Overvoltage Protection and Insulation Coordination

FachgebietHochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 2 - 1 -

Voltage Stress in Power Systems - Classification

IEC 60071-1


Classification of real stressClassification of real stress

Voltage Stress in Power Systems

"Continuous (power-frequency) voltage"

Power-frequency voltage, considered having constant r.m.s. value, continuously applied to any pair of terminals of an insulation configurationf = 50 Hz or 60 HzT1 ≥ 3 600 s

Any power-frequency voltage lasting for 1 h or more is considered a continuous voltage!

Standard voltageStandard voltage "Standard power-frequency voltage"

A sinusoidal voltage with frequency of 50 Hz or 60 HzT1 to be specified by the apparatus committees T1 up to 2 years! see next slides

Conversioninto



Example: Cable tests at power-frequency voltageExample: Cable tests at power-frequency voltage

Voltage Stress in Power SystemsVoltage Stress in Power Systems - Classification

Lifetime characteristic:


Example: Cable tests at power-frequency voltageExample: Cable tests at power-frequency voltage

Voltage Stress in Power SystemsVoltage Stress in Power Systems - Classification

Source: Brugg Cables

11.4 years



"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 Hz3 600 s ≥ T1 ≥ 0.02 sHighest values by following main reasons:

• phase-to-earth earth faults and load rejection• phase-to-phase load rejection• longitudinal phase opposition during synchronization of two grids

Standard voltageStandard voltage "Standard short-duration power-frequency voltage"

A sinusoidal voltage with frequency between 48 Hz and 62 HzT1 = 60 s

Conversioninto


Example [THI-01]




"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.

Standard voltageStandard voltage "Standard switching impulse"

An impulse voltage ofTp = 250 µsT2 = 2 500 µs

Conversioninto


"Slow-front overvoltage"Transient overvoltage, usually unidirectional5000 µs ≥ Tp > 20 µsT2 ≤ 20 msMain reasons: line faults, switching

Example [THI-01]






Standard voltageStandard voltage "Standard lightning impulse"

An impulse voltage ofT1 = 1.2 µsT2 = 50 µs

Conversioninto


"Fast-front overvoltage"Transient overvoltage, usually unidirectional20 µs ≥ T1 > 0.1 µsT2 ≤ 300 µsMain reasons: lightning strokes, switching

Example [THI-01]






Standard voltageStandard voltage not standardized

Conversioninto


"Very-fast-front overvoltage"Transient overvoltage, usually unidirectionalTf < 100 ns(Tt ≤ 3 ms)basic oscillation (1st harmonics) 30 kHz < f < 300 kHzsuperimposed oscillations 300 kHz < f < 100 MHzMain reasons: switching of disconnectors in GIS

Example [THI-01]



Voltage Stress in Power Systems"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.

Standard voltageStandard voltage "Standard combined switching impulse"

Conversioninto


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.



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 parameter: Earth fault factor kImportant parameter: Earth fault factor k

LE

b / 3Uk

U=... in other "words": ULE ... phase-to-earth voltage of sound phase during fault

Ub ... phase-to-phase voltage at same location before fault

IEC 60071-1



The earth fault factor depends on the ratio of the complex impedances Z1 and Z0 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 X0 and X1:

( )20 1 0 1

0 1

1 / /3

2 /X X X X

kX X

+ += ⋅

+

a ratio of X0/X1 = -2 must be avoided!

solid

ly e

arth

ed n

eutra

l

resonant earthedneutral,isolated neutral

resonant earthedneutral,isolated neutral

not forpractical use!

according to [BAL-04]


Temporary Overvoltages – Earth FaultsTreatment of neutral in Germany (VDEW, 1998):


treatment of neutral 10 kV 20 kV 110 kV 380 kV isolated 8.6% < 0.1% 0.0% 0.0% resonant earthed 77.8% 92.8% 80.9% 0.7% solidly earthed 13.6% 2.2% 19.1% 99.3%

Earthing reactor (Petersen coil):fixed or switchable type

Earthing reactor (Petersen coil):variable core type

Pictures: VATech

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

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



Active part of a high-voltage reactor with variable core

Fixed part of the core

Drive

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

core

mov

emen

t


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




faultaccording to [BAL-04]



fault clearing

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


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

new fault after initial fault clearing

voltage of faulty phaseaccording to [BAL-04]



new fault after initial fault clearing

voltage of sound phaseaccording to [BAL-04]



voltage of the zero sequence systemaccording to [BAL-04]



3 ... 2k ≈

1.4k ≤

1.4 1.8k< <≈

3 ...1.85k ≈

IEC 60071-1


Temporary Overvoltages – Load Rejection (Example 1)

Example according to [ETG-93]

Increase in generator voltage of 120% voltage increase on high-voltage side of generator transformer:from 380 kV 460 kV for 1.4 s duration!

Increase in generator voltage of 120% voltage increase on high-voltage side of generator transformer:from 380 kV 460 kV for 1.4 s duration!

Increase in frequency leads to repeated phase oppositions at the open circuit breaker for several minutes, see next slide

Increase in frequency leads to repeated phase oppositions at the open circuit breaker for several minutes, see next slide



Example according to [ETG-93]

Phase opposition between open circuit breaker terminals – stress of longitudinal insulationPhase opposition between open circuit breaker terminals – stress of longitudinal insulation



Example according to [DOR-81]

1: Excitation by rotating rectifiers

2: Constant excitation (manual regulation)

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



TOV at the end of a long transmission lineTOV at the end of a long transmission line

• caused by capacitive currents• can be controlled by parallel compensation

e1cos

aUUβ

=

Ue ... voltage at end of lineUa ... voltage at line entrance

11

avωβ =β1 ... phase angle of the positive system

11 1

1vL C

=′ ′

v1 ... phase velocity of the positive system

[DOR-81]

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


Temporary Overvoltages – Load Rejection (Summary)

Voltage increase factors due to load rejection:• moderately extended systems: < 1.2 p.u. for up to several minutes• widely extended systems: ≈ 1.5 p.u. for some seconds• close to turbo generator: ≈ 1.3 p.u.• close to salient pole (German: "Schenkelpol") generator: ≈ 1.5 p.u.

Voltage increase factors due to load rejection:• moderately extended systems: < 1.2 p.u. for up to several minutes• widely extended systems: ≈ 1.5 p.u. for some seconds• close to turbo generator: ≈ 1.3 p.u.• close to salient pole (German: "Schenkelpol") generator: ≈ 1.5 p.u.

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 highvalues of the short-circuit power at the terminal stations.High values of temporary overvoltages in systems with long lines and low values of short-circuit power at the generating side (= typical situation of extra-high voltage systems in their initial stage).


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


Temporary Overvoltages – Resonance Phenomena (Example 1)

Energizing a transformer in a grid tuned to resonance at 3rd harmonics (150 Hz)Energizing a transformer in a grid tuned to resonance at 3rd harmonics (150 Hz)

Grid tuned to 150 Hz TOV of 1.9 p.u. Grid tuned to (150 Hz – 7%) TOV of 1.2 p.u.

[DOR-81]


Temporary Overvoltages – Resonance Phenomena (Example 2)

Load rejection with transformer in a grid tuned to resonance at 5th harmonics (250 Hz)Load rejection with transformer in a grid tuned to resonance at 5th harmonics (250 Hz) [DOR-81]

length of line: a

Length of line: 174 km fr = 250 Hz5th harmonics 33% TOV = 1.7 p.u.

Length of line: 116 km fr = 300 Hz5th harmonics 10% TOV = 1. p.u.


Temporary Overvoltages and Surge Arresters

Surge arresters cannot limit TOV!Exception: resonance effects may be suppressed or even avoided by MO arresters.Care has then to be taken not to thermally overload the arresters!

Surge arresters cannot limit TOV!Exception: resonance effects may be suppressed or even avoided by MO arresters.Care has then to be taken not to thermally overload the arresters!

0,8

0,85

0,9

0,95

1

1,05

1,1

1,15

1,2

1,25

1,3

0,1 1 10 100 1000

t / s

k tov

= U

/Ur

Time duration of (over-)voltage

Possible voltages without arresters

Voltages limited by arresters

Withstand voltage of equipment

Lightning overvoltages(Microseconds)

Switching overvoltages(Milliseconds)

Temporary overvoltages(Seconds)

Highest voltage of equipment(Continuously)

Mag

nitu

deof

(ove

r-)vo

ltage

/ p.u

.

1

2

3

4

0

5

Time duration of (over-)voltage

Possible voltages without arresters

Voltages limited by arresters

Withstand voltage of equipment

Lightning overvoltages(Microseconds)

Switching overvoltages(Milliseconds)

Temporary overvoltages(Seconds)

Highest voltage of equipment(Continuously)

Mag

nitu

deof

(ove

r-)vo

ltage

/ p.u

.

1

2

3

4

0

5

region of impressed voltagecurrent develops according to

U-I-characteristics

region of impressed voltagecurrent develops according to

U-I-characteristics

region of impressed currentvoltage develops according to

U-I-characteristics

region of impressed currentvoltage develops according to

U-I-characteristics

Overvoltage Protection and Insulation Coordination

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