrestrictions

the continuous operating voltage is greater than the

highest continuously occurring phase-to-earth voltage.An allowance of at least 5% (IEC 60099-5, clause 3.2.1)

is recommended. With this, possible harmonics in the

system voltage are taken into account.

Rated voltage (Ur):

is somewhat misleading. Instead it characterizes the

capability of the arrester to deal with temporary

overvoltages in the system. It can only be applied

temporarilyfor a time period of 10 seconds.


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Arrester parametersRated voltage (Ur):

The actual cause of the temporary time limit is thesudden great increase in the temperature and the

frequent rise in leakage current (the UI-characteristic is

temperature-dependence), after, for example, the

arrester has diverted a current impulse to the ground

(that is, after it had to "operate").

In this case an extensive application of the rated

voltage could render the arrester incapable of

recooling; instead it would become thermally unstable

and would continually heat up until it reached self-destruction (so-called thermal runaway).


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Arrester parameters Lightning impulse protective level

This depicts the voltage which drops across the arresterterminals when the nominal discharge current flows through the

arrester.

The aforementioned is a lightning current impulse of a

standardized shape, whose amplitude is assigned to different

classes from 1.5 kA to 20 kA, according to the IEC standard60099-4. For high-voltage arresters (in systems with Us 123 kV)only classes 10 kA and 20 kA are common.

Two "10-kA-arresters" can have very different properties.When selecting an arrester the nominal discharge currenttherefore cannot be considered on its own.

The statement "lightning impulse protective level = 823 kV"means the following: a voltage at a maximum of 823 kVdrops across the terminals when impressing a lightningcurrent impulse of 8 sof virtual front time, 20 sof virtualtime to half-value on the tail and a peak value of 10 kA


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Arrester parameters Lightning impulse protective level

This depicts the voltage which drops across the arresterterminals when the nominal discharge current flows through the

arrester.

The aforementioned is a lightning current impulse of a

standardized shape, whose amplitude is assigned to different

classes from 1.5 kA to 20 kA, according to the IEC standard60099-4. For high-voltage arresters (in systems with Us 123 kV)only classes 10 kA and 20 kA are common.

Two "10-kA-arresters" can have very different properties.When selecting an arrester the nominal discharge currenttherefore cannot be considered on its own.

The statement "lightning impulse protective level = 823 kV"means the following: a voltage at a maximum of 823 kVdrops across the terminals when impressing a lightningcurrent impulse of 8 s of virtual front time, 20 s of virtualtime to half-value on the tail and a peak value of 10 kA


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