Voltage Stress in Power Systems -

7/30/2019 Voltage Stress in Power Systems -

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HochspannungstechnikOvervoltage Protection and Insulation Coordination / Chapter 2 - 1 -

Voltage Stress in Power Systems - Classification

IEC 60071-1


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Classification of real stress

Classification 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 configuration

f= 50 Hz or 60 Hz

T1 3 600 s

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

Standard voltage

Standard voltage "Standard power-frequency voltage"

A sinusoidal voltage with frequency of 50 Hz or 60 Hz

T1 to be specified by the apparatus committees T1 up to 2 years! see next slides

Conversioninto



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Example: Cable tests at power-frequency voltage

Example: Cable tests at power-frequency voltage

Voltage Stress in Power SystemsVoltage Stress in Power Systems - Classification

Lifetime characteristic:


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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


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"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 T1 0.02 s

Highest 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 Hz

T1 = 60 s

Conversioninto

Classification of real stressClassification of real stress

Example [THI-01]



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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.

Standard voltageStandard voltage "Standard switching impulse"

An impulse voltage of

Tp = 250 s

T2 = 2 500 s

Conversioninto


"Slow-front overvoltage"

Transient overvoltage, usually unidirectional

5000 s Tp > 20 s

T2 20 ms

Main reasons: line faults, switching

Example [THI-01]



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separate events.

Standard voltageStandard voltage "Standard lightning impulse"

An impulse voltage of

T1 = 1.2 s

T2 = 50 s

Conversioninto


"Fast-front overvoltage"


20 s T1 > 0.1 s

T2 300 s

Main reasons: lightning strokes, switching

Example [THI-01]



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separate events.

Standard voltageStandard voltage not standardized

Conversioninto


"Very-fast-front overvoltage"


Tf< 100 ns

(Tt 3 ms)

basic oscillation (1st harmonics) 30 kHz < f< 300 kHzsuperimposed oscillations 300 kHz < f< 100 MHz

Main reasons: switching of disconnectors in GIS

Example [THI-01]



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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.

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. Bothimpulses 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.



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

LE

b/ 3

Uk

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


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The earth fault factor depends on the ratio of the complex impedances Z1 andZ0 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:

( )2

0 1 0 1

0 1

1 / /3

2 /

X X X Xk

X X

+ +=

+

a ratio ofX0/X1 = -2 must be avoided!solidlyearth

edneutral

resonant earthedneutral,isolated neutral

resonant earthedneutral,isolated neutral

not forpractical use!

according to [BAL-04]


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Treatment 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 resonantearthed mode in order to increase

reliability of power supply. [Information

from a Cigr 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 Cigr meeting in Frankfurt,

October 2005]


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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)

coremove

ment


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Earth fault in case of isolated neutral system:



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faultaccording to [BAL-04]


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fault clearing

k= 2 due to capacitances of zero sequence system, charged to a direct voltage



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Intermitting earth fault in case of isolated neutral system:new fault after initial fault clearing

voltage offaulty phaseaccording to [BAL-04]


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Intermitting earth fault in case of isolated neutral system:new fault after initial fault clearing

voltage ofsound phaseaccording to [BAL-04]


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Intermitting earth fault in case of isolated neutral system:

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


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3 ... 2k

1.4k

1.4 1.8k<


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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


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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


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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.


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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

e

1cos

aU

U

=

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

1

1

av

=1 ... phase angle of the positive system

1

1 1

1v

L C

=

v1 ... phase velocity of the positive system

[DOR-81]

Not an issue for "normal"

length transmission lines

Not an issue for "normal"

length transmission lines


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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 intheir initial stage).


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Temporary Overvoltages Resonance Phenomena

Temporary overvoltages caused by resonance phenomena generally arise when circuitswith large capacitive elements, such as

lines

cables

series compensated linesand 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!


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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]


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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.


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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

ktov=

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)

Magnitudeo

f(over-)voltage

/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)

Magnitudeo

f(over-)voltage

/p.u.

1

2

3

4

0

5

region of impressed voltage current develops according to

U-I-characteristics

region of impressed voltage current develops according toU-I-characteristics

region of impressed current

voltage develops according toU-I-characteristics

region of impressed current

voltage develops according toU-I-characteristics

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Voltage Stress in Power Systems -