Overvoltage Protection and Insulation Coordination

Voltage Voltage Stress Stress in Power Power Systems Systems - Classifica Classification tion IEC 60071-1

Fachgebiet

Over Ov ervo volt ltag age e Pr Prot otec ecti tion on an and d In Insu sula lati tion on Co Coor ordi dina nati tion on / Ch Chap apte terr 2

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Voltage Voltage Stress Stress in Power Power Systems Systems - Classifica Classification tion Classification Classification of of real real stress stress

" Contin uous (powerpower-freque frequency) ncy) volt volta age"

Power-frequency voltage, considered having constant r.m.s. value, continuously applied to any pair of terminals of an insulation configuration f = 50 Hz or 60 Hz T 1 ≥ 3 600 s Æ Any power-frequency voltage lasting for 1 h or more is considered a continuous voltage! Conversion into

Standard Standard voltage voltage

"Standard power-frequency voltage"

A sinusoidal voltage with frequency of 50 Hz or 60 Hz T 1 to be specified by the apparatus committees Æ T1 up to t o 2 years! years!

Fachgebiet

Æ

see next slides

Over Ov ervo volt ltag age e Pr Prot otec ecti tion on an and d In Insu sula lati tion on Co Coor ordi dina nati tion on / Ch Chap apte terr 2

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Voltage Stress in Power Systems - Classification Example: Example: Cable Cable tests tests at at power-frequency power-frequency voltage voltage

Lifetime characteristic:

Fachgebiet

Overvoltage Protection and Insulation Coordination / Chapter 2

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Voltage Stress in Power Systems - Classification Example: Example: Cable Cable tests tests at at power-frequency power-frequency voltage voltage

11.4 years Source: Brugg Cables

Fachgebiet


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Voltage Stress in Power Systems - Classification Classification Classification of of real real stress stress

"Temporary overvoltage"

Power-frequency overvoltage of relatively long duration. The overvoltage may be damped or undamped. In some cases its frequency may be several times smaller or higher than power frequency. 10 Hz < f < 500 Hz 3 600 s ≥ T 1 ≥ 0.02 s Highest values by following main reasons: • phase-to-earth Æ earth faults and load rejection • phase-to-phase Æ load rejection • longitudinal Æ phase opposition during synchronization of two grids Conversion into


Example [THI-01]

" Standard s hort-duration power-frequency voltage"

A sinusoidal voltage with frequency between 48 Hz and 62 Hz T 1 = 60 s Fachgebiet


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" 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. " Slow-front ov ervolt age" Transient overvoltage, usually unidirectional 5000 µs ≥ T p > 20 µs T 2 ≤ 20 ms Main reasons: line faults, switching Conversion into


"Standard switching impulse"

Example [THI-01]

An impulse voltage of T p = 250 µs T 2 = 2 500 µs Fachgebiet


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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. " Fast-front overvoltage" Transient overvoltage, usually unidirectional 20 µs ≥ T 1 > 0.1 µs T 2 ≤ 300 µs Main reasons: lightning strokes, switching Conversion into


"Standard lightning impulse"

An impulse voltage of T 1 = 1.2 µs T 2 = 50 µs Fachgebiet

Example [THI-01] Overvoltage Protection and Insulation Coordination / Chapter 2

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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. "Very-fast-front overvoltage" Transient overvoltage, usually unidirectional T f < 100 ns (T t ≤ 3 ms) basic oscillation (1st harmonics) 30 kHz < f < 300 kHz superimposed oscillations 300 kHz < f < 100 MHz Main reasons: switching of disconnectors in GIS Conversion into


not standardized Example [THI-01]

Fachgebiet


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" Combined (temporary, slow -front , fast-front , very-fast-front) overvoltage"

Consisting of two voltage components simultaneously applied between each of the two phase terminals of a phase-to-phase (or longitudinal) insulation and earth. It is classified by the component of the higher peak value. Conversion into


" Standard combi ned switchi ng impulse"

Combined impulse voltage having two components of equal peak value and opposite polarity. The positive component is a standard switching impulse and the negative one is a switching impulse whose times to peak and half value should not be less than those of the positive impulse. Both impulses should reach their peak values at the same instant. The peak value of the combined voltage is, therefore, the sum of the peak values of the components.

Fachgebiet


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Temporary Overvoltages – Earth Faults Reasons for temporary overvoltages: • earth faults • load rejection • resonance phenomena In case of earth faults the overvoltage amplitudes depend on • neutral earthing • fault location. Important Important parameter: parameter: Earth Earth fault fault factor factor kk

IEC 60071-1

... in other "words": k = Fachgebiet

U LE U b /

3

U LE ... phase-to-earth voltage of sound phase during fault U b ... phase-to-phase voltage at same location before fault


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Temporary Overvoltages – Earth Faults The earth fault factor depends on the ratio of the complex impedances Z 1 and Z 0 of the positive and zero sequence systems (German: "Mitsystem", "Nullsystem"). In case of neglecting the resistances (possible in high-voltage systems) it depends on the ratio of the reactances X 0 and X 1: k =

3⋅

(

1 + X 0 / X 1 + X 0 / X 1

)

2

2 + X 0 / X 1

a ratio of X 0/ X 1 = -2 must be avoided!

resonant earthed neutral, isolated neutral

not for practical use!

l a r t u e n d e h t r a e y l d i l o s

resonant earthed neutral, isolated neutral

according to [BAL-04] Fachgebiet


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Temporary Overvoltages – Earth Faults Treatment of neutral in Germany (VDEW, 1998): treatment of neutral isolated resonant earthed

10 kV 8.6% 77.8%

20 kV < 0.1% 92.8%

110 kV 0.0% 80.9%

380 kV 0.0% 0.7%

solidly earthed

13.6%

2.2%

19.1%

99.3% according to [BAL-04]

Pictures: VATech

Earthing reactor (Petersen coil): fixed or switchable type Fachgebiet

Earthing reactor (Petersen coil): variable core type

Caused Caused by by several several recent recent blackouts blackouts itit has has been been considered considered internationally internationally to to increasingly increasingly operate operate sub-transmission sub-transmission systems systems (U (U ss ≤≤ 170 170 kV) kV) in in the the resonant resonant earthed earthed mode mode in in order order to to increase increase reliability reliability of of power power supply. supply. [Information [Information from from a Cigré Cigré meeting meeting in in Frankfurt, Frankfurt, October October 2005] 2005]


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Temporary Overvoltages – Earth Faults Drive

t n e m e v o m e r o c

Lead screw (the core is actually in 100% position)

Fixed part of the core

Active part of a high-voltage reactor with variable core Fachgebiet


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Temporary Overvoltages – Earth Faults Earth fault in case of isolated neutral system:

according to [BAL-04] Fachgebiet


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fault Fachgebiet


according to [BAL-04] - 15 -


fault clearing

k = 2 due to capacitances of zero sequence system, charged to a direct voltage according to [BAL-04] Fachgebiet


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Temporary Overvoltages – Earth Faults Intermitting earth fault in case of isolated neutral system: new fault after initial fault clearing

voltage of faulty phase Fachgebiet



Temporary Overvoltages – Earth Faults Intermitting earth fault in case of isolated neutral system: new fault after initial fault clearing

voltage of sound phase Fachgebiet



Temporary Overvoltages – Earth Faults Intermitting earth fault in case of isolated neutral system:

voltage of the zero sequence system Fachgebiet



Temporary Overvoltages – Earth Faults k ≈

3 ... 2

k ≤ 1.4

1.4 < k <≈ 1.8

k ≈

3 ...1.85

IEC 60071-1 Fachgebiet


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Temporary Overvoltages – Load Rejection (Example 1) Increase Increase in in generator generator voltage voltage of of 120% 120% Æ Æ voltage voltage increase increase on on highhighvoltage voltage side side of of generator generator transformer: transformer: from from 380 380 kV kV Æ 460 kV kV Æ 460 for for 1.4 1.4 ss duration! duration!

Example according to [ETG-93] Fachgebiet

Increase Increase in in frequency frequency leads leads to to repeated repeated phase phase oppositions oppositions at at the the open open circuit circuit breaker breaker for for several several minutes, minutes, see see next next slide slide Æ Æ Overvoltage Protection and Insulation Coordination / Chapter 2

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Temporary Overvoltages – Load Rejection (Example 1) Phase Phase opposition opposition between between open open circuit circuit breaker breaker terminals terminals –– stress stress of of longitudinal longitudinal insulation insulation

Example according to [ETG-93] Fachgebiet


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Temporary Overvoltages – Load Rejection (Example 2)

Voltage Voltage increase increase by by factor factor of of 1.35; 1.35; decrease decrease to to factor factor of of 1.2 1.2 after 2 s.

2: Constant excitation (manual regulation) 1: Excitation by rotating rectifiers

Example according to [DOR-81] Fachgebiet


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Temporary Overvoltages – Load Rejection (Example 3) TOV TOV at at the the end end of of aa long long transmission transmission line line • caused by capacitive currents • can be controlled by parallel compensation

U e =

U a cos β 1

U e ... voltage at end of line U a ... voltage at line entrance β1 ... phase angle of the positive system β 1 =

ω

a

v1

v 1 ... phase velocity of the positive system v1 =

1 L1′C 1′

Not Not an an issue issue for for "normal" "normal" length length transmission transmission lines lines

[DOR-81] Fachgebiet


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Temporary Overvoltages – Load Rejection (Summary) Temporary overvoltages caused by load rejection depend on • the rejected load • the system layout after disconnection • the characteristics of the sources (short-circuit power, generator type and regulation) Extremes: Low values of temporary overvoltages in systems with relatively short lines and high values of the short-circuit power at the terminal stations. High values of temporary overvoltages in systems with long lines and low values of shortcircuit power at the generating side (= typical situation of extra-high voltage systems in their initial stage). Voltage Voltage increase increase factors factors due due to to load load rejection: rejection: •• moderately moderately extended extended systems: systems: << 1.2 1.2 p.u. p.u. for for up up to to several several minutes minutes •• widely widely extended extended systems: systems: ≈≈ 1.5 1.5 p.u. p.u. for for some some seconds seconds •• close close to to turbo turbo generator: generator: ≈≈ 1.3 1.3 p.u. p.u. •• close close to to salient salient pole pole (German: (German: "Schenkelpol") "Schenkelpol") generator: generator: ≈≈ 1.5 1.5 p.u. p.u. Fachgebiet


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Temporary Overvoltages – Resonance Phenomena 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.

Can Can easily easily be be avoided avoided by by de-tuning de-tuning the the system system from from the the resonance resonance frequency! frequency!

Fachgebiet


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Temporary Overvoltages – Resonance Phenomena (Example 1) rd harmonics Energizing Energizing aa transformer transformer in in aa grid grid tuned tuned to to resonance resonance at at 33rd harmonics (150 (150 Hz) Hz)

Grid tuned to 150 Hz Fachgebiet

Æ

TOV of 1.9 p.u.

[DOR-81]

Grid tuned to (150 Hz – 7%) Æ TOV of 1.2 p.u.


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Temporary Overvoltages – Resonance Phenomena (Example 2) Load Load rejection rejection with with transformer transformer in in aa grid grid tuned tuned to to resonance resonance at at 55thth harmonics harmonics (250 (250 Hz) Hz)

length of line: a

Length of line: 174 km Æ f r = 250 Hz 5th harmonics 33% Æ TOV = 1.7 p.u. Fachgebiet

Length of line: 116 km Æ f r = 300 Hz 5th harmonics 10% Æ TOV = 1. p.u.


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[DOR-81]

Temporary Overvoltages and Surge Arresters Surge Surge arresters arresters cannot cannot limit limit TOV! TOV! Exception: Exception: resonance resonance effects effects may may be be suppressed suppressed or or even even avoided avoided by by MO MO arresters. arresters. Care Care has has then then to to be be taken taken not not to to thermally thermally overload overload the the arresters! arresters! 5 . u . p / 4 e g a t l o v 3 ) r e v o ( f o 2 e d u t i n g a 1 M

region region of of impressed impressed current current Æ voltage develops develops according according to to Æ voltage U-I-characteristics U-I-characteristics

Possible voltages without arresters

Withstand voltage of equipment

region region of of impressed impressed voltage voltage Æ current develops develops according according to to Æ current U-I-characteristics U-I-characteristics 1,3 1,25 1,2

Voltages limited by arresters r

0

1,15

U 1,1 /

Lightning overvoltages (Microseconds)

Switching overvoltages (Milliseconds)

Temporary overvoltages Highest voltage of equipment (Seconds) (Continuously)

Time duration of (over-)voltage

U

= v o

t

k

1,05 1 0,95 0,9 0,85 0,8 0,1

1

10

t / s

Fachgebiet


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1000

Overvoltage Protection and Insulation Coordination

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