slow-front overvoltages

7/29/2019 Slow-Front Overvoltages

1/30

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.


2/30

Fachgebiet


d


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


3/30

Fachgebiet



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"


4/30

Fachgebiet



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:


5/30

Fachgebiet



u f(u) P(u)

u

u

u

f(u)

P(u)

u

u

2%-value2%-value

truncation valuetruncation value


6/30

Fachgebiet



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


7/30

Fachgebiet



Example: normal distributions of SFO on overhead lines phase-to-earth

Cumu

lativedistribution/%

ue2

uet

ue ue


8/30

Fachgebiet



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


9/30

Fachgebiet



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


10/30

Fachgebiet



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


11/30

Fachgebiet



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])


12/30

Fachgebiet


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.


13/30

Fachgebiet



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!


14/30

Fachgebiet



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.


15/30

Fachgebiet


Time instants of max. UpTime instants of max. Up



16/30

Fachgebiet


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


17/30

Fachgebiet


Time instants of max. UeTime instants of max. Ue



18/30

Fachgebiet


The 2% phase-to-phase overvoltage can approximately be determined from thephase-to-earth overvoltage:

three-phase re-energization

three-phase energization



19/30

Fachgebiet


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


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


20/30

Fachgebiet



Possible causes of line switching overvoltages (continued next slide)


21/30

Fachgebiet



Possible causes of line switching overvoltages


22/30

Fachgebiet



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]


23/30

Fachgebiet



Measures against line switching overvoltages (continued next slide)


24/30

Fachgebiet



Measures against line switching overvoltages (continued next slide)


25/30

Fachgebiet



Measures against line switching overvoltages


26/30

Fachgebiet



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


27/30

Fachgebiet


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.


28/30

Fachgebiet


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


29/30

Fachgebiet


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.


30/30

Fachgebiet


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.

slow-front overvoltages

Documents