Volt
Voltage St
age Stress
ress in Pow
in Power Sys
er Systems
tems -- Class
Classifica
ification
tion
IEC 60071-1 IEC 60071-1
Classification of real stress
Classification of real stress
Classification of real stress
Classification of real stress
Voltage Stress in Power
Voltage Stress in Power Systems
Systems
"
"CContinontinuous uous ((power-power-frequefrequency) ncy) voltvoltaage"ge"
Power-frequency voltage, considered having constant r.m.s. value, continuously applied to any pair of Power-frequency voltage, considered having constant r.m.s. value, continuously applied to any pair of terminals of an insulation configuration
terminals of an insulation configuration f f = 50 Hz or 60 Hz= 50 Hz or 60 Hz
T
T 11 ≥≥ 3 600 s3 600 s
Æ
Æ Any power-frequency voltage lasting for 1 h or more is considered a continuous voltage! Any power-frequency voltage lasting for 1 h or more is considered a continuous voltage!
Standard voltage
Standard voltage
Standard voltage
Standard voltage
"Standard power-frequency voltage""Standard power-frequency voltage"A sinusoidal voltage with frequency of 50 Hz or 60 Hz A sinusoidal voltage with frequency of 50 Hz or 60 Hz
T
T 11 to be specified by the apparatus committeesto be specified by the apparatus committees ÆÆ TT11 up tup to 2 yeao 2 years!rs! ÆÆ see next slidessee next slides
Conversion Conversion
into into
Volt
Example: Cable tests at power-frequency voltage
Example: Cable tests at power-frequency voltage
Voltage Stress in Power Systems
Voltage Stress in Power Systems - Classification
Example: Cable tests at power-frequency voltage
Example: Cable tests at power-frequency voltage
Voltage Stress in Power Systems
Voltage Stress in Power Systems - Classification
Source: Brugg Cables
Voltage Stress in Power Systems
"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
Standard voltage
Standard voltage
" Standard short-duration power-frequency voltage"A sinusoidal voltage with frequency between 48 Hz and 62 Hz
T 1 = 60 s
Conversion into
Classification of real stress
Classification of real stress
Example [THI-01]
Voltage Stress in Power Systems
" 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 voltage
Standard voltage
"Standard switching impulse"An impulse voltage of
T p = 250 µs
T 2 = 2 500 µs
Conversion into
Classification of real stress
Classification of real stress
" Slow-front ov ervolt age" Transient overvoltage, usually unidirectional
5000 µs ≥ T p > 20 µs
T 2 ≤ 20 ms
Main reasons: line faults, switching
Example [THI-01]
Voltage Stress in Power Systems
" 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 voltage
Standard voltage
"Standard lightning impulse"An impulse voltage of
T 1 = 1.2 µs
T 2 = 50 µs
Conversion into
Classification of real stress
Classification of real stress
" Fast-front overvoltage" Transient overvoltage, usually unidirectional
20 µs ≥ T 1 > 0.1 µs
T 2 ≤ 300 µs
Main reasons: lightning strokes, switching
Example [THI-01]
Voltage Stress in Power Systems
" 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 voltage
Standard voltage
not standardizedConversion into
Classification of real stress
Classification of real stress
"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
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 voltage
Standard voltage
" Standard combi ned switchi ng impulse"Conversion into
Classification of real stress
Classification of real stress
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
kLE b / 3 U k U =
... in other "words":
U LE... phase-to-earth voltage of sound phase during faultU b ... phase-to-phase voltage at same location before fault
Temporary Overvoltages – Earth Faults
The earth fault factor depends on the ratio of the complex impedances Z
1and
Z
0of 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
0and X
1:
(
)
2 0 1 0 1 0 1 1 / / 3 2 / X X X X k X X + + = ⋅ +a ratio of X
0/ X
1= -2 must be avoided!
so l i d l y e a r t h e d n e u t r a l resonant earthed neutral, isolated neutral resonant earthed neutral, isolated neutral not for practical use! according to [BAL-04]
Temporary Overvoltages – Earth Faults
Treatment of neutral in Germany (VDEW, 1998):
according to [BAL-04]
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 (U s ≤ 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 (U s ≤ 170 kV) in the resonant
earthed mode in order to increase reliability of power supply. [Information from a Cigré meeting in Frankfurt, October 2005]
Temporary Overvoltages – Earth Faults
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)
c o r e m o v e m e n t
Temporary Overvoltages – Earth Faults
Earth fault in case of isolated neutral system:
Temporary Overvoltages – Earth Faults
Earth fault in case of isolated neutral system:
fault
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
Temporary Overvoltages – Earth Faults
Intermitting earth fault in case of isolated neutral system:
new fault after initial fault clearing
voltage of faulty phase
Temporary Overvoltages – Earth Faults
Intermitting earth fault in case of isolated neutral system:
new fault after initial fault clearing
voltage of sound phase
Temporary Overvoltages – Earth Faults
Intermitting earth fault in case of isolated neutral system:
voltage of the zero sequence system
Temporary Overvoltages – Earth Faults
3 ... 2 k ≈ 1.4 k ≤ 1.4 < <≈k 1.8 3 ...1.85 k ≈ IEC 60071-1You're Reading a Preview
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Temporary Overvoltages – Load Rejection (Example 1)
Example according to [ETG-93]
Phase opposition between open circuit breaker terminals – stress of longitudinal insulation Phase opposition between open circuit breaker terminals – stress of longitudinal insulation
Temporary Overvoltages – Load Rejection (Example 2)
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.
Temporary Overvoltages – Load Rejection (Example 3)
TOV at the end of a long transmission line TOV at the end of a long transmission line
• caused by capacitive currents
• can be controlled by parallel compensation
e 1
cos
aU
U
β
=
U e ... voltage at end of line
U a ... voltage at line entrance
1 1 a v
ω
β
=β1 ... phase angle of the positive system
1 1 1 1 v L C = ′ ′
v 1 ... phase velocity of the positive system
[DOR-81]
Not an issue for "normal"
length transmission lines
Not 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 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
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 Æ TOV of 1.9 p.u. Grid tuned to (150 Hz – 7%) Æ TOV of 1.2 p.u.
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 Æ f r = 250 Hz
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 t o v = U / U r
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) M a g n i t u d e o f ( o v e r - ) v o l t a g e / 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) M a g n i t u d e o f ( o v e r - ) v o l t a g e / p . u . 1 2 3 4 0 5
region of impressed voltage
Æ current develops according to
U-I-characteristics
region of impressed voltage
Æ current develops according to
U-I-characteristics
region of impressed current
Æ voltage develops according to
U-I-characteristics
region of impressed current
Æ voltage develops according to
