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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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overvoltage Transient overvoltage
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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overvoltage Transient overvoltage
Fast front
Transient overvoltage, usually unidirectional
20 s T1 > 0.1 s
T2 300 s
Main reasons: lightning strokes
Conversion into standard:
An impulse voltage of
Tf = 1.2s
tT= 50sCurrent standard impulse:
Tf = 8 s
tT = 20 s
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overvoltage Transient overvoltage
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
Conversion into standard:
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:
Temporary Overvoltages
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Load Rejection:
Temporary Overvoltages
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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.
Temporary Overvoltages
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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.
Temporary Overvoltages
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Resonance:
Temporary Overvoltages
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Resonance:
Temporary Overvoltages
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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
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
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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Slow front overvoltages
SOV for a 500kV line
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Slow front overvoltages
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
an assumed maximum overvoltage or
a probability distribution of the overvoltage amplitudes.
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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