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Fachgebiet
Hochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 2 - 1 -
Voltage Stress in Power Systems - Classification
IEC 60071-1
Fachgebiet
Hochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 2 - 2 -
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 configuration
f = 50 Hz or 60 Hz
T1 ≥ 3 600 s
Æ 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 Hz
T1 to be specified by the apparatus committees Æ T1 up to 2 years!
Conversion
into
Voltage Stress in Power Systems - Classification
Fachgebiet
Hochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 2 - 3 -
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 ≥ T1 ≥ 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 voltageStandard voltage "Standard short-duration power-frequency voltage"
A sinusoidal voltage with frequency between 48 Hz and 62 Hz
T1 = 60 s 
Conversion
into
Classification of real stressClassification of real stress
Example [THI-01]
Voltage Stress in Power Systems - Classification
Fachgebiet
Hochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 2 - 4 -
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 voltageStandard voltage "Standard switching impulse"
An impulse voltage of
Tp = 250 µs
T2 = 2 500 µs 
Conversion
into
Classification of real stressClassification of real stress
"Slow-front overvoltage"
Transient overvoltage, usually unidirectional
5000 µs ≥ Tp > 20 µs
T2 ≤ 20 ms
Main reasons: line faults, switching
Example [THI-01]
Voltage Stress in Power Systems - Classification
Fachgebiet
Hochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 2 - 5 -
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 voltageStandard voltage "Standard lightning impulse"
An impulse voltage of
T1 = 1.2 µs
T2 = 50 µs 
Conversion
into
Classification of real stressClassification of real stress
"Fast-front overvoltage"
Transient overvoltage, usually unidirectional
20 µs ≥ T1 > 0.1 µs
T2 ≤ 300 µs
Main reasons: lightning strokes, switching
Example [THI-01]
Voltage Stress in Power Systems - Classification
Fachgebiet
Hochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 2 - 6 -
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 voltageStandard voltage Æ not standardized
Conversion
into
Classification of real stressClassification of real stress
"Very-fast-front overvoltage"
Transient overvoltage, usually unidirectional
Tf < 100 ns
(Tt ≤ 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 - Classification
Fachgebiet
Hochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 2 - 7 -
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"
Conversion
into
Classification of real stressClassification 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.
Voltage Stress in Power Systems - Classification
Fachgebiet
Hochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 2 - 8 -
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
Fachgebiet
Hochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 2 - 9 -
Temporary Overvoltages – Earth Faults
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
k
X X
+ += ⋅ +
a ratio of X0/X1 = -2 must be avoided!
s
o
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l
y
 
e
a
r
t
h
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n
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r
a
l
resonant earthed
neutral,
isolated neutral
resonant earthed
neutral,
isolated neutral
not for
practical use!
according to [BAL-04]
Fachgebiet
Hochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 2 - 10 -
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 (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 Cigrémeeting in Frankfurt, 
October 2005]
Fachgebiet
Hochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 2 - 11 -
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
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e
 
m
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e
m
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n
t
Fachgebiet
Hochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 2 - 12 -
Temporary Overvoltages – Earth Faults
Earth fault in case of isolated neutral system: 
according to [BAL-04]
Fachgebiet
Hochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 2 - 13 -
Temporary Overvoltages – Earth Faults
Earth fault in case of isolated neutral system: 
fault
according to [BAL-04]
Fachgebiet
Hochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 2 - 14 -
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
Hochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 2 - 15 -
Temporary Overvoltages – Earth Faults
Intermitting earth fault in case of isolated neutral system: 
new fault after initial fault clearing 
voltage of faulty phase
according to [BAL-04]
Fachgebiet
Hochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 2 - 16 -
Temporary Overvoltages – Earth Faults
Intermitting earth fault in case of isolated neutral system: 
new fault after initial fault clearing 
voltage of sound phase
according to [BAL-04]
Fachgebiet
Hochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 2 - 17 -
Temporary Overvoltages – Earth Faults
Intermitting earth fault in case of isolated neutral system: 
voltage of the zero sequence system
according to [BAL-04]
Fachgebiet
Hochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 2 - 18 -
Temporary Overvoltages – Earth Faults
3 ... 2k ≈
1.4k ≤
1.4 1.8k< <≈
3 ...1.85k ≈
IEC 60071-1
Fachgebiet
Hochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 2 - 19 -
Temporary Overvoltages – Load Rejection (Example 1)
Example according to [ETG-93]
Increase in generator 
voltage of 120% Æ
voltage increase on high-
voltage side of generator 
transformer:
from 380 kV Æ 460 kV 
for 1.4 s duration!
Increase in generator 
voltage of 120% Æ
voltage increase on high-
voltage side of generator 
transformer:
from 380 kV Æ 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 Æ
Increase in frequency 
leads to repeated phase 
oppositions at the open 
circuit breaker for several 
minutes, see next slide Æ
Fachgebiet
Hochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 2 - 20 -
Temporary Overvoltages – Load Rejection (Example 1)
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
Fachgebiet
Hochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 2 - 21 -
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.
Fachgebiet
Hochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 2 - 22 -
Temporary Overvoltages – Load Rejection (Example 3)
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
aUU β=
Ue ... voltage at end of line
Ua ... voltage at line entrance
1
1
a
v
ωβ =β1 ... phase angle of the positive system
1
1 1
1v
LC
= ′ ′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
Fachgebiet
Hochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 2 - 23 -
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).
Fachgebiet
Hochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 2 - 24 -
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!
Fachgebiet
Hochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 2 - 25 -
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.
[DOR-81]
Fachgebiet
Hochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 2 - 26 -
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 fr = 250 Hz
5th harmonics 33% Æ TOV = 1.7 p.u.
Length of line: 116 km Æ fr = 300 Hz
5th harmonics 10% Æ TOV = 1. p.u.
Fachgebiet
Hochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 2 - 27 -
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 overloadthe 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
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f
 
(
o
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-
)
v
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t
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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
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n
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t
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f
 
(
o
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-
)
v
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a
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/
 
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
U-I-characteristics