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

Overvoltage Protection and Insulation Coordination / Chapter 2

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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

Standard Standard voltage voltage

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

Overvoltage Protection and Insulation Coordination / Chapter 2

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

" 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 Standard voltage voltage

"Standard switching impulse"

Example [THI-01]

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

Overvoltage Protection and Insulation Coordination / Chapter 2

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

" 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. " 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 Standard voltage voltage

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

" 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. "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

Standard Standard voltage voltage

not standardized Example [THI-01]

Fachgebiet

Overvoltage Protection and Insulation Coordination / Chapter 2

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

" 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 Standard voltage voltage

" 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

Overvoltage Protection and Insulation Coordination / Chapter 2

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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

Overvoltage Protection and Insulation Coordination / Chapter 2

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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

Overvoltage Protection and Insulation Coordination / Chapter 2

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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]

Overvoltage Protection and Insulation Coordination / Chapter 2

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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

Overvoltage Protection and Insulation Coordination / Chapter 2

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

according to [BAL-04] Fachgebiet

Overvoltage Protection and Insulation Coordination / Chapter 2

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

fault Fachgebiet

Overvoltage Protection and Insulation Coordination / Chapter 2

according to [BAL-04] - 15 -

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

fault clearing

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

Overvoltage Protection and Insulation Coordination / Chapter 2

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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

Overvoltage Protection and Insulation Coordination / Chapter 2

according to [BAL-04] - 17 -

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

voltage of sound phase Fachgebiet

Overvoltage Protection and Insulation Coordination / Chapter 2

according to [BAL-04] - 18 -

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

voltage of the zero sequence system Fachgebiet

Overvoltage Protection and Insulation Coordination / Chapter 2

according to [BAL-04] - 19 -

Temporary Overvoltages – Earth Faults k  ≈

3 ... 2

k  ≤ 1.4

1.4 < k