Download - Slow-Front Overvoltages
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Fachgebiet
HochspannungstechnikOvervoltage Protection and Insulation Coordination / Chapter 3 - 1 -
Slow-Front Overvoltages
Slow-front overvoltages have front durations of some tens to some thousandsof microseconds and tail durations in the same order of magnitude and areoscillatory by nature. They generally arise from:
line energization and re-energization; faults and fault clearing; load rejections; switching of capacitive or inductive currents; distant lightning strikes to the conductor of overhead lines.
Slow-front overvoltages have front durations of some tens to some thousandsof microseconds and tail durations in the same order of magnitude and areoscillatory by nature. They generally arise from:
line energization and re-energization; faults and fault clearing; load rejections; 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.
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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Fachgebiet
HochspannungstechnikOvervoltage Protection and Insulation Coordination / Chapter 3 - 2 -
d
Slow-Front Overvoltages
The representative amplitude is the amplitude ofthe overvoltage considered independently from its
actual time to peak. However, in some systems in
range II, overvoltages with very long fronts may
occur and the representative amplitude may be
derived by taking into account the influence of the
front duration upon the dielectric strength of the
insulation.
The representative voltage shape is the standardswitching impulse: Tp = 250 s, T2 = 2500 s.
s10 3020
m
0
1
3
2
4
MV
U
1
+
2
T = 250 s
0
s
3
d
cr
T = 850 scr
T = 750 scr
T = 650 scr
T = 450 scr
T = 250 scr
see HVT II, Chapter 9:
3: curve of minimum strength
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Fachgebiet
HochspannungstechnikOvervoltage Protection and Insulation Coordination / Chapter 3 - 3 -
Slow-Front Overvoltages
The probability distribution of the overvoltages without surge arrester operationis characterized by *)
its 2 % values ue2, up2
its deviations e, p
its truncation values uet, upt.
Although not perfectly valid, the probability distribution can be approximated by a
Gaussian distribution between the 50 % value and the truncation value abovewhich no values are assumed to exist. see next slides
Alternatively, a modified Weibull distribution may be used.
(see: IEC 60071-2, Annex C, Annex D)
*) Indices: e "phase-to-earth"p "phase-to-phase"
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Fachgebiet
HochspannungstechnikOvervoltage Protection and Insulation Coordination / Chapter 3 - 4 -
Slow-Front Overvoltages
f(u)
P(u)
u
u
Normal distribution (Gaussian distribution)Normal distribution (Gaussian distribution)
2
121( ) e
2
u
f u
=
standard deviation expectation average mean value ofui
( ) ( ) du
P u f u u
=
Probability density function of voltage occurrence:
Cumulative distribution function of voltage occurrence:
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Fachgebiet
HochspannungstechnikOvervoltage Protection and Insulation Coordination / Chapter 3 - 5 -
Slow-Front Overvoltages
u f(u) P(u)
u
u
u
f(u)
P(u)
u
u
2%-value2%-value
truncation valuetruncation value
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Fachgebiet
HochspannungstechnikOvervoltage Protection and Insulation Coordination / Chapter 3 - 6 -
Slow-Front Overvoltages
P(ue)
ue / p.u.
2%
Example: normal distribution of phase-to-earth overvoltages, definitions acc. to IEC 60071-2(for phase-to-phase voltages accordingly)
ue2ue2 uet = ue2 + euet = ue2 + e
50%
0.1%
1
All overvoltages are higher than 1 p.u.
Overvoltages are characterized by their2% value ue2.
The difference between the minimum value and the
2% value is equivalent to 4 standard deviations:
e 2 e1 4u = ( )e e 20.25 1u =
ue50 = ue2 - 2eue50 = ue2 - 2e
All relevant information can be derived from ue2.All relevant information can be derived from ue2.
4e4e
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Fachgebiet
HochspannungstechnikOvervoltage Protection and Insulation Coordination / Chapter 3 - 7 -
Slow-Front Overvoltages
Example: normal distributions of SFO on overhead lines phase-to-earth
Cumu
lativedistribution/%
ue2
uet
ue ue
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Fachgebiet
HochspannungstechnikOvervoltage Protection and Insulation Coordination / Chapter 3 - 8 -
Slow-Front Overvoltages
The assumed maximum value of the representative overvoltage stress is equal
to the truncation value of the overvoltages or to the switching impulse protective level Upsof the surge arrester
whichever is lower.
The assumed maximum value of the representative overvoltage stress is equal
to the truncation value of the overvoltages or to the switching impulse protective level Upsof the surge arrester
whichever is lower.
see next slide
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Fachgebiet
HochspannungstechnikOvervoltage Protection and Insulation Coordination / Chapter 3 - 9 -
Slow-Front Overvoltages
0
100
200
300
400
500
600
700
800
900
1000
11001200
10-4 10-2 10 2 10 41
Peakvalueo
fvoltage
/kV
Peak value of current / A
residual voltage at switching impulse current 1 kA
= switching impulse protection level = 680 kV
Nominal discharge current In = 10 kA
Switching impulse current = 1 kA
residual voltage at In = lightning impulse protection level = 823 kV
Standard switching impulse current values acc. to IEC60099-4; switching impulse protection level Ups =residual voltage at the highest current amplitude each
Example forUs = 420 kV
= 2 Ur= 2 336 kV = 475 kV
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Fachgebiet
HochspannungstechnikOvervoltage Protection and Insulation Coordination / Chapter 3 - 10 -
Slow-Front Overvoltages
0 1 2 3 4 5 6
ue / p.u.
Probability
density
Note:
In case of overvoltage limitation by surge arresters increase of probability density at ups!
ups
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Fachgebiet
HochspannungstechnikOvervoltage Protection and Insulation Coordination / Chapter 3 - 11 -
Slow-Front Overvoltages
Phase-peak method: from each switching operation the highest peak value ofthe overvoltage on each phase-to-earth or between each combination ofphases is included in the overvoltage probability distribution, i.e. each operationcontributes three peak values to the representative overvoltage probability
distribution. This distribution then has to be assumed to be equal for each of thethree insulations involved in each part of insulation, phase-to-earth, phase-to-phase or longitudinal.
Phase-peak method: from each switching operation the highest peak value ofthe overvoltage on each phase-to-earth or between each combination ofphases is included in the overvoltage probability distribution, i.e. each operationcontributes three peak values to the representative overvoltage probability
distribution. This distribution then has to be assumed to be equal for each of thethree insulations involved in each part of insulation, phase-to-earth, phase-to-phase or longitudinal.
Case-peak method: from each switching operation the highest peak value of theovervoltages of all three phases to earth or between all three phases isincluded in the overvoltage probability distribution, i.e. each operation
contributes one value to the representative overvoltage distribution. Thisdistribution is then applicable to one insulation within each type.
Case-peak method: from each switching operation the highest peak value of theovervoltages of all three phases to earth or between all three phases isincluded in the overvoltage probability distribution, i.e. each operation
contributes one value to the representative overvoltage distribution. Thisdistribution is then applicable to one insulation within each type.
IEC recommended practice
Common practice in the US and Canada [HIL-99]
(Both methods give only slightly different results; see IEC 60071-2, Annex D and [HIL-99])
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Fachgebiet
HochspannungstechnikOvervoltage Protection and Insulation Coordination / Chapter 3 - 12 -
Slow-Front Overvoltages Line Energization and Re-Energization
A three-phase line energization or re-energization produces switchingovervoltages on all three phases of the line. Therefore, each switching operation
produces three phase-to-earth and, correspondingly, three phase-to-phaseovervoltages.
A three-phase line energization or re-energization produces switchingovervoltages on all three phases of the line. Therefore, each switching operation
produces three phase-to-earth and, correspondingly, three phase-to-phaseovervoltages.
TNA studies have to be performed with several switching operations at randomdistribution of the time instants.
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Fachgebiet
HochspannungstechnikOvervoltage Protection and Insulation Coordination / Chapter 3 - 13 -
Slow-Front Overvoltages Line Energization and Re-Energization
Range of 2% slow-front phase-to-earth overvoltages at the receiving end due to line energizationand re-energization (IEC 60071-2, Figure 1)
Values just for estimation purposes; detailed studies required!Values just for estimation purposes; detailed studies required!
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HochspannungstechnikOvervoltage Protection and Insulation Coordination / Chapter 3 - 14 -
Slow-Front Overvoltages Line Energization and Re-Energization
Phase-to-phase overvoltagesIn the evaluation of the phase-to-phase overvoltages, an additional parameter needs tobe added. As the insulation is sensitive to the subdivision of a given phase-to-phaseovervoltage value into two phase-to-earth components, the selection of a specificinstant shall take into account the insulation characteristics.
Time instant of phase-to-phase overvoltage peak: this instant gives the highest phase-to phase overvoltage value. It represents the highest stress for all insulation configurations,
for which the dielectric strength between phases is not sensitive to the subdivisioninto components. Typical examples are the insulation between windings or short airclearances.
Time instant of phase-to-phase overvoltage peak: this instant gives the highest phase-to phase overvoltage value. It represents the highest stress for all insulation configurations,for which the dielectric strength between phases is not sensitive to the subdivisioninto components. Typical examples are the insulation between windings or short airclearances.
Two particular time instants are of importance (see also next two slides):
Phase-to-phase overvoltage at the instant of the phase-to-earth overvoltage peak:although this instant gives lower overvoltage values than the instant of the phase-to-phase overvoltage peak, it may be more severe for insulation configurations for which thedielectric strength between phases is influenced by the subdivision into components.Typical examples are large air clearances for which the instant of the positive phase-to-earth peak is most severe, or gas-insulated substations (three-phase enclosed)
for which the negative peak is most severe.
Phase-to-phase overvoltage at the instant of the phase-to-earth overvoltage peak:although this instant gives lower overvoltage values than the instant of the phase-to-phase overvoltage peak, it may be more severe for insulation configurations for which thedielectric strength between phases is influenced by the subdivision into components.Typical examples are large air clearances for which the instant of the positive phase-to-earth peak is most severe, or gas-insulated substations (three-phase enclosed)for which the negative peak is most severe.
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Fachgebiet
HochspannungstechnikOvervoltage Protection and Insulation Coordination / Chapter 3 - 15 -
Time instants of max. UpTime instants of max. Up
Slow-Front Overvoltages Line Energization and Re-Energization
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HochspannungstechnikOvervoltage Protection and Insulation Coordination / Chapter 3 - 16 -
Dielectric Breakdown of Gases
Breakdown voltage of positive tip is always lower
than that of a negative tip (derived for air):
Ud, positive < Ud, negativeUd, positive < Ud, negative
memory hook: "positive is negative"
At alternating voltage stress the breakdown of a strongly inhomogeneousasymmetrical electrode configuration in air generally occurs in the positive half cycle
At alternating voltage stress the breakdown of a strongly inhomogeneousasymmetrical electrode configuration in air generally occurs in the positive half cycle
Recall from HVT II:
Extension of this rule: this is valid only for air insulation!In SF
6under high pressure (GIS): just the other way round
At alternating voltage stress the breakdown of a strongly inhomogeneous
asymmetrical electrode configuration in SF6 under high pressure generallyoccurs in the negative half cycle
At alternating voltage stress the breakdown of a strongly inhomogeneous
asymmetrical electrode configuration in SF6 under high pressure generallyoccurs in the negative half cycle
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Fachgebiet
HochspannungstechnikOvervoltage Protection and Insulation Coordination / Chapter 3 - 17 -
Time instants of max. UeTime instants of max. Ue
Slow-Front Overvoltages Line Energization and Re-Energization
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Fachgebiet
HochspannungstechnikOvervoltage Protection and Insulation Coordination / Chapter 3 - 18 -
The 2% phase-to-phase overvoltage can approximately be determined from thephase-to-earth overvoltage:
three-phase re-energization
three-phase energization
Slow-Front Overvoltages Line Energization and Re-Energization
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Fachgebiet
HochspannungstechnikOvervoltage Protection and Insulation Coordination / Chapter 3 - 19 -
factor2.83factor2.83
factor2.5factor2.5
factor2.0
factor2.0
Standard insulation levels for range II(IEC 60071-1, Table 3):
The smaller the factor Ue/Um, thehigher the factor Up/Ue
Slow-Front Overvoltages Line Energization and Re-Energization
Comparison with the slide before:
Um = 300 kV 1 p.u. = 245 kV 850 kV = 3.47 p.u.
Up2/Ue2 = 1.45
Um = 300 kV 1 p.u. = 245 kV 850 kV = 3.47 p.u.
Up2/Ue2 = 1.45
Um = 420 kV 1 p.u. = 343 kV 1050 kV = 3.06 p.u.
Up2/Ue2 = 1.5
Um = 420 kV 1 p.u. = 343 kV 1050 kV = 3.06 p.u. U
p2/U
e2= 1.5
Um = 765 kV 1 p.u. = 625 kV 1550 kV = 2.48 p.u. Up2/Ue2 = 1.6
Um = 765 kV 1 p.u. = 625 kV 1550 kV = 2.48 p.u. Up2/Ue2 = 1.6
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Fachgebiet
HochspannungstechnikOvervoltage Protection and Insulation Coordination / Chapter 3 - 20 -
Slow-Front Overvoltages Line Energization and Re-Energization
Possible causes of line switching overvoltages (continued next slide)
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Fachgebiet
HochspannungstechnikOvervoltage Protection and Insulation Coordination / Chapter 3 - 21 -
Slow-Front Overvoltages Line Energization and Re-Energization
Possible causes of line switching overvoltages
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Fachgebiet
HochspannungstechnikOvervoltage Protection and Insulation Coordination / Chapter 3 - 22 -
Slow-Front Overvoltages Line Energization and Re-Energization
Sending end
Sending end
Receiving end
Receiving end
Energizing at voltage peakin phase R (tR = 0)
Energizing 2 ms after voltagepeak in phase R
tR = 1 ms, ts = 5 ms, tt = 3 ms tR = 0 ms, ts = 2 ms, tt = 2 ms
Synchronousswitching
Non-synchronousswitching
(by pre-strikingof the contacts)
ueT =
1.35
ueR =1.60
ueR =
1.35
ueR =1.70
ueS =
1.40
ueT=
1.95
ueT =
1.35
ueT =
1.85
ue = 1.35ue = 1.35
+15%
ue = 1.70ue = 1.70
ue = 1.40ue = 1.40
ue = 1.95ue = 1.95
+26%
+39%
Example: 420-kV line, length 340 km,
resonant frequency (100200) Hz [DOR-81]
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Fachgebiet
HochspannungstechnikOvervoltage Protection and Insulation Coordination / Chapter 3 - 23 -
Slow-Front Overvoltages Line Energization and Re-Energization
Measures against line switching overvoltages (continued next slide)
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HochspannungstechnikOvervoltage Protection and Insulation Coordination / Chapter 3 - 24 -
Slow-Front Overvoltages Line Energization and Re-Energization
Measures against line switching overvoltages (continued next slide)
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HochspannungstechnikOvervoltage Protection and Insulation Coordination / Chapter 3 - 25 -
Slow-Front Overvoltages Line Energization and Re-Energization
Measures against line switching overvoltages
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HochspannungstechnikOvervoltage Protection and Insulation Coordination / Chapter 3 - 26 -
Slow-Front Overvoltages Line Energization and Re-Energization
Measures against line switching overvoltages
IEC 60071-2: "It should be noted that whenarresters are installed at the ends of longtransmission lines for the purpose of limiting slow-front overvoltages, the overvoltages in the middleof the line may be substantially higher than atthe line ends."
IEC 60071-2: "It should be noted that whenarresters are installed at the ends of longtransmission lines for the purpose of limiting slow-
front overvoltages, the overvoltages in the middleof the line may be substantially higher than atthe line ends."
For this reason AEP (American Electric Power)installed one set of 800-kV transmission linearresters in the middle of the line.
ABB
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Fachgebiet
HochspannungstechnikOvervoltage Protection and Insulation Coordination / Chapter 3 - 27 -
Slow-Front Overvoltages Earth Faults
Highest slow-front overvoltages due to earth faults in isolated neutral systems!Example:
[DOR-81]
ue = 2.7 p.u.ue = 2.7 p.u.
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Fachgebiet
HochspannungstechnikOvervoltage Protection and Insulation Coordination / Chapter 3 - 28 -
Slow-Front Overvoltages Switching Cap. or Ind. Currents
Restrike of the circuit breakerRestrike of the circuit breaker
ue = 2.1 p.u.ue = 2.1 p.u.
Begin of opening of the circuit breakerBegin of opening of the circuit breaker
Measure against: use ofrestrike-free breakersMeasure against: use ofrestrike-free breakers
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Fachgebiet
HochspannungstechnikOvervoltage Protection and Insulation Coordination / Chapter 3 - 29 -
Slow-Front Overvoltages Limitation by Arresters
MO arresters limit switching overvoltages (current peak values 500 A 2 kA)to about:
Ups (peak value) 2Ur (r.m.s. value) (see slide 9: Ur= 336 kV; Ups = 680 kV)
Conclusions:
MO arresters do limit slow-front overvoltages due to line energization and re-
energization and switching of inductive and capacitive currents. MO arresters usually cannot limit slow-front overvoltages caused by earth
faults and fault clearing (exception: isolated neutral systems, series
compensated lines), as their amplitudes are too low.
Separation effects (protective distance) have not to be taken into account(overvoltages too slow)
But: exception for long transmission lines voltages in middle and/or end ofline can take considerably higher values than arrester's protection level!
Separation effects (protective distance) have not to be taken into account(overvoltages too slow)
But: exception for long transmission lines voltages in middle and/or end ofline can take considerably higher values than arrester's protection level!
Ur (r.m.s. value) 1 p.u. Ups 2 p.u.
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HochspannungstechnikOvervoltage Protection and Insulation Coordination / Chapter 3 - 30 -
Slow-Front Overvoltages Limitation by Arresters
Representative voltages in case of MO surge arresters:
Phase-to-earth: Ure = UpsPhase-to-earth: Ure = Ups
Phase-to-phase: the lower value of Urp = 2 Ups Urp = Upt (truncation value determined acc. to IEC 60071-2, Annex D)
Phase-to-phase: the lower value of Urp = 2 Ups Urp = Upt (truncation value determined acc. to IEC 60071-2, Annex D)
If arresters limit phase-to-earth voltages to less than 70% of their unaffectedUe2-values, the resulting phase-to-phase voltages will be Up 2 Ups of thearrester.
If arresters limit phase-to-earth voltages to less than 70% of their unaffectedUe2-values, the resulting phase-to-phase voltages will be Up 2 Ups of thearrester.