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Slow-Front Overvoltages Slow-front overvoltages have front front durations durations of of some some tens to some thousands of microseconds and tail durations durations in in the the same same order order of of magnitude magnitude and and are are oscillatory by nature. They generally arise from: – line energization and re-energization; – faults and fault fault clearing; clearing; – load rejections; – switching switching of of capacitive capacitive or or inductive inductive currents; currents; – distant lightning strikes to the conductor of overhead lines. The representative voltage stress is characterized by: – a representative voltage shape Æ 250/2500 µs; – a representative amplitude which can be either • an assumed maximum overvoltage or • a probability distribution of the overvoltage amplitudes.

Fachgebiet Hochspannungstechnik

Overvoltage Protection and Insulation Coordination / Chapter 3

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Slow-Front Overvoltages The representative voltage shape is the standard switching impulse: Tp = 250 µs, T2 = 2500 µs. d

see HVT II, Chapter 9: 3: curve of minimum strength

4

MV Tcr = 250 µs

1 3

2

The representative amplitude is the amplitude of the 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. Fachgebiet Hochspannungstechnik

Tcr = 850 µs

3

Tcr = 750 µs Tcr = 650 µs

2 Ud

Tcr = 450 µs

+

Tcr = 250 µs

1

s

0 0

Overvoltage Protection and Insulation Coordination / Chapter 3

10

-2-

s

20

m

30

Slow-Front Overvoltages The probability distribution of the overvoltages without surge arrester operation is characterized by *) • its 2 % values ue2, up2 • its deviations σe, σp • 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 above which no values are assumed to exist. Æ 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"

Fachgebiet Hochspannungstechnik

Overvoltage Protection and Insulation Coordination / Chapter 3

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Slow-Front Overvoltages f(u)

Normal Normal distribution distribution (Gaussian (Gaussian distribution) distribution) Probability density function of voltage occurrence:

u P(u)

1 f (u ) = e σ 2π

1 ⎛ u−µ ⎞ − ⎜ ⎟ 2⎝ σ ⎠

2

σ … standard deviation

µ … expectation ≈ average mean value of ui

Cumulative distribution function of voltage occurrence: u

P (u ) =

∫

f (u ) d u

−∞

u Fachgebiet Hochspannungstechnik

Overvoltage Protection and Insulation Coordination / Chapter 3

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Slow-Front Overvoltages f(u) u

f(u)

P(u)

u truncation truncation value value 2%-value 2%-value

u P(u) u u

u Fachgebiet Hochspannungstechnik

Overvoltage Protection and Insulation Coordination / Chapter 3

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Slow-Front Overvoltages Example: normal distribution of phase-to-earth overvoltages, definitions acc. to IEC 60071-2 (for phase-to-phase voltages accordingly) • Overvoltages are characterized by their 2% value ue2. • All overvoltages are higher than 1 p.u. • The difference between the minimum value and the 2% value is equivalent to 4 standard deviations:

P(ue)

ue 2 − 1 = 4σ e

σ e = 0.25 ⋅ ( ue 2 − 1)

50%

All . All relevant relevant information information can can be be derived derived from from uue2 e2. 2% ue / p.u.

0.1% 1

uue50 = u e2 -- 22σσee e50 = ue2

uue2 e2

uuetet == uue2 +σ e2 + σee

44σσee Fachgebiet Hochspannungstechnik

Overvoltage Protection and Insulation Coordination / Chapter 3

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Slow-Front Overvoltages

Cumulative distribution / %

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

Æ ue2 Æ uet ue Fachgebiet Hochspannungstechnik

ue

Overvoltage Protection and Insulation Coordination / Chapter 3

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Slow-Front Overvoltages The The assumed assumed maximum maximum value value of of the the representative representative overvoltage overvoltage stress stress is is equal equal •• to to the the truncation truncation value of the overvoltages or or of the surge arrester •• to the switching switching impulse impulse protective protective level level U Ups ps whichever is lower.

see next slide……

Fachgebiet Hochspannungstechnik

Overvoltage Protection and Insulation Coordination / Chapter 3

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Slow-Front Overvoltages Peak value of voltage / kV

1200 1100

Example for Us = 420 kV

1000

residual voltage at In = lightning impulse protection level = 823 kV

900

residual voltage at switching impulse current 1 kA = switching impulse protection level = 680 kV

800 700 600

û = √2 · Ur = √2 · 336 kV = 475 kV

500 400 300 Switching impulse current = 1 kA

200 100

Nominal discharge current In = 10 kA

0 10-4

10-2

1

10 4 10 2 Peak value of current / A Standard switching impulse current values acc. to IEC 60099-4; switching impulse protection level Ups = residual voltage at the highest current amplitude each

Fachgebiet Hochspannungstechnik

Overvoltage Protection and Insulation Coordination / Chapter 3

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Slow-Front Overvoltages

Probability density

Note: In case of overvoltage limitation by surge arresters increase of probability density at ups!

0

1

2

3

u e / p.u. Fachgebiet Hochspannungstechnik

4

5

6

ups

Overvoltage Protection and Insulation Coordination / Chapter 3

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Slow-Front Overvoltages Phase-peak Phase-peak method: from each switching operation the highest peak value of the the overvoltage on each phase-to-earth or between each each combination combination of of phases phases is included in the the overvoltage overvoltage probability probability distribution, distribution, i.e. each operation contributes contributes three peak values to to the the representative representative overvoltage probability probability distribution. distribution. This This distribution distribution then then has has to to be be assumed assumed to be equal for for each each of the three three insulations involved involved in in each each part part of of insulation, insulation, phase-to-earth, phase-tophase phase or longitudinal. IEC recommended practice Case-peak method: from each switching operation the highest peak value of the overvoltages of all three phases to earth or between all three phases is included in the overvoltage probability distribution, i.e. each operation contributes one value to the representative overvoltage distribution. This distribution is then applicable to one insulation within each type. 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]) Fachgebiet Hochspannungstechnik

Overvoltage Protection and Insulation Coordination / Chapter 3

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Slow-Front Overvoltages – Line Energization and Re-Energization A A three-phase three-phase line energization energization or or re-energization re-energization produces produces switching overvoltages overvoltages on on all all three three phases phases of of the line. Therefore, each switching operation produces produces three three phase-to-earth and, correspondingly, three phase-to-phase overvoltages. overvoltages. TNA studies have to be performed with several switching operations at random distribution of the time instants.

Fachgebiet Hochspannungstechnik

Overvoltage Protection and Insulation Coordination / Chapter 3

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Slow-Front Overvoltages – Line Energization and Re-Energization Range of 2% slow-front phase-to-earth overvoltages at the receiving end due to line energization and re-energization (IEC 60071-2, Figure 1)

Values just for estimation estimation purposes; purposes; detailed detailed studies studies required! Fachgebiet Hochspannungstechnik

Overvoltage Protection and Insulation Coordination / Chapter 3

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Slow-Front Overvoltages – Line Energization and Re-Energization Phase-to-phase overvoltages In the evaluation of the phase-to-phase overvoltages, an additional parameter needs to be added. As the insulation is sensitive to the subdivision of a given phase-to-phase overvoltage value into two phase-to-earth components, the selection of a specific instant shall take into account the insulation characteristics. Two particular time instants are of importance (see also next two slides): Time Time instant instant of of phase-to-phase phase-to-phase overvoltage overvoltage peak: peak: this this instant instant gives gives the the highest highest phasephaseto to phase phase overvoltage overvoltage value. value. ItIt represents represents the the highest highest stress stress for for all all insulation insulation configurations, configurations, for for which which the the dielectric dielectric strength strength between between phases phases is is not not sensitive sensitive to to the the subdivision subdivision into into components. components. Typical Typical examples examples are are the the insulation insulation between between windings windings or or short short air air clearances. clearances. Phase-to-phase Phase-to-phase overvoltage overvoltage at at the the instant instant of of the the phase-to-earth phase-to-earth overvoltage overvoltage peak: peak: although although this this instant instant gives gives lower lower overvoltage overvoltage values values than than the the instant instant of of the the phase-tophase-tophase phase overvoltage overvoltage peak, peak, itit may may be be more more severe severe for for insulation insulation configurations configurations for for which which the the dielectric dielectric strength strength between between phases phases is is influenced influenced by by the the subdivision subdivision into into components. components. Typical Typical examples examples are are large large air air clearances clearances for for which which the the instant instant of of the the positive positive phasephaseto-earth to-earth peak peak is is most most severe, severe, or or gas-insulated gas-insulated substations substations (three-phase (three-phase enclosed) enclosed) for for which which the the negative negative peak peak is is most most severe. severe. Fachgebiet Hochspannungstechnik

Overvoltage Protection and Insulation Coordination / Chapter 3

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Slow-Front Overvoltages – Line Energization and Re-Energization Time Time instants of max. U Upp

Fachgebiet Hochspannungstechnik

Overvoltage Protection and Insulation Coordination / Chapter 3

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Dielectric Breakdown of Gases Recall from HVT II: • Breakdown voltage of positive tip is always lower than that of a negative tip (derived for air):

UUd,d,positive < U < Ud,d,negative positive negative memory hook: "positive is negative" •• At At alternating alternating voltage voltage stress stress the the breakdown breakdown of of aa strongly strongly inhomogeneous inhomogeneous asymmetrical asymmetrical electrode electrode configuration configuration in in air air generally generally occurs occurs in in the the positive positive half half cycle cycle Extension of this rule: this is valid only for air insulation! In SF6 under high pressure (GIS): just the other way round Æ •• At At alternating alternating voltage voltage stress stress the the breakdown breakdown of of aa strongly strongly inhomogeneous inhomogeneous asymmetrical asymmetrical electrode electrode configuration configuration in in SF SF66 under under high high pressure pressure generally generally occurs occurs in in the the negative negative half half cycle cycle Fachgebiet Hochspannungstechnik

Overvoltage Protection and Insulation Coordination / Chapter 3

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Slow-Front Overvoltages – Line Energization and Re-Energization Time Time instants of max. U Uee

Fachgebiet Hochspannungstechnik

Overvoltage Protection and Insulation Coordination / Chapter 3

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Slow-Front Overvoltages – Line Energization and Re-Energization The 2% phase-to-phase overvoltage can approximately be determined from the phase-to-earth overvoltage:

three-phase re-energization three-phase energization

Fachgebiet Hochspannungstechnik

Overvoltage Protection and Insulation Coordination / Chapter 3

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Slow-Front Overvoltages – Line Energization and Re-Energization Standard insulation levels for range II (IEC 60071-1, Table 3):

ffaaccttoor 2 r 2..8833

ffaacct toorr 2 2..55

ffaacc ttoorr 22..00 Fachgebiet Hochspannungstechnik

The smaller the factor Ue/Um, the higher the factor Up/Ue Comparison with the slide before: U 300 kV kV Umm == 300 Æ Æ 11 p.u. p.u. == 245 245 kV kV Æ Æ 850 850 kV kV == 3.47 3.47 p.u. p.u. Æ /U e2 == 1.45 ÆU Up2 1.45 p2/Ue2 U Umm == 420 420 kV kV Æ Æ 11 p.u. p.u. == 343 343 kV kV Æ Æ 1050 1050 kV kV == 3.06 3.06 p.u. p.u. Æ /U e2 == 1.5 ÆU Up2 1.5 p2/Ue2 U Umm == 765 765 kV kV Æ Æ 11 p.u. p.u. == 625 625 kV kV Æ Æ 1550 1550 kV kV == 2.48 2.48 p.u. p.u. Æ /U e2 == 1.6 ÆU Up2 1.6 p2/Ue2 Overvoltage Protection and Insulation Coordination / Chapter 3

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Slow-Front Overvoltages – Line Energization and Re-Energization Possible causes of line switching overvoltages (continued next slide)

Fachgebiet Hochspannungstechnik

Overvoltage Protection and Insulation Coordination / Chapter 3

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Slow-Front Overvoltages – Line Energization and Re-Energization Possible causes of line switching overvoltages

Fachgebiet Hochspannungstechnik

Overvoltage Protection and Insulation Coordination / Chapter 3

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Slow-Front Overvoltages – Line Energization and Re-Energization ueT = 1.35

Synchronous switching

ueR = 1.35

Sending end

uuee == 1.35 1.35 +26%

ueR = 1.60

Energizing at voltage peak in phase R (tR = 0)

ueS = 1.40

Non-synchronous switching (by pre-striking of the contacts)

ueR = 1.70

Receiving end uuee = = 1.70 1.70

Energizing 2 ms after voltage peak in phase R

ueT = 1.35

+15%

Sending end

uuee == 1.40 1.40 +39%

ueT= 1.95

tR = 1 ms, ts = 5 ms, tt = 3 ms Fachgebiet Hochspannungstechnik

ueT = 1.85

tR = 0 ms, ts = 2 ms, tt = 2 ms

Receiving end uue = 1.95 e = 1.95 Example: 420-kV line, length 340 km, resonant frequency (100…200) Hz [DOR-81]

Overvoltage Protection and Insulation Coordination / Chapter 3

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Slow-Front Overvoltages – Line Energization and Re-Energization Measures against line switching overvoltages (continued next slide)

Fachgebiet Hochspannungstechnik

Overvoltage Protection and Insulation Coordination / Chapter 3

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Slow-Front Overvoltages – Line Energization and Re-Energization Measures against line switching overvoltages (continued next slide)

Fachgebiet Hochspannungstechnik

Overvoltage Protection and Insulation Coordination / Chapter 3

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Slow-Front Overvoltages – Line Energization and Re-Energization Measures against line switching overvoltages

Fachgebiet Hochspannungstechnik

Overvoltage Protection and Insulation Coordination / Chapter 3

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Slow-Front Overvoltages – Line Energization and Re-Energization Measures against line switching overvoltages IEC IEC 60071-2: 60071-2: "It "It should should be be noted noted that that when when arresters arresters are are installed installed at at the the ends ends of of long long transmission transmission lines lines for for the the purpose purpose of of limiting limiting slowslowfront front overvoltages, overvoltages, the the overvoltages overvoltages in in the the middle middle of of the the line line may may be be substantially substantially higher higher than than at at the the line line ends." ends." For this reason AEP (American Electric Power) installed one set of 800-kV transmission line arresters in the middle of the line.

ABB Fachgebiet Hochspannungstechnik

Overvoltage Protection and Insulation Coordination / Chapter 3

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Slow-Front Overvoltages – Earth Faults Highest slow-front overvoltages due to earth faults in isolated neutral systems! Example:

uee = 2.7 p.u.

[DOR-81] Fachgebiet Hochspannungstechnik

Overvoltage Protection and Insulation Coordination / Chapter 3

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Slow-Front Overvoltages – Switching Cap. or Ind. Currents

Begin Begin of of opening opening of of the the circuit circuit breaker breaker Restrike Restrike of of the the circuit circuit breaker breaker

uuee == 2.1 2.1 p.u. p.u.

Measure Measure against: against: use use of of restrike-free restrike-free breakers breakers Fachgebiet Hochspannungstechnik

Overvoltage Protection and Insulation Coordination / Chapter 3

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Slow-Front Overvoltages – Limitation by Arresters MO arresters limit switching overvoltages (current peak values 500 A … 2 kA) to about: • Ups (peak value) ≈ 2·Ur (r.m.s. value) (see slide 9: Ur = 336 kV; Ups = 680 kV) • Ur (r.m.s. value) ≈ 1 p.u. Æ

Ups ≈ 2 p.u.

Conclusions: • MO arresters do limit slow-front overvoltages due to line energization and reenergization 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 Separation effects effects (protective (protective distance) distance) have have not not to to be be taken taken into into account account (overvoltages (overvoltages too too slow) slow) But: But: exception exception for for long long transmission transmission lines lines –– voltages voltages in in middle middle and/or and/or end end of of line line can can take take considerably considerably higher higher values values than than arrester's arrester's protection protection level! level! Fachgebiet Hochspannungstechnik

Overvoltage Protection and Insulation Coordination / Chapter 3

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Slow-Front Overvoltages – Limitation by Arresters If arresters limit phase-to-earth voltages to less than 70% of their unaffected -values, the resulting phase-to-phase voltages will be U ≈ 2 · U ps of the Ue2 e2-values, the resulting phase-to-phase voltages will be pp ≈ 2 · Ups arrester. Representative voltages in case of MO surge arresters: Phase-to-earth: = Ups Phase-to-earth: U Ure re ps Phase-to-phase: the lower value of • Urp = 2 · U ps rp = 2 · ps • Urp = U pt (truncation (truncation value determined acc. to IEC IEC 60071-2, 60071-2, Annex Annex D) D) rp = pt

Fachgebiet Hochspannungstechnik

Overvoltage Protection and Insulation Coordination / Chapter 3

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