Application guidelines
Metal-oxide surge arresters
in medium-voltage systems
M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S |
First published November 1994
2nd revised edition: September 1995
3rd revised edition: May 1999
4th revised and expanded edition: February 2009
5th revised edition: May 2011
6th revised edition: June 2018
All rights reserved. Neither the complete brochure
nor parts of it are to be copied, reproduced, transmitted
in any way or translated into other languages without the
express written consent of Hitachi Energy Switzerland Ltd.
© Hitachi Energy Switzerland Ltd.
Surge Arresters
Wettingen, Switzerland
3
Foreword to the sixth edition
The first edition of our guidelines for the dimensioning, testing and application of
metal-oxide surge arresters (MO surge arresters) for use in medium-voltage systems
appeared in 1994.
A number of developments in technology and application of MO surge arresters as well
as in standardization have taken place in the past years. The standards produced by
TC 37 of IEC have undergone radical changes, based on recent research work initialized
and supervised by Cigré working groups WG A3.17 and WG A3.25 of SC A3 High
Voltage Equipment. Edition 3.0 of IEC 60099-4 contains important changes to the
definitions and test requirements of the energy-handling capability. All related standards
of the IEC 60099 series were adapted accordingly, and new standards were published.
Consequently, it was necessary to completely revise the selection principles and
application recommendations. In principle, this revised brochure keeps the concept
of the previous editions. The design, function and application of MO surge arresters
are described, taking into consideration the new definitions and test procedures.
Some chapters have been shortened and are more concentrated, for better readability.
Additional and more detailed information will be given in separate documents with
respect to theory, background information, and specific applications.
We hope that you as a reader will be satisfied with the new appearance of our new
edition, and that you will find it useful for your purposes. We welcome amendments
and suggestions that help us to better understand and meet all possible customer
needs. Further, we would like to thank everyone who contributed their valuable
comments on this brochure.
Bernhard Richter
Hitachi Energy Switzerland Ltd.
Surge Arresters
Wettingen, May 2018
4
| M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S
Table of contents
003
Foreword to the sixth edition
006
01
Introduction
008 02
Surge arrester technology
2.1
General
2.2
Arrester design
2.3
Metal-oxide resistors
2.4
High-field MO resistors and GIS arresters
2.5
Influence of different frequencies and DC voltage on MO resistors
Different frequencies
DC voltage
2.6
Microvaristors and field grading
018
03
Function and performance of MO surge arresters
3.1
General
3.2
Currents and voltages
3.3
Charge transfer and energy absorption capability
3.4
Cool-down time
3.5
Stability of an MO surge arrester
Thermal stability
Long-term stability
3.6
Protective characteristics
3.7
Temporary overvoltage
024 04
4.1
4.2
4.3
4.4
4.5
4.6
4.7
Service conditions
Normal service conditions
Special service conditions
Overload behavior
Mechanical stability
Elevated ambient temperature
Pollution and cleaning
Altitude adjustment of the arrester housing
02605
5.1
5.2
5.3
5.4
5.5
5.6
Tests
General
Type tests (design tests)
Routine tests
Acceptance tests
Special tests
Commissioning and on-site tests
M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S |
032 06
Neutral earthing methods and determination of Uc
6.1
General considerations
6.2
Systems with insulated star point or with earth fault compensation
6.3
Systems with high-ohmic insulated neutral and automatic earth fault clearing
6.4
Systems with direct or low-ohmic star point earthing
Systems with direct star point earthing
Systems with low-ohmic star point earthing
6.5
Four-wire, multi-earthed-wye systems
6.6
Distribution systems with delta connection
6.7
Arresters between phases
Six-arrester arrangement
Neptune design
6.8
Operating voltage with harmonic oscillation
037
07
7.1
7.2
7.3
7.4
Coordination of insulation and selection of MO surge arresters
General considerations
Selection of nominal discharge current, charge and energy
Protection level
Selection of arrester housing
04108
8.1
8.2
8.3
8.4
Protective distance of MO surge arresters
General considerations
Traveling waves
Protective distance
Induced voltages
044 09
9.1
9.2
9.3
9.4
9.5
9.6
9.7
9.8
9.9
9.10
Equipment protection
General considerations
Protection of transformers
Protection of cables
Cable sheath protection
Arresters in metal-enclosed medium-voltage substations (cubicles)
Generator connected to a lightning-endangered MV line
Protection of motors
Arresters parallel to a capacitor bank
Line traps (parallel protection)
Line arresters
050
10
10.1
10.2
10.3
MO surge arresters in parallel connection
General considerations
Parallel connections to increase the energy handling capability
Coordination of parallel-connected MO surge arresters
052
11
11.1
11.2
11.3
11.4
Accessories
Spark prevention unit
Disconnectors
Indicators
Brackets, ground plates and clamping devices
053
12
Monitoring of MO surge arresters
054
13
Overload and failure analysis
055
14
Summary and developments
056
Acronyms/Abbreviations
057
Literature
5
6
| M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S
01 Introduction
Hitachi Energy in Switzerland produces MO surge arresters for the protection of equipment
against transient overvoltages. Decades of experience in design and development gives the
competence for specific solutions.
Overvoltages in electrical supply systems result from the
effects of lightning incidents and switching actions and cannot
be avoided. They endanger the electrical equipment because,
for economic reasons, the insulation cannot be designed
to withstand all possible cases. An economical and safe on-line
system calls for extensive protection of the electrical equipment
against unacceptable overvoltage stresses. This applies generally to high-voltage systems as well as to medium- and
low-voltage systems.
The most effective protection against
overvoltages in medium-voltage systems is
therefore the use of surge arresters in the
vicinity of the electrical equipment.
Overvoltage protection can be basically achieved in two ways:
A surge arrester is a protective device for limiting surge
voltages on equipment by diverting surge current and returning
the device to its original status. A surge arrester is capable of
repeating these functions a large number of times as specified.
1. Avoiding lightning overvoltage at the point of origin, such
as through shielding wires in front of the substation that
intercept lightning
2. Limiting overvoltage near the electrical equipment, for
instance through surge arresters in the vicinity of the
electrical equipment
Today’s technology for surge arresters intended for use in
medium-voltage systems is the gapless metal-oxide surge
arrester (MO surge arrester) with a synthetic housing. Therefore,
this brochure concentrates only on MO surge arresters without
gaps with silicone housing, as developed and produced
by Hitachi Energy in Switzerland.
In high-voltage systems, both methods of protection are
common. Shielding wire protection of overhead lines in
medium-voltage systems is not generally used.
Hitachi Energy in Switzerland has concentrated all surge
arrester activities under one roof in Wettingen. This ensures that
all steps in development and production, from raw material
qualification up to shipment of the final product, are under the
same management and quality control.
M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S |
Close cooperation with Hitachi Energy’s corporate research
center, which is located a short distance from the surge arrester
factory, ensures that the current state of the art is considered in
material technology and processing of MO resistors and surge
arresters. This, and decades of experience in the design and
development of MO surge arresters gives the competence for
specific solutions and applications in overvoltage protection.
Due to the variety of MO resistors and surge arresters produced, applications in AC and DC power systems, e.g. traction
systems and high-voltage direct current (HVDC) systems are
possible with products adapted to the system and environmental requirements.
In this brochure, the basics of MO surge arrester technology
are described, covering the function and performance of MO
material and MO surge arresters. Service conditions and tests
according to the current international standard IEC 60099-4,
Ed. 3.0 are listed and briefly explained. Then follows a section
on neutral earthing methods in medium-voltage power systems,
which is important for the selection of the power frequency voltages that can be applied to the MO surge arresters. Installation
principles and the protective distance of MO surge arresters is
addressed, followed by a more detailed section describing the
protection of various equipment in medium-voltage systems.
7
Special applications, like parallel connection of MO resistors
and surge arresters, and coordination of surge arresters, are
addressed in a separate chapter. Accessories like disconnectors and indicators, etc., are mentioned. Finally some remarks
are made on the overload performance of MO surge arresters
and failure analysis. An overview of ongoing developments in
MO surge arrester technology and standardization closes this
brochure. Acronyms, abbreviations and a list of literature are
given at the end.
In the large number of publications on MO resistors and
MO surge arresters, different terms are used for basically
the same object: ZnO varistor, ZnO resistor, MO varistor, MO
resistor, varistor, ZnO or MO arrester, MO surge arrester, etc.
This has historical reasons, and also depends on the technical
community or the kind of research and development performed.
In this brochure, the technical terms MO resistor and MO surge
arrester are mainly used, following the wording in the international standards of IEC TC 37, which are directly related to the
subjects of this brochure.
8
| M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S
02 Surge arrester technology
Based on experience in surge arrester design and application from the very beginning
of arrester technology, Hitachi Energy in Switzerland produces today MO surge arresters,
MO resistors and microvaristors for various applications in overvoltage protection and
field grading.
2.1 General
Hitachi Energy in Switzerland has a long history in MO technology and surge arrester design. In the 1980s, BBC (now Hitachi
Energy) started producing MO resistors and gapless MO surge
arresters. In 1986, the first MO surge arresters with patented
direct molding for medium-voltage systems were delivered.
In the same year, the first gapless MO surge arrester for SF6
gas insulated substations (GIS) came to market.
Since the beginning, continuous development of products and
process technology has taken place. High-field MO resistors
have been developed, along with MO resistors for DC applications and microvaristors for field grading applications, to name
a few. Applications in specific fields, like traction systems,
power electronics and wind power parks, are possible with
products adapted to the specific requirements.
2.2 Arrester design
Generally, an MO surge arrester is made up of two parts:
the active part, consisting of one or more piled up MO resistors,
and an insulating housing, which guarantees both the insulation
and the mechanical strength.
Fundamentally, there are three different possibilities for
construction:
• The active part is held mechanically together with glass-fiber
reinforced loops or straps. The polymeric material (such as
silicone) is directly molded on to the MO resistors. This direct
molding has the advantage that no gas volume remains in
the arrester. Sealing problems and inner partial discharges
are thus out of the question. There are no interfaces between
the polymeric materials into which humidity can penetrate.
The danger of violent shattering of the housing is negligible.
MO surge arresters designed according this principle belong
to Group I, see Figure 1a
• The active part is wrapped with glass-fiber material and is
soaked with resin, which turns the whole into a rigid body.
The insulating polymeric housing is then slipped over the resin
block or shrunk onto it. This construction has the disadvan-
All MO surge arresters produced by Hitachi Energy in Switzerland used in medium-voltage systems are designed according
to the same principle. This construction concept of silicone
direct molding, which was patented by Hitachi Energy, consists
of two electrodes connected together through two or more
glass-fiber reinforced elements. It results in a stiff cage or
frame, which guarantees the mechanical strength. The MO
resistors are arranged within this frame. Additional metal cylinders with the same diameter as the MO resistors fill the inside
completely, forming a uniformly round active part. The MO
resistors are pressed together with a bolt in the center of
the lower electrode; the bolt is secured in the end position,
thereby providing each arrester with the same contact pressure.
The active part is placed into a mold and completely sealed with
silicone. As a result, the surge arrester, which is completely
sealed and tight, has no internal void.
Figure 2 shows an MO surge arrester of the POLIM-D type
manufactured according to this technique. It is shown before
and after being molded in silicone. The flexible method of
construction (modular concept) makes it possible to change the
form of the arrester to meet any necessity.
1a
02
The demands on the arresters depend on the operational conditions and the type of the electrical equipment to be protected.
Figure 3 gives an overview of the variety of MO surge arresters
developed and produced by Hitachi Energy in Switzerland,
covering station and distribution types intended for use in
medium-voltage systems, as well as arresters for application
in traction systems (AC and DC) and for special applications.
tage that it forcibly breaks apart when the MO blocks are
overloaded. Another disadvantage is the fact that there are
different insulating materials, which also means that there are
more boundary layers. Therefore, it is necessary to take special measures for sealing. This principle of design belongs in
Group II, see Figure 1b
• In a glass-fiber reinforced tube made of synthetic material,
which is covered with an insulating polymeric material, the
active part is installed, similarly to insulators made of porcelain. These “hollow” insulators have the same disadvantages
as the porcelain insulators: they need a sealing and pressure
relief system and they can have internal partial discharges.
This is considered Group III, see Figure 1c
Silicone rubber (usually simply referred to as “silicone”) is
an excellent insulating material for high-voltage insulators.
In high-voltage technologies, silicone has been successfully
used for about 50 years for long rod insulators and bushings,
for example. The first MO surge arresters with the typical Hitachi Energy direct molding were used in 1986. Millions of these
arresters have been, and are still being, used trouble-free all
over the world and under all climate conditions.
The basic Si-O-Si-O matrix with additional CH3-groups (methyl)
is characteristic of silicone. The filling materials and special
additives cause the arcs and creep resistance necessary for
use in high-voltage technology. The qualities of silicone include
very high elasticity and resistance to tearing, high temperature
stability, very low combustibility (silicone is a self-extinguishing
material) and high electrical disruptive strength. Besides all
these qualities, the most remarkable one is hydrophobicity:
water simply rolls off the silicone surface. Silicone insulators are
water-repellent even if they are polluted. This means that the
hydrophobicity is also transmitted into the pollution layer on the
surface. All this provides excellent performance properties for
high-voltage equipment insulated with silicone. The hydrophobicity of the silicone can be diminished under the influence of a
long period of humidity or electrical discharges on the surface;
it is however completely restored in a short period of time (from
a couple of hours to a couple of days). As much as we can say
today this mechanism works for unlimited time.
1b
1c
01 Design principles for MO surge
arresters: a) Group I, b) Group II,
c) Group III. For explanation see
text above.
02 POLIM-D type MO surge arrester
(design Group I).
Left: active part before molding.
Middle: schematic design.
Right: complete arrester.
03 Range of MO surge arresters
developed and produced by
Hitachi Energy in Switzerland.
03
10
| M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S
The MO resistor stack of the surge arrester behaves in an
almost pure capacitive manner with applied continuous operating voltage Uc. The stray capacitance of each resistor against
the earth causes an uneven voltage distribution along the
arrester axis under applied Uc. This unevenness increases with
the length of the resistor stack. High-voltage MO surge arresters therefore need grading elements, such as grading rings,
which mostly compensate the unfavorable influence of the stray
capacitance. The resistor stack with medium-voltage arresters
is, however, so short that the uneven voltage distribution can be
neglected. Therefore, medium-voltage arresters do not require
any grading elements.
As a rule, the mechanical loads are low with medium-voltage
arresters. All Hitachi Energy medium-voltage arresters can be
installed in regions where earthquakes occur. Horizontal and
hanging installation is possible. If the arresters have to bear
additional mechanical loads, besides their own weight and the
normal wind and ice loads, that exceed the guarantee data,
then the manufacturer should be contacted.
M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S |
2.3 Metal-oxide resistors
MO resistors are made of different metal-oxides in powder
form, which are compressed and sintered in the form of round
blocks. Figure 4 shows the principle of the manufacturing
process. The diameter of the MO resistors produced by Hitachi
Energy in Switzerland lie between 38 mm and 108 mm. The
height of the MO resistor blocks is typically between 23 mm and
46 mm. For special applications, the MO resistors can be sliced
to a height as small as 0.8 mm. The diameter of the MO resistors determines the current; the height of the MO resistors
(or resistor stack) determines the voltage in continuous operation and the volume of the blocks determines the energy
handling capability and charge transfer capability.
Figure 5 shows a selection of MO resistors. Figure 6 shows in
an enlarged form the inner structure of the MO material. It is
absolutely necessary to obtain a very homogeneous structure
of the material in order to achieve a high specific energy
handling capability for the MO resistor. The energy handling
capability of an MO resistor and of an MO surge arrester
respectively, depend on the volume of the active part, the
design (heat transfer) and the electrical dimensioning.
Metal-oxide resistors have an extreme non-linear voltage-current characteristic, which is described as
I = k × Uα
α is variable between α ≤ 5 and α ≈ 50.
k is a material depending factor.
An exact value for α can only be provided for a very restricted
range of the current in the characteristic curve.
The lateral surface of the MO resistors is passivated with glass,
the contact areas are laser cleaned and activated before metallizing with soft aluminum. The metallization reaches up to the
edge of the MO resistors. In this way, the MO material of the
MO resistors produced by Hitachi Energy in Switzerland is
completely covered.
05 Range of MO resistors produced by
Hitachi Energy in Switzerland.
1 Mixing of the
metal-oxide powders
2 Spray-drying of
the powder mixture
8 MO resistors ready to
be installed in the arrester
04 Manufacturing process of MO resistors
7 Final tests of
the MO resistors
3 Pressing of
the MO resistors
4 Sintering
6 Laser cleaning,
activation and metallizing
5 Coating the surface
passivation with glass
06 Surface electron microscope
image of the MO structure. Fracture
surface, enlarged 2’000 ×. The MO
grains and the boundaries between
the single grains can clearly be seen.
11
| M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S
12
The typical U-I characteristic of such an MO resistor (or MO
surge arrester) is shown in Figure 7. Some important terms are
explained below.
Region A describes the part of the U-I characteristic curve
relevant to the power frequency voltage. It is also considered to
be the pre-breakdown or low-current region. The continuous
operating voltage Uc is the power frequency voltage that can be
applied to the MO surge arrester (or MO resistor) continuously
without any restrictions. The current flowing through the MO
surge arrester is the “leakage” current ic, which is almost purely
capacitive (see Figure 8). The power losses at Uc can be
neglected, assuming standard ambient conditions and the correct choice of arrester.
The rated voltage Ur is the voltage value that is applied for
t = 10s in the operating duty test in order to simulate a temporary overvoltage in the system. The relationship between the
rated voltage and the continuous operating voltage is generally
Ur /Uc = 1.25. This is understood as a given fact, but it is not
defined anywhere. Other ratios are possible. The rated voltage
has no other importance, although it is often used in type
designations or when choosing an arrester.
The reference current iref is the peak value of the resistive
component of a power frequency current, and is chosen by the
manufacturer. Usually, the same current density is used for all
MO resistors in production. The reference voltage Uref is the
peak value of the power frequency voltage divided by √2, which
is applied to the arrester to obtain the reference current (see
Figure 9). Because of the dominant ohmic component of the reference current, the influence of stray capacitances of the MO
resistors or MO surge arresters at the measurement of the
U
Region A
α≤5
Region B
I = k x U α with α ≈ 50
reference voltage is negligible. Therefore, the reference voltages, which are measured at single MO resistors, can be added
to give the reference voltage of the entire arrester. The measurement of the reference voltage is a routine test for each MO
resistor and each MO surge arrester produced by Hitachi
Energy in Switzerland. The measurement of the reference
voltage Uref at iref, and the residual voltage Upl at In ensures a
control of the U-I characteristic of each MO resistor.
It is important to note that in Region A,
the resistive part of the current, and
therefore the power losses, depend
strongly on the temperature of the
MO resistors.
Due to the negative temperature coefficient in this region,
there is a strong increase in power losses with increasing
temperature. This may be critical for the thermal stability of the
MO surge arrester in service, and it has to be considered in
the relevant type tests, as well as in applications at elevated
ambient temperatures.
M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S |
Region C is the area of currents greater than about 100 A,
and it describes the protective characteristic of the MO surge
arrester. It is considered to be the high-current region. The most
important parameter is the lightning impulse protective level Upl.
This is the maximum permissible peak voltage on the terminals
of an arrester subjected to the nominal discharge current In.
The amplitude of the nominal discharge current In, with a wave
shape of 8/20 μs, together with the arrester class prescribe the
test parameters, see also section 3.3, Table 2. Figure 10 shows
as an example the nominal discharge current In and the residual
voltage Upl of an MO resistor. It needs to be mentioned that in
Region C we have a positive temperature coefficient. The influence of the temperature on the residual voltage of the MO
resistors is in the range of only a few percent and can be
neglected in standard applications.
2.4 High-field MO resistors and
GIS arresters
The field strength (voltage per unit height of the MO resistor)
is generally in the range of 2 kV/cm at a given current iB in the
breakdown range, considered to be the “normal” field strength.
The field strength of MO resistors is determined by the number
Region B is the breakdown region. It is the part of the U-I curve
in which even minimal voltage increases lead to a significant rise
in the current. Only transient events in the time range of
milli- and micro-seconds (switching overvoltages) can be
handled by the arrester. A continuous application of power
frequency voltage in this area of the characteristic would
destroy the arrester in a fraction of a second.
t [ms]
5
10
15
20
25
30
35
40
45
50
Ur
Uc
In
log I
i [kA]
18
12
15
10
12
8
9
6
6
4
3
2
0
0
0
U
10
10
15
20
25
30
35
40
45
50
iref
09
t [μs]
07 Non-linear voltage-current characteristic of an MO resistor (principle)
A: Region relevant to power frequency voltage.
B: Region with the highest non-linearity.
C: Region describing the protection characteristic.
5
Uref
ic
u [kV]
Uref
t [ms]
0
08
Upl
Iref
Hitachi Energy in Switzerland developed a specific recipe for
high-field MO resistors that provides lower power losses, especially at higher temperatures. This opens advantages in the
design of MO surge arresters with high-field MO resistors.
In arrester designs with SF6 gas as insulating medium (GIS
arresters) the use of high-field MO resistors can bring really big
advantages. As the MO resistor stack can be reduced by up to
50 percent, the size of the vessel can be reduced accordingly.
This reduces the volume of the vessel and the amount of SF6
gas needed. Further, high-field MO resistors can find their
application in liquid-immersed arresters and in arresters
with solid insulation, e.g. completely encapsulated arresters.
High-field MO resistors for DC applications are also available.
The use of high-field MO resistors in standard applications, e.g.
air-insulated MO surge arresters (AIS), brings little or no benefit,
because the height of such a surge arrester is given by the
external flashover withstand capability of the housing.
i, u
Region C
α≤5
Uc
Ic
of boundary layers per unit height. By increasing the number of
boundary layers, i.e. reducing the size of the grains in a given
MO resistor, the field strength can be increased up to 4 kV/cm,
considered to be “high-field” (HF).
i, u
0
13
10
i
20
30
40
08 Continuous operating voltage Uc and
leakage current ic of an MO surge arrester.
09 Reference current iref and reference
voltage Uref.
10 Nominal discharge current I n = 10 kA
and residual voltage Upl.
14
| M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S
Figure 11 shows the relation between an MO resistor with
normal field strength and a high-field MO resistor, developed
and produced by Hitachi Energy in Switzerland, as used in GIS
arresters. The high-field MO resistor has the same rated voltage
of 8.8 kV as the normal-field MO resistor. The diameter of the
high-field MO resistor is, at 108 mm, the same as for the MO
resistors with standard field strength. The height of the MO
resistor with normal field strength is 46 mm, the one for the
high-field MO resistor is 24 mm.
M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S |
frequency, because the MO surge arrester behaves in an almost
purely capacitive manner considering the continuous operating
voltage. Because of
XC =
1
ωxC
XC = capacitive impedance
Additional DC voltage applications are to be found in converter
stations, drives and photovoltaic systems. It is absolutely necessary to get into contact with the manufacturer if MO surge
arresters are to be used in such installations.
ω = 2 × π × f = angular frequency
C = capacity of the MO surge arrester
Hitachi Energy in Switzerland has developed and produces
SF6 gas-insulated (GIS) MO surge arresters for all transmission
system voltages. Figure 12 illustrates the reduction in volume of
the arrester vessel if high-field MO resistors are used instead
of MO resistors with standard field strength.
It has to be understood that doubling the field strength means
doubling the energy under a given current impulse, and consequently the temperature rise. Therefore, the increase in field
strength means that the energy absorption capability, thermal
stability and voltage withstand capability is decreased. These
disadvantages can be technically covered by increasing the
diameter of the MO resistors or by the use of heat sinks in an
arrester design. As mentioned above, Hitachi Energy in Switzerland developed a specific recipe for high-field MO resistors that
avoids such drawbacks in the design of MO surge arresters for
gas insulated substations (GIS).
2.5 Influence of different frequencies and
DC voltage on MO resistors
2.5.1 Different frequencies
Beside the system frequency of f = 50 Hz and f = 60 Hz,
the “railway frequency” of f = 16.7 Hz also has technical
importance. MO surge arresters without spark-gaps can be
used without any problem with these frequencies. It is to be
noted that the continuous current ic will change with the
11 MO resistor with normal
field strength (left, field strength
approximately 2 kV/cm) and
a high-field MO resistor with
approximately 4 kV/cm
field strength (right).
DC voltage systems are broadly used for traction systems.
The nominal voltages in the public DC traction systems lie
between Un = 600 V (urban traction systems) and Un = 3’000 V
(long-distance trains). It is necessary to observe both the high
electrical requirements for MO surge arresters in the traction
systems, as well as the mechanical and safety-relevant requirements.
the capacitive impedance becomes smaller with increased
frequency, and consequently the capacitive current increases
with increasing frequency. Table 1 shows typical values as
examples.
15
2.6 Microvaristors and field grading
Microvaristors (μvar) are small spherical particles that behave
like a varistor (see Figure 13). The materials of the microvaristors
and the production process are similar to the materials and
production of MO resistors.
Microvaristors are used in polymeric materials, e.g. silicone,
for field grading purposes. Typical applications can be in HV
and MV terminations, long rod insulators for AC and DC,
grading tape for stator windings, or semi-conducting varnish.
The advantage of microvaristor filled polymers (compounds) is
that the non-linear characteristic of the compound is given by
the U-I characteristic of the microvaristors and not by the filling
grade only, as is the case with other functional filler materials,
like e.g. carbon black.
01 Power losses P v and continuous current ic for an MO surge
arrester of class SL with Uc = 20 kV.
Frequency f in Hz
1
150
1
Power losses PV in W
2
150
1
Continuous current iC in mA, rms
3
150
1
This makes it possible to adjust the properties of the
compound to the required applications. The large variety of the
field strength of the compound allows tailor-made solutions for
different products. Hitachi Energy in Switzerland produces
microvaristors for the various applications mentioned and
consults on the development and production of field grading
products.
The dimensioning and application of MO surge arresters for
railway systems with f = 50 Hz and f = 16.7 Hz is precisely
described in a separate brochure.
The manufacturer must be contacted if the MO resistors or
arresters are to be used for frequencies rated higher than 60 Hz.
A special case are test transformers and resonance circuits with
450 Hz, which are sometimes used for on-site insulation tests.
In this case the capacitive current of the arrester is approximately nine times higher than with 50 Hz.
2.5.2 DC voltage
In principle, in DC voltage systems, there also appear
overvoltages produced by lightning or switching activities,
which may endanger the equipment and the insulation. In this
case, it is also necessary to use an arrester as protection
against overvoltages. MO surge arresters without spark gaps
are particularly suitable, because they do not conduct any
follow current after the limiting of the overvoltage, except a
leakage current of a few μA, and therefore it is not necessary
to extinguish any DC current arc.
It is to be observed that only MO resistors with proved DC longterm stability are to be used for MO surge arresters in DC voltage systems (see section 3.5.2). It goes without saying that all
the type tests using continuous voltage should be performed
with DC voltage. Typical DC voltage stresses are to be found in
the high-voltage DC transmission (HVDC). The various voltage
stresses in HVDC systems and the relevant tests are given in
the standard IEC 60099-9, Ed. 1.0, Surge arresters – Part 9:
Metal-oxide surge arresters without gaps for HVDC converter
stations, published in June 2014.
12 GIS arresters. The arresters have the same ratings.
Left: design with MO resistors with standard field strength.
Right: with MO resistors with high-field strength.
13 Photograph of sintered microvaristors determined by electron
microscopy. The diameter of the microvaristors is in the range of
20 to 150 μm.
16
| M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S
M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S |
Metal-oxide resistors –
at the heart of modern
surge arresters.
17
| M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S
03 Function and performance
of MO surge arresters
MO surge arresters are devices that protect electrical equipment and installations by limiting
surge voltages and diverting surge currents to earth.
3.1 General
The function of a surge arrester with an active part consisting of
a series connection of MO resistors is very simple. In the event
of a voltage increase at the arrester’s terminals, the current rises
according to the characteristic curve (see Figure 14) continually
and without delay, which means that the arrester skips over to
the conducting condition. After the overvoltage subsides, the
current becomes smaller according to the characteristic curve.
The subsequent current after the MO surge arrester protected is
an almost pure capacitive leakage current ic of about 1 mA.
In is the nominal discharge current, and Upl is the lightning impulse protection level of the surge arrester. It is defined as the
maximum voltage between the terminals of the surge arrester
during the flow of In.
The following paragraph shows, and briefly explains, typical
current and voltage waveforms in the high-current region
(protection characteristics) of the characteristic curve. For the
low-current region, see Figure 7 in section 2.3.
3.2 Currents and voltages
Residual voltage Ures
Peak value of voltage that appears between the arrester terminals during the passage of discharge current.
The residual voltage of an MO resistor or MO surge arrester is
determined with surges having different wave forms and current
heights, and it is given in tables or as a voltage-current characteristic on a curve (see Figure 14). The measurements are generally performed on MO resistors. As the measurement is mostly
performed in regions of the characteristic where the ohmic part
of the current is dominant, the capacitive stray influences can
be ignored. The residual voltages measured on single MO
resistors can be summed as the residual voltages of the
whole arrester.
High current impulse Ihc
Peak value of discharge current having a 4/10 μs impulse shape.
The high current impulse represents not only an energetic
stress, but also a dielectric one, taking into consideration the
high residual voltage that occurs with a high current impulse
with a peak value of 100 kA. However, it is necessary to strongly
emphasize that a high current impulse with an amplitude of 100
kA is not the same as a real lightning current of the same amplitude. The real lightning current of this amplitude measured
during a thunderstorm lasts longer than several hundred microseconds. Such strong lightning currents and impulse shapes
are very rare and appear only under special conditions, such as
during winter lightning in hilly coastal areas.
Switching current impulse Isw
Peak value of discharge current having a virtual front time
greater than 30 μs and less than 100 μs, and a virtual time
to half-value on the tail of roughly twice the virtual front time.
The switching current impulses are used to determine the voltage-current characteristic. The current amplitudes lie between
500 A and 2 kA for station class arresters, and roughly reproduce the load of an arrester caused by overvoltages due to
circuit breaker operation.
M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S |
Long-duration current impulse Ild
Also called rectangular wave (Irw) or square wave, a long-duration current impulse is a rectangular impulse that rises rapidly
to its peak value and remains constant for a specified period
of time before it falls rapidly to zero. The length of the current
pulse duration is correlated to the line length in transmission
and distribution systems. Rectangular impulses are used in
laboratories during type tests. The current amplitudes are typically up to 2 kA and reproduce the load of an arrester when a
charged transmission line discharges into the arrester in case
of an overvoltage occurrence. See Figure 15 for an example of
a rectangular current impulse with a virtual time duration of
2.15 ms.
For comparison of different MO surge arresters, it is regarded
as a matter of course to use a rectangular wave of 2 ms duration, although there is no norm established for doing so. Specified is either the amplitude of the rectangular wave for a specific
MO surge arrester, or the energy transferred into the arrester
during the flow of the rectangular current.
3.3 Charge transfer and energy
absorption capability
classification based on repetitive charge transfer rating (Qrs), as
well as on thermal energy rating (Wth) for station class and thermal charge transfer rating (Qth) for distribution class arresters.
Station and distribution class arresters are classified as
indicated in Table 2. The letters “H”, “M” and “L” in the designation stand for “high”, “medium” and “low” duty, respectively.
In medium-voltage systems, distribution arresters are mainly
used. For specific applications, where higher energy requirements apply, such as protection of cables, rotating machines or
capacitor banks and other important equipment, station class
arresters may also be needed for medium-voltage systems.
The repetitive charge transfer rating Qrs is defined as the
maximum specified charge transfer capability of an arrester,
in the form of a single event or groups of surges that may be
transferred through the arrester without causing mechanical
failure or unacceptable electrical degradation to the MO resistors. This rating is verified in a type test on single MO resistors
in open air and, therefore, is an MO resistor-related material
test.
u[kV]
1.5
i[A]
14
700
12
600
10
500
8
400
6
300
4
200
2
100
0
0
4/10 μs
1/9 μs
30/69 μs
AC
Uref
8/20 μs
0.5 Ur
Steep current impulse
Current impulse with a virtual front time of 1 μs and a virtual
time to half-value on the tail not longer than 20 μs. The steep
current impulses are used to determine the voltage-current
characteristic. They have amplitudes up to 20 kA and roughly
reproduce steep current impulses like those which may appear
with disconnector operation, re-striking, back flashovers, and
vacuum circuit breakers. All the current impulses described
above (except the high current impulse) are used to determine
the voltage-current characteristic of an MO surge arrester.
It is to be considered that only the virtual front time and the
amplitude of the current impulses are decisive for the residual
voltage and not the virtual time to half-value on the tail. That is
the reason why the tolerance for the virtual front times is very
tight, and contrastingly, the virtual times to half-value on the tail
are very broad.
19
With Ed.3.0 of IEC 60099-4, a new concept of arrester
classification and energy withstand testing was introduced:
the line discharge classification was replaced with a
U/Upl
18
Uc
iref
isw
in
I[A]
0
10-4
10-3
10-2
10-1
100
101
102
103
104
0
105
U
14 Voltage-current characteristic of an MO surge arrester with I n = 10 kA,
type SL. The voltage is normalized to the residual voltage of the arrester
at In. The values are given as peak values for the voltage (linear scale)
and the current (logarithmic scale). Shown are typical values.
0.5
1.0
1.5
2.0
2.5
3.0
i
15 Long-duration current impulse Ild = 506 A with a virtual
duration of the current of t 90% = 2.15 ms. The residual voltage
is U res ≈ 10.8 kV.
3.5
4.0
4.5
t[ms]
20
| M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S
M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S |
02 MO surge arresters made by Hitachi Energy in Switzerland, classification according to IEC 60099-4, Ed. 3.0.
Arrester class designation
Station SH
Station SM
Station SL
Station SL
Distribution DH
Hitachi Energy type (choice)
POLIM-H..N
POLIM-S..N
POLIM-I..N MWK
POLIM-K
POLIM-D
Nominal discharge current In
20 kA
10 kA
10 kA
10 kA
10 kA
2 kA
1 kA
0.5 kA
0.5 kA
–
2.4
2.0
1.6
1.0
0.5
12
8
5
4.5
–
-
-
-
–
1.1
Switching impulse discharge current
Q rs (C)
Wth (kJ/kV rated)
Q th (C)
The thermal energy rating Wth is the maximum specified
energy, given in kJ/kV of Ur that may be injected into an arrester
or arrester section within three minutes in a thermal recovery
test without causing a thermal runaway. This rating is verified
by the operating duty test for station class arresters. This test is
a thermal stability test for MO surge arresters of classes SH, SM
and SL.
The thermal charge transfer rating Qth is the maximum specified
charge that may be transferred through the arrester or arrester
section within one minute in a thermal recovery test without
causing a thermal runaway. This rating is verified by the operating duty test for distribution class arresters. This test is a
thermal stability test for MO surge arresters of classes DH,
DM and DL.
The operating duty tests are performed on thermally prorated
sections representing the arrester being modelled. The purpose
of this test is to verify the arrester’s ability to thermally recover
after injection of the rated thermal energy Wth or transfer of the
rated thermal charge Qth under applied temporary overvoltage
and following continuous operating voltage conditions.
3.4 Cool-down time
The arresters in the system can work reliably and safely if their
energy absorption or charge transfer capability is greater than
the energy strain expected in the system operation. In case of
multiple surges, one after another, the injected energy accumulates in the arrester, and therefore an intermediary cool-down
time can be ignored. But if the energy reaches the guaranteed
value, which is applied in the operating duty test, the arrester
must have enough time to cool down. The necessary cool-down
time for the arrester depends on the construction, the ambient
temperature and the applied voltage. The cool-down time typically lies between 45 and 60 minutes, depending on the arrester
type and the ambient conditions.
3.5 Stability of a MO surge arrester
There are two situations to take into account: the thermal stability of the MO surge arrester after adiabatic energy absorption
(sometimes known as “short-time stability”) and the long-time
stability of the MO surge arrester in system operation.
3.5.1 Thermal stability
In Figure 16, P represents the power losses of the MO resistors
in an arrester when Uc is applied. It is evident that P exponentially increases with the MO-temperature T, which also results in
increased heating of the active component. The cooling down
of the MO resistors occurs with the heat flow Q˙ from the active
part of the arrester to the exterior. P is greater than Q˙ at temperatures above the critical point (thermal stability limit). Here,
the cooling is not sufficient to dissipate the heat produced by
the power losses to the exterior. The MO resistors would continue to heat up and the arrester would be destroyed by overheating. This occurrence is called “thermal run-away” or
“thermal instability”.
If the power losses P stay under the critical point (i.e. P < Q˙),
it is possible to eliminate the warmth faster than it is produced,
and the active part cools down until it returns to the stable
working condition after the cool-down time (stable operating
point). This is the area of thermal stability. As long as the critical
point is not exceeded, the arrester can branch off the loaded
energy as often as is necessary, which means that it can limit
the overvoltage just as often as is required. It is possible to raise
the critical point to such a level, that even if the highest energies
are likely to occur during the operation, this critical point cannot
possibly be reached. This can be achieved through suitably
dimensioning of the MO resistors and through design measures
that enable them to cool down.
3.5.2 Long-term stability
An MO surge arrester can operate absolutely reliably if the
voltage-current characteristics curve of the MO resistors under
applied continuous voltage does not change. The continuous
current ic should not be allowed to shift to higher values to also
prevent increases in power losses. A change of the electrical
characteristic curve due to applied continuous voltage Uc is not
to be expected with MO resistors that are produced by leading
international manufacturers, considering the present state of
technology.
Under certain circumstances, a change (or, more precisely, deterioration) of the voltage-current characteristic curve can occur
due to extreme stresses, such as very high or very steep current
impulses. Another cause that can lead to a change of the electrical characteristics close to the rim may be different components of the materials in which the MO resistors are embedded.
This is the reason why the surface area of the MO resistors is
passivated, which means that they are coated with a gas-proof
glass that is also highly robust.
All these reasons make it indispensable to permanently control
the long-term behavior of MO resistors during their manufacture. This is achieved with the long-term stability test according
to IEC 60099-4 (Ed. 3.0). In addition to type tests of over 1,000
hours, there are also accelerated ageing tests according to
internal manufacturer instructions to be conducted on each
production batch.
It should be emphasized that the long-term stability test must
be performed with the same kind of voltage that is applied to
the MO surge arrester in the system. Thus, the MO resistors
for AC systems must be tested with AC voltage, and the MO
resistors for DC systems must be tested with DC voltage. Experience shows, however, that DC-stable MO resistors are usually
also stable under AC loads, but AC-stable MO resistors are not
necessarily stable under DC loads. That is why it is particularly
important to use DC-stable MO resistors with MO surge arresters in DC systems.
Figure 17 shows an example of a long-term stability test.
Temperature and test voltage are to be kept constant over the
whole test time. The power losses P are recorded and should
decrease constantly or remain constant. The test duration of
1,000 h at 115 °C is considered to correspond to an operating
time of 110 years in the system at an environmental temperature
of 40 °C.
W
Thermal runway
P, Q˙
21
Thermal
stability
limit
T
Q˙
Stable
operating
point
Q˙
P
T[°C]
16 Power losses P of the MO resistors and the heat flow Q˙
from the active part of an arrester to the exterior, as a function
of the temperature (T) of the MO resistors at continuous operating
voltage Uc.
P
1.1
1.0
0.9
0.8
0.7
0.6
0.5
t[h]
0
200
400
17 Example of a long-term stability test
(type test over 1,000 h).
600
800
1000
22
| M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S
3.6 Protective characteristics
The protective characteristic of an arrester is given by the maximum voltage Ures at the terminals of an arrester during the flow
of a current surge. Generally, a lightning impulse protective level
of Upl ≤ 4 p.u. is considered acceptable. This is a value that
is generally accepted for the insulation coordination. The real
residual voltage with nominal discharge current In (thus Upl) can
lie above or below that, depending on the type of arrester. If Upl
is set in a relationship with Uc of an arrester, it is possible to get
very good information about the quality of the arrester performance with regard to the protective level. The smaller the Upl/Uc
ratio, the better the protection.
In addition to the residual voltage at In, the residual voltages at
steep current impulse and at switching current impulse are also
important. The residual voltage increases slightly with the current, but also with the steepness of the current impulse, as can
be seen from the data sheets of each arrester, and also from the
voltage-current characteristic. Depending on the application,
the residual voltage at the steep current impulse and at switching current impulse must be taken into account, apart from the
residual voltage at In.
3.7 Temporary overvoltage
Temporary (short-time) overvoltages UTOV are power frequency
overvoltages of limited time duration. They appear during
switching operations or earth faults in the system and they can
stay in medium-voltage systems with insulated transformer
M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S |
neutrals for several hours. Their height depends on the system
configuration and the treatment of the star point. The duration is
given by the time that elapses until the registration and the
switching off of the system failure.
MO surge arresters are able to withstand an increased operating voltage for a certain period of time. The factor of resistance
(T) of the arrester against such temporary overvoltages can be
seen as an example in Figure 18. T = UTOV/Uc is the extent of the
permissible height of UTOV.
The following example should explain the use of TOV curves
in Figure 18. An arrester with Uc = 24 kV is operated with Uc
in a normally functioning, undisturbed system for an unlimited
period of time. At time t = 0 the arrester is stressed with an
energy of Wth = 5.625 kJ/kVUc. Immediately afterwards, the
temporary overvoltage UTOV = 31 kV occurs. Therefore, it is
T = UTOV/Uc = 31 kV/24 kV = 1.29. T = 1.29 results in a time of
t = 20 s according to curve b. That means that the arrester
can withstand an increased voltage of 31 kV for 20 s without
becoming thermally instable. After 20 s, the voltage must go
back to Uc so that the arrester does not become overloaded.
If the arrester is not loaded with the energy Wth before the
appearance of the temporary overvoltage, it is curve a that
counts, and the arrester can withstand UTOV for 90 s. Therefore,
the height and duration of the admissible temporary overvoltage
directly depend on the previous energy load of the arrester.
T = UTOV/UC
1.60
1.50
1.40
a
1.30
b
1.20
1.10
1.00
t[s]
0.1
1
18 Resistance T = UTOV/Uc against temporary overvoltages
depending on the time t. Curve a is valid for an arrester without
energy pre-stress, curve b with a pre-stress of the guaranteed energy
Wth, and t is the time duration of the overvoltage at power frequency.
Example for type MWK.
10
100
1,000
10,000
Long term tests of the MO
resistors in modern automated
test stations ensure reliable
performance.
23
24
| M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S
04 Service conditions
MO surge arresters must perform reliable under normal and special service conditions.
Adaptations in design may be necessary to meet specific applications.
M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S |
4.5 Elevated ambient temperature
Hitachi Energy arresters (AC and DC voltage) are guaranteed
to function flawlessly up to 40 °C ambient air temperature.
This also includes maximum solar radiation of 1.1 kW/m2 for
outdoor arresters. If there are heat sources in the vicinity of the
arrester, the increased ambient temperature has to be taken into
account, and the value of Uc increased if necessary. If the ambient temperature exceeds 40 °C, Uc must be increased by 2 percent for every 5 °C of temperature elevation. This correction is
possible up to maximum of 80 °C ambient temperature.
If it is not acceptable to increase the continuous operating voltage Uc, and consequently the protection level Upl, in a specific
application, then a reduction of the thermal energy rating has to
be considered.
4.1 Normal service conditions
The service life of an arrester made by Hitachi Energy in
Switzerland is 30 years or more under normal operating
conditions and if it is correctly chosen according to the system
voltages and the expected electrical and mechanical loads.
The normal service conditions for an arrester are listed in
IEC 60099-4 (Ed. 3.0).
• Ambient air temperature within the range of -40 °C to +40 °C
• Solar radiation of 1.1 kW/m2
• An altitude not exceeding 1,000 m above sea level
• Frequency of AC voltage between 48 Hz and 62 Hz
• A power frequency voltage at the arrester terminals not higher
than the continuous operating voltage Uc of the arrester
• Wind speed ≤ 34 m/s
• Vertical erection, not suspended
All arresters made by Hitachi Energy in Switzerland meet,
or even exceed, these operating conditions. For example:
• The ambient air temperature can be up to 55 °C
(at derated thermal energy capability)
• The AC power frequency can be between 15 Hz and 62 Hz
• The altitude can be up to 1,800 m without altitude correction
• The arresters can be mounted in any position,
including hanging
4.2 Special service conditions
The following examples are typical special service conditions
(referred to as abnormal serviced conditions in IEC 60099-4
(Ed. 3.0)) that may require special consideration in the manufacture or application of surge arresters and should be called to
the attention of the manufacturer.
• Ambient temperatures in excess of + 40 °C or below – 40 °C
• Service at altitudes above 1,000 m
• Fumes or vapors that may cause deterioration of the insulating
surface or mounting hardware
• Excessive contaminations by smoke, dirt, salt spray, or other
conducting materials
• Excessive exposure to moisture, humidity, dropping water
or steam
• Live washing of arresters
• Areas with a risk of explosion
• Unusual mechanical conditions
• Voltage distortions or voltages with superimposed contents
of high frequencies that are caused by the system
Further special conditions are listed in IEC 60099-5 (Ed. 3.0).
The following paragraphs illustrate a few special cases. It is
advisable to contact the manufacturer should conditions appear
that are not covered here.
4.3 Overload behavior
Any arrester can be overloaded. The causes can be extremely
high lightning currents, lightning currents with a very large
charge, or a “voltage-transition”. This is to be understood as
a short-circuit between two different voltage levels. In all these
situations, there is in fact an energy overload. In the case of an
overload, the MO resistors either spark-over or break down and
tend to create a permanent short-circuit. An arc results inside
the arrester, and the current in this arc is defined by the
short-circuit power of the system. Hitachi Energy arresters with
directly molded silicone housings do not face the risk of explosion or violent shattering in the case of an overload. There is no
air space between the active part of the arrester and its silicone
insulation: thus, there is no space for the pressure to build up.
The occurring arc (or sparks) escapes the silicone insulation
as soon as it occurs and is freed. Because of their special
construction, the arresters are protected from violent shattering
up to the highest short-circuit currents.
4.4 Mechanical stability
Hitachi Energy’s arresters are operationally reliable even in
areas of high earthquake activity. The arresters may partially
take on the support function or serve as line arresters, or they
may have the function of suspension insulators. The manufacturer should be informed about such operational situations.
The values given in the data sheets of the individual arresters
are not to be exceeded. Arrester types that are to be applied
for rolling stock are delivered with a reinforced base plate and
are tested under vibration and shock conditions.
4.6 Pollution and cleaning
Silicone is the best insulating material in case of pollution.
This is mainly because the material is water-repellent (hydrophobic). Silicone arresters behave more favorably under conditions of heavy pollution than porcelain-housed arresters or other
polymeric insulation materials, e.g. EPDM. Decisive for the longterm behavior under pollution of an insulation made of a polymeric material is the dynamic behavior of the hydrophobicity,
which is originally always very good.
Depending on the material, a loss of
hydrophobicity can be permanent or
temporary. In contrast to other polymeric
materials, silicone is able to regain its
hydrophobicity after losing it temporarily.
In our operation instructions the best way to clean silicone
surfaces, if needed, is described.
25
4.7 Altitude adjustment of the arrester
housing
Correction factors for altitude adjustment of external insulation
are given in several IEC and IEEE standards. The correction factors differ from standard to standard, depending on the type of
equipment and mainly due to assumptions and safety margins
considered. In IEC standards the normal service conditions are
valid up to 1,000 m above sea level, while in IEEE standards
1,800 m are mentioned. Above these “standard altitudes” an
adjustment of the arrester housing has to be considered.
MO surge arresters made by Hitachi Energy in Switzerland can
be used without any housing adjustment up to a height of 1,800
m above sea level. At higher altitudes, the air density may be so
low that the withstand voltage of the arrester housing (external
flashover) is no longer sufficient. In this case, the unaltered
active part of the arrester (same protection level) must be
placed in an elongated housing with a longer flashover distance.
As a reference value, one may consider that for every 1,000 m
above 1,800 m above sea level the flashover distance must be
increased by 10 percent. For example, at an altitude of 3,300 m
above sea level the flashover distance of the housing must
be 15 percent longer than that of a standard arrester. It is
necessary to observe here that the flashover distances of
surge arresters for lower voltage levels are initially relatively
large, exceeding the minimum requirements of the withstand
voltage. Thus, in each individual case it should be checked
whether the normal housing possesses a sufficient withstand
voltage for application at higher altitudes.
26
| M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S
05 Tests
Constant quality and the guaranteed performance of the products is ensured by a number
of tests performed during development and production of MO resistors and surge arresters.
M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S |
Heat dissipation behavior of test sample
In the operating duty test and the power frequency voltage-versus-time test, the behavior of the tests sample is to a great
extent dependent on the ability of the sample to dissipate heat,
i.e. to cool down after being stressed by a discharge. Therefore,
the thermal equivalency between the complete arrester and the
arrester section shall be demonstrated by a test.
Operating duty tests
The purpose of the operating duty tests is to verify the arrester’s
ability to thermally recover after injection of the rated thermal
energy Wth or transfer of the rated thermal charge Qth under
applied temporary overvoltage and following continuous
operating voltage conditions.
5.1 General
Tests have to demonstrate that an MO surge arrester can
survive the rigors of reasonable environmental conditions and
system phenomena, while protecting equipment and/or the
system from damaging overvoltage caused by lightning, switching, and other system disturbances. Arresters manufactured by
Hitachi Energy in Switzerland are tested according to the
current international IEC standards. IEC 60099-4: 2014 (Ed. 3.0)
is applicable for the MO surge arresters with polymer housings.
Below, the main tests relevant for MO surge arresters with polymeric housings for medium-voltage systems are addressed in
brief. If in doubt, only consider the text in the current edition
(English version) of the relevant standard.
5.2 Type tests (design tests)
The development of an arrester design ends with type tests.
They are the proof that the arrester construction fulfills the
applicable standards. These tests need be repeated only if
changes in the design also cause changes in the properties
or characteristics. In such cases, only the affected tests need
be repeated.
The type tests that are to be performed on MO surge arresters
with polymer housing are listed and briefly explained in the
following paragraphs.
Insulation withstand tests
The insulation withstand tests demonstrate the voltage
withstand capability of the external insulation of the arrester
housing. The withstand values to be proved are calculated
from the residual voltages of the arrester. The withstand
values of arresters intended for use on systems of Us ≤ 245 kV
(this means all arresters used in medium-voltage systems) are
tested with the lightning impulse voltage (wave shape 1.2/50 μs)
under dry conditions, and with a one-minute AC voltage test.
The AC voltage test is performed under wet conditions for arresters intended for outdoor use; arresters intended for indoor
use are tested in a dry environment with the AC voltage test.
Naturally, the tests are performed with arrester housings without
active part inside.
Residual voltage tests
These tests determine the voltage-current characteristic in the
high current range. The residual voltage for steep current impulse, lightning current impulse and switching current impulse
at different amplitudes is determined and given either in tables
or in a curve form. The residual voltage tests are generally
performed on MO resistors.
Test to verify long term stability under continuous
operating voltage
The test is an accelerated ageing test performed on individual
MO resistors to provide insurance that they will exhibit stable
operating conditions in terms of power loss over the anticipated
lifetime of the arrester. The test is performed on MO resistors
including all material (solid or liquid) in direct contact with them,
e.g. in air, SF6 gas or molded in silicone. Therefore, MO resistors
of directly molded arresters also have to be molded with the
same material during the accelerated ageing test. The test is
passed if the power losses of the MO resistors during the
1,000-h test under elevated conditions (115 °C and increased
voltage) do not increase above 1.3 time the lowest power
losses, Pmin, and all measurements of power losses are not
greater than 1.1 times the power losses at the start of the test,
Pstart. MO resistors made by Hitachi Energy typically show a
constant decrease in power losses over the whole test time and,
therefore, have long-term stability under AC and DC conditions.
Test to verify the repetitive charge transfer rating, Qrs
The purpose of this test is to verify the maximum impulse
charge (and, indirectly, the maximum energy) that can be handled by an arrester in an event that may be repeated many times
over its lifetime. The charge has been chosen as a test basis for
the purpose of better comparison between different makes
of MO resistors and arresters. Because surge arresters are
subjected to current impulses from lightning and switching
events, it is necessary to know the capability of the arresters
in terms of the charge transferred by the arresters during such
events. In addition, the withstand capability of MO resistors is
a statistical parameter, and a high-voltage arrester can contain
a significant number of MO resistors. If a single MO resistor
fails, the probability is high that the complete arrester would
fail. Distribution arresters contain only a few MO resistors,
but the installed number of distribution arresters is very high.
If an arrester absorbs energy from a system event (e.g. lightning
impulse, switching surge, temporary overvoltage), the temperature of the MO resistors may rise to a point that is beyond the
arrester’s ability to thermally recover to its previous steady state
condition. Arrester manufacturers are required to specify values
for energy (Wth for station class arresters) or charge (Qth for
distribution class arresters) that represent the thermal limit for
each arrester type. This implies that the arrester must always
remain thermally stable after duty while in service over its
expected lifetime. It is also the purpose of this test to ensure
that the protective characteristic is not significantly changed
by such duty. The test consists of two parts:
1. The characterization and conditioning part of the test,
which may be performed at an ambient temperature of 20 °C
(± 15 K) on the MO resistors or pro-rated sections in still air
2. The thermal recovery part of this test shall be performed on
thermally pro-rated sections
With station class arresters, the thermal energy Wth is brought
in within three minutes by one or more long-duration current
impulses or by unipolar sine half-wave current impulses. Station
class arresters have to absorb energy that may be stored in the
system in the moment the overvoltage occurs. Line arresters
without gaps (NGLAs) may be tested with lightning current
impulses.
With distribution class arresters the thermal charge transfer Qth
is injected with two lightning current impulses of 8/20 μs within
one minute. Distribution arresters are mainly stressed by lightning events, meaning that they have to absorb a charge, which
is why they are tested with lightning current impulses.
27
Power frequency voltage-versus-time test (TOV curve)
The purpose of the test is to demonstrate the temporary
overvoltage (TOV) withstand capability of an MO surge arrester.
The test is performed with prior duty (injection of rated thermal
energy Wth or rated thermal charge Qth) and without prior duty.
The test procedure is the same as the thermal recovery tests
in the operating duty tests (second part of the operating duty
tests). But instead of applying only the rated voltage Ur, the
TOV test has to be performed with four different overvoltages
at different time durations each (test with prior duty), and two
overvoltages at different time durations (without prior duty).
The published data must cover the time range between
0.1 s and 3,600 s.
Tests of arrester disconnector
This test applies principally to distribution arresters and nongapped line arresters. The test is to verify that the disconnector
of an arrester can withstand all stresses related to their application in arresters without operation such as charge transfer and
operating duty. The test also demonstrates that the disconnector will perform according to the time-current characteristic
published by the manufacturer. Furthermore, the water tightness
and the mechanical strength of the disconnector have to be
verified.
Short-circuit tests
Surge arresters are not allowed to fail with violent shattering
in case of overloading and should self-extinguish any open
flames within a defined period of time. This is to be proved
with a short-circuit test. The way the short-circuit is initiated
in the arrester depends on its construction. Directly molded
medium-voltage arresters are electrically pre-damaged, that
is they are made low-ohmic by applying an increased voltage,
and afterwards they are connected to the actual test so that
the short-circuit develops inside the arrester. This is a form of
overload that looks very much alike the one taking place in
the arrester under real conditions in service.
The short-circuit ratings for MO surge arresters are tabled
in IEC 60099-4, Ed. 3.0 and must be declared by the
manufacturer.
28
| M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S
Test of the bending moment
This test demonstrates the ability of the arrester to withstand
the manufacture’s declared values for bending loads. As a rule,
an arrester is not designed for torsional loading. If an arrester is
subjected to torsional loads, a specific test may be necessary
by agreement between the manufacturer and the user.
A test in two steps (for Us ≤ 52 kV) shall be performed one after
the other: a thermomechanical test, and a water immersion test.
These tests demonstrate the ability of the arrester to resist
ingress of moisture after being subjected to mechanical
stresses.
Radio interference voltage (RIV) test
This test is applicable only for arresters intended for use in
systems with Us ≥ 72.5 kV. For arresters in medium-voltage
systems this test is performed as a routine test (internal partial
discharge test) on each complete arrester.
Test to verify the dielectric withstand of internal
components
The purpose of this test is to verify the internal dielectric
withstand of an arrester even under impulse currents of amplitudes higher than the nominal discharge current. The test is
required only, if the conditioning part of the operating duty
test was not performed on a dielectrical prorated section. If the
dielectrical prorated section is identical to the thermal prorated
section as used in the operating duty test, this test can be
omitted.
Weather ageing test
This test demonstrates the ability of a polymeric-housed
arrester to withstand specific climatic conditions. The test
consists of two parts: 1,000 h test under salt fog conditions and
a 1,000 h UV test. The former must be performed on the highest
electrical unit with the minimum specific creepage distance and
the later on shed and housing materials. As a rule, the largest
arrester is tested with the medium-voltage arresters.
5.3 Routine tests
Routine tests are performed on each arrester or parts of an
arrester (for example, on MO resistors). According to the IEC,
at least the following tests must be performed:
Measurement of reference voltage
The reference voltage is measured with the reference current
specified by the manufacturer, and should be within the range
specified. This measurement is performed on each MO
resistor and on each MO surge arrester.
Residual voltage tests
The residual voltage is measured on each MO resistor at a
current value of 10 kA with a current rise time of 8 μs, which
is normally a lightning current impulse (or the nominal current).
The residual voltages of the MO resistors inside an arrester
can be directly added up, and they represent the total residual
voltage of the arrester.
M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S |
Leakage Current (μA)
100
90
80
70
Internal partial discharge test
In case of medium-voltage arresters, the test is normally
performed on each complete arrester. This test is performed
at 1.05 × Uc after the rated voltage was applied for 2 to 10 s.
The measured value of the internal partial discharges is not
allowed to exceed 10 pC according to the IEC. Hitachi Energy’s
internal guidelines require a value less than 5 pC, which means
virtually no partial discharges. During this test, the arrester can
be screened off from the external partial discharges.
60
Tightness test (leakage check)
This test demonstrates that the construction of the arrester is
tight. The manufacturer must choose a procedure that is sensitive enough. This test is not applicable for arresters that are
completely molded in silicone.
0
Current distribution test
The current distribution test is to be performed on MO surge
arresters with parallel MO resistors or parallel columns of MO
resistors. Arresters with one column only are naturally not to be
subjected to such a test.
Proper assembly of disconnectors
The proper assembly of each disconnector has to be demonstrated by either measurement of resistance/ capacitance or
partial discharges.
Apart from the routine tests considered
as the minimum requirement by the IEC,
Hitachi Energy performs additional routine
tests on MO resistors and arresters to
ensure high quality.
These include:
• Measurement of the total leakage current on each arrester
at Uc
• Regular measurement of the power losses on the MO
resistors and arresters
• Examination of the energy handling capability of MO
resistors with current impulses
• A reduced accelerating ageing test on some MO resistors
from each production lot
29
50
40
30
20
10
Testing time [Days]
0
Group I
100
Group II
200
300
400
500
600
700
800
Group III
19 Internal leakage currents of MO surge arrester
design principles during a long-term humidity test.
5.4 Acceptance tests
Standard acceptance tests performed include:
• Measurement of the reference voltage on the arrester
• Measurement of the lightning impulse residual voltage on
the arrester or arrester unit
• Test of internal partial discharges
The acceptance tests are to be agreed upon when the
products are ordered. The tests are performed on the nearest
lower whole number to the cube root of the number of arresters
to be supplied.
The proof of the thermal stability of an arrester as part of the
acceptance test requires additional agreement between manufacturer and purchaser, and it is to be explicitly specified in
the order. This is necessary, because proof of thermal stability
means that part of the operating duty test has to be performed.
This test is expensive and can be performed only in laboratories
that have the necessary equipment, and they have to be booked
in advance.
5.5 Special tests
As part of the development of the arresters, additional tests
were performed in cooperation with users and research institutes. These tests were performed to examine the behavior
of MO surge arresters with silicone housings under special
conditions.
Temperature cycles
The construction and also the materials used for the MO
surge arresters manufactured by Hitachi Energy in Switzerland
tolerate temperatures up to -60 °C and extreme changes in temperature between -40 °C and +40 °C without any changes to
the mechanical or electrical qualities. The construction of the
arrester, and especially the surface of the silicone, were not
harmed in any way by ice during cyclic freezing.
Humidity tests
The electrical behavior of the arresters directly molded with
silicone were not influenced by humidity during long-duration
tests that lasted more than two years, and during which the
arresters were subjected to a relative humidity of more than
90 percent, and also to regular rain.
Figure 19 shows the results of a long-term test at high humidity.
Design principle Group I (directly molded arresters, see Figure
1) behaved best in this test. No significant increase in the internal leakage current was observed during the test period of
more than two years.
Behavior in fire
Silicone is a self-extinguishing material. If silicone catches
fire as a result of a flame or an electric arc and the cause of
the fire is removed or switched off, then the burning silicone
extinguishes itself in about one minute. Only non-toxic burnt
silicone remains in the burned patch, which is in fact nothing
but fine quartz sand. Smoke analyses show no toxic gases
occur as a result of fire.
30
| M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S
Shock and vibration
Surge arresters of type POLIM-I/S/H 36 N were subjected to a
vibration test according to IEC 61373: 2010, category 1, class B.
Due to the variable installation position of the surge arresters,
testing was performed with the highest long-term test level
(z-direction) and shock test level (x-direction). No significant
damage was found by visual examination after the vibration
test (lifetime test) and shock test in the z-, y- or x-coordinate
directions.
Finally, repeated electrical tests showed no significant changes
compared to the pre-measurements.
The surge arresters of type POLIM-I/S/H 36 N meet the
requirements of IEC 61373: 2010.
Wind tunnel test
MO surge arresters for application in traction systems on rolling
stock, e.g. high-speed trains, may be subjected to very high
wind loads. To ensure the mechanical integrity of the MO surge
arrester and the stiffness of the silicone housing under extreme
wind loads, MO surge arresters of type POLIM-S (class SM) and
POLIM-H (class SH) were subjected to aerodynamic forces in a
wind tunnel test. As a result of the high-speed visualization, it
can be stated that at a wind speed of 100 m/s (360 km/h), one
does not observe oscillations of the sheds. This proves that the
MO surge arresters for application on rolling stock can be used
without restrictions on high-speed trains.
5.6 Commissioning and on-site tests
All MO surge arresters undergo a routine test (factory test)
before shipping. The routine test report contains all relevant
results and is delivered together with the MO surge arresters.
No on-site or commissioning test is necessary, and it is not
advised.
It has to be noted that on-site tests, e.g. insulation tests on
cables or gas-insulated substations (GIS), cannot be correctly
performed if MO surge arresters are connected to the system
under test. This is because MO surge arresters will carry a
current in the mA range and will limit the test voltage. In a worst
case the MO surge arresters can be destroyed by the application of AC withstand voltage for a prolonged time. For this reason, MO surge arresters must be disconnected when on-site
tests are performed. If in any doubt, the manufacturer must be
contacted before such tests are performed.
M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S |
100 per cent factory tests
are performed on each MO
resistor and MO surge arrester
in automated test stations.
31
32
| M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S
06 Neutral earthing methods
and determination of Uc
For correct choice of MO surge arresters, the system preconditions must be known.
The handling of the transformer neutral and the failure conditions in the system determine
the continuous operating voltage Uc.
6.1 General considerations
The earthing method of the star point of a transformer has
a direct influence on the choice of the continuous operating
voltage Uc of all MO surge arresters to be installed in the
system.
The manner in which the star point is treated affects directly the
height of the current, which occurs in cases of failure with the
earth connection, on temporary overvoltages with power
frequency and transient overvoltages. Single-phase-to-earth
faults (earth fault, earth short-circuit) are the most frequent failures in medium and high-voltage systems.
Low currents at the failure point tend to be connected with high
and long existing temporary overvoltages of the sound phases.
This is the case with systems having an insulated star point
or earth fault compensation. The single-phase earth fault is
registered and quickly switched off by the system protection
in systems with low-ohmic star point earthing.
In Figure 20, the basic circuit of a medium-voltage transformer
with a star connection with open star point (Mp) is shown.
Specified are the voltages and currents in case of a symmetrical
load, i.e. in an undisturbed service case. All line-to-earth
voltages ULE are equally high. The voltage of the star point UMp-E
relative to earth is zero. The voltage triangle is provided on the
right side for better understanding.
If a single earth fault occurs in the described system, e.g. line
L3 touches the earth, an asymmetry occurs, the voltage at L3
becomes almost zero, and the voltage at the sound phases
shifts to the voltage ULL, which is the system voltage Us.
The consequence is that a failure current ICe flows through the
failure point back into the system. The value of the failure current is determined by the impedance in the current path.
Further, a TOV occurs on the sound phases as long as the
failure lasts, which the installed equipment (the MO surge
arresters) has to withstand. Figure 21 reflects this situation.
In the following chapters, different star point treatments are
briefly explained and the choice of the continuous operating
voltage Uc as the most important characteristic for a safe application of the MO surge arrester in the system is specified.
While choosing the continuous operating voltage Uc, it is
necessary to ensure that the arrester will not be overloaded
under any circumstances due to the voltage with power
frequency. In this way, the arrester meets the requirements
of the operating system. Therefore, Uc of the arrester is to be
chosen in such a way that the arrester cannot become unstable
either through the continuous applied voltage coming from the
system, or through temporary overvoltages that may occur.
In selecting the Uc of an arrester in a three-phase system,
the location of the arrester plays the deciding role: between
conductor and earth, between the transformer neutral and earth
or between two phases.
The maximum operating voltage at the arrester terminals can
be calculated with the help of the maximum system voltage Us.
M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S |
That is why it always makes sense to set
the continuous operating voltage Uc of an
MO surge arrester somewhat higher than
the calculated minimal value that is required.
voltage ULE in absence of the fault at the same location in the
system. The earth fault factor only refers to a particular point of
a three-phase system, and to a particular system condition.
The magnitude of the earth fault factor depends on the way
the neutrals of a system are earthed.
k=
This “safety margin” contributes to a secure and reliable operational system. A safety margin of 10 percent is recommended
when choosing the Uc unless there are explicit technical reasons for not doing so. The thermal stability of the surge arrester
in the system is always to be preferred over a fully optimized
protection level. The examination of the residual voltage of the
chosen arrester and eventually the examination of the resulting
protection distance is necessary in any case.
ULE, f
ULE
A system is considered effectively earthed if the earth fault
factor (k) does not have a value higher than 1.4 anywhere in the
system. This is the case in systems that are described as solid
or directly earthed. If the earth fault factor is higher than 1.4
at any point in the system, then this is considered ineffectively
earthed. In such systems, the star point is insulated (also
described as open) or compensated.
The earth fault factor k is the ratio of the highest
power-frequency phase to earth voltage ULE,f on a healthy phase
during an earth fault to the power frequency phase to earth
Trafo
33
i
L1
Mp
U LL = U s
L1
L2
U LE
L2
L3
U LL
U Mp-E = 0
U LE
L3
As mentioned above, in medium-voltage systems special
attention must be paid to potential temporary overvoltages
UTOV. They occur during earth faults, and they depend on the
treatment of the star point of the transformers and the system
management.
20 Basic circuit of a medium-voltage
transformer with connected lines
(star connection with isolated star point).
Thus, generally the demand for the continuous operating voltage results are as follows:
Trafo
k x Us
Uc ≥
T x √3
i
L1
Mp
L2
L2
U L2-E
U L1-E
T is the factor given in the TOV curves, supplied by the
manufacturer.
As a rule, in medium-voltage systems the withstand voltage
values of the insulation are rather high in relation to the system
voltage (see Table 3). This means that the distance between
the lightning impulse withstand voltage (LIWV) and the lightning
impulse protection level (Upl ) of an MO surge arrester is in most
cases sufficient. On the other hand, the system conditions and
the maximum system voltage Us are not always clearly known.
L1
L3
CK
U Mp-E = U LE
I Ce
U=0
U = U s = √3 x U LE
U LL = U s
I Ce
L3
I Ce ≤ 30 A
RE
21 Situation in a medium-voltage
system with insulated star point and
single earth fault (line L3).
U Mp-E
34
| M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S
6.2 Systems with insulated star point or
with earth fault compensation
6.3 Systems with high-ohmic insulated
neutral and automatic earth fault clearing
As a rule, systems with an insulated star point are systems of
small extension, auxiliary power systems for power stations or
station services. A capacitive earth failure current ICe of about
5 A to 30 A flows in case of failure. The earth fault factor is:
The same voltages occur as described in section 6.2 in the case
of an earth fault. However, immediate automatic fault clearing
enables a reduction of Uc by the factor T. Naturally, it is decisive
to know the level of the possible temporary overvoltage, as well
as the maximum time for the clearing of the earth fault.
k ≈ √3
M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S |
In these systems a short-circuit appears in case of a failure.
In medium-voltage systems the short-circuit current can be
as high as IK = 20 kA, and consequently, the failure has to be
cleared in less than 0.5 seconds. However, under worst case
conditions, and considering some safety margins, it can be assumed that in medium-voltage systems the clearing time of the
earth fault is t = 3 s at the most. In Figure 18 the described TOV
curve for an MO surge arrester with class SL (e.g. an arrester
type MWK) lists T = 1.343 as a result, so that it may be written
Making use of the TOV curve, this results in:
In case of intermittent earth faults, the earth fault factor can
reach values up to k = 1.9. The duration of the failure lies
between several minutes and several hours.
Uc ≥
Us
T
Systems with earth fault compensation are mostly overhead
line systems with system voltages between Us = 10 kV and
Us = 110 kV. One or more transformer star points in these systems are earthed with high-ohmic Petersen coils. An earth fault
current of approximately 5 A to 60 A can flow in case of an earth
fault. The earth fault factor is in this case
for the arrester between phase and earth.
k ≈ (1.0 … 1.1) √3
6.4 Systems with direct or low-ohmic
star point earthing
In both cases, the voltage increases in the “healthy” phases to
a maximum of Us under earth-fault conditions. This results in:
Uc ≥ U S
for the arrester between phase and earth.
The voltage at transformer neutral can reach a maximum of
Us /√3. This results in:
Uc ≥
Us
√3
Uc ≥
Us
T x √3
for the arrester between transformer neutral and earth.
A system with low-ohmic star point earthing is provided if the
star point of one or more transformers are directly earthed or
through current limiting impedances. The system protection is
set up so that even a single line-to-earth fault at any place in
the system causes an automatic fault clearing. These are typical
cable systems in towns with system voltages between 10 kV
and 110 kV. In the case of a failure, the earth short-circuit current (Ik) flows, which leads to an immediate automatic clearing
of the fault. As a rule, the duration of the failure is limited to
tk < 0.5 s. In unfavorable situations, the duration of the failure
can last up to 3 s in medium-voltage systems.
for the arrester between transformer neutral and earth.
In every system, there exist inductances and capacitances
that produce oscillating circuits. If their resonant frequency is
close to that of the operating frequency, the voltage between
the phase conductor and earth could basically become higher
than that of Us in single-pole earth faults. The system management should avoid the occurrence of such resonances. If this is
not possible, then the Uc should be correspondingly increased.
In systems with earth fault compensation the earth fault factor
can reach a value of 1.9 in unfavorable conditions. This is to be
taken into account by increasing the continuous voltage by
10 percent.
6.4.1 Systems with direct star point earthing
“Direct” or “solid” star point earthing (earth conductor) is principally used for all systems with system voltages of 220 kV and
above, but it can also be found in medium-voltage systems. In
these types of systems, there are so many transformers with
direct neutral earthing that during an earth fault, the phase
voltage in the complete system never exceeds 1.4 p.u.
Therefore, the earth fault factor is:
k = (0.75 …≤ 0.8) × √3, that is k ≤ 1.4
Uc ≥
K x Us
T x √3
=
1.4 x Us
1.343 x √3
≈
1.05 x Us
√3
for arresters between phase and earth.
This simple equation can be generally used as a rule of thumb
for systems with direct earthed neutral.
35
voltage should be chosen according to
Uc ≥ 1.25 × Us / √3 = 0.72 × Us
for the MO surge arrester between phase and earth.
6.6 Distribution systems with delta
connection
Transformers in delta connection naturally have no neutral or
star point. In the case of an earth fault of one of the phases in
such systems, the arresters connected to the sound phases will
be stressed with the system voltage Us. An earth fault factor of
k = √3 = 1.732 must be considered. The continuous operating
voltage should be chosen according to
Uc ≥ Us
The voltage of the neutral of the unearthed transformers in the
system reaches a maximum of
UTOV = 0.4 × Us
This results in:
Uc ≥
0.4 x Us
T
=
0.4 x Us
1.343
= 0.3 × Us
for arresters between transformer neutral and earth.
6.4.2 Systems with low-ohmic star point earthing
In case of low-ohmic earthing, one has to distinguish between
inductive earthing (neutral reactor) and resistive earthing (earthing resistor). The fault current is in the range of 500 A to 2,000
A. The fault duration is in the range of a few seconds maximum.
The earth fault factor is:
k = (0.8…1.0) × √3
For arresters in the vicinity of low-ohmic earthed transformers,
an earth fault factor of k ≤ 1.4 is applicable, and the same equations for Uc as for direct earthed transformers can be chosen,
see 6.4.1.
Care is required if the arresters are located just a few kilometers
from the transformer. In unfavorable earthing conditions, e.g.
desert regions or mountains, the earthing resistance can be
very high, and consequently the earth fault factor higher than
1.4. In the case of single pole earth faults with resistive current
limitation, earth fault factors of 2.0 can appear. In such cases
the procedure described in Section 6.3 should be followed.
6.5 Four-wire, multi-earthed-wye systems
In some countries, a four-wire system is used in special cases.
In this system, a fourth wire is connected to the earthed neutral
point of the transformer and connected additionally to earth at
several points along the line. In such systems, an earth fault
factor of k = 1.25 can be assumed. The continuous operating
6.7 Arresters between phases
Considerable overvoltages between the phase terminals of
transformers or reactors may occur when a reactor or a reactive
loaded transformer is switched off. The withstand voltage of the
reactor or transformer between the phases may be exceeded
without operation of the phase-to-earth arresters. If such
switching overvoltages are expected, surge arresters should
be applied between phases in addition to the phase-to-earth
arresters.
6.7.1 Six-arrester arrangement
In special cases, such as in arc furnace installations, switching
overvoltages occur that are insufficiently limited by arresters
between phase and earth. In such cases, it is necessary to
install additional arresters between the phases. The arresters
between the phases should have a continuous operating voltage of Uc ≥ 1.05 Us. The continuous operating voltage Uc of the
phase-to-earth arresters depends on the earthing of the transformer neutral. A typical arrangement for systems with insulated
transformer neutral is given in Figure 22a. In this case all six
arresters should have a continuous operating voltage of
Uc ≥ 1.05 × Us
The factor 1.05 takes account of possible harmonics in the
system voltage Us.
The continuous operating voltage Uc of the phase-to-earth
arresters depends on the earthing of the transformer neutral.
In case of a system with low-ohmic star point earthing (directly
earthed) of the transformer the continuous operating voltage
can be chosen to
Uc ≥ (1.05 × Us) / √3
for the phase-to-earth arresters.
36
| M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S
6.7.2 Neptune design
A variation of the six-arrester arrangement is the “Neptune”
(or “candle”) design, because of its arrangement of arresters.
It consists of four similar arresters. Two arresters in series are
fitted between the phases and the earth and also between the
phases, as shown in Figure 22b. This arrangement permits
overvoltage protection both between the phases, and between
the phases and the earth. This kind of arrangement however,
has a fundamental disadvantage in comparison to the six-arrester arrangement. Since the arresters behave in a capacitive
manner at continuous operating voltage, if there is an earth
fault, all four arresters form an asymmetrical system. In the
case where each arrester has identical capacitance (meaning
that the arresters have identical ratings), arresters A1 to A3
would be stressed with 0.661 × Us and A4 with 0.433 × Us.
However, a simple solution is to use four arresters of the same
type and rating. For this case:
Uc ≥ 0.661 × Us
The protection level of this arrangement, which has always
two arresters in series, is therefore similar to that offered by
the arrester with Uc ≥ 1.322 × Us. The residual voltage of this
arrester combination is therefore also 32 percent higher than
that of the six-arrester arrangement. If a lower protection level
is required between phase and earth, a lower continuous operating voltage for arrester A4 may be chosen compared to A1 to
A3. Since the arrester capacitance is inversely proportional to
the arrester Uc, the final steady-state voltages need to be
calculated individually for each specific case.
M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S |
6.8 Operating voltage with harmonic
oscillation
Harmonic currents generate harmonic oscillations superimposed upon the power frequency voltage. For this reason, it is
possible that the peak value of phase-to-phase voltage (Us) can
be higher than √2 × Us. If this difference is less than 5 percent,
then a correspondingly higher Uc must be used. On the other
hand, if due to the harmonics the voltage increase is higher
than 5 percent, the choice of Uc should be discussed with the
arrester manufacturer. The same applies for forms of voltage
that can often be seen in the vicinity of thyristor converters:
voltage steps, ignition peaks, and asymmetries in the two half
cycles. Commutation overshoots with a high repetition rate, or
other voltage spikes, which are common in drives and converters, cannot generally be limited by gapless MO surge arresters.
This is not a typical application for MO surge arresters. In the
case of commutation overshoots and other superimposed
voltage spikes, special criteria for the dimensioning of MO surge
arresters have to be considered. This makes close cooperation
and detailed discussion between the user and manufacturer
necessary.
Uc ≥ 1.05 x US
T
7.1 General considerations
The principle of insulation coordination for an electricity system
is given in the IEC 60071-1 and IEC 60071-2 standards. It is the
matching between the dielectrical withstand of the electrical
equipment taking into consideration the ambient conditions and
the possible overvoltages in a system.
1. It has to limit the occurring overvoltage to a value that is not
critical for the electrical equipment
2. It has to guarantee a safe and reliable service in the system
L3
L2
A1
A2
A3
A4
Uc ≥ 0.661 x US
L3
Uc ≥ 1.05 x US
Insulation coordination is a balance between stresses from the system vs. strength of the
equipment. The MO surge arresters are matched to the system preconditions and the
insulation levels of the equipment to be protected.
L2
T
L1
07 Coordination of insulation and
selection of MO surge arresters
For economic reasons, it is not possible to insulate electrical
equipment against all overvoltages that may occur. That is why
surge arresters are installed to limit the overvoltages up to a
value that is not critical for the electrical equipment. An MO
surge arrester ensures that the maximum voltage that appears
at the electrical equipment always stays below the guaranteed
withstand value of the insulation of an electrical device.
Therefore, an arrester has to fulfill two fundamental tasks:
L1
37
The continuous operating voltage Uc is to be chosen in such
a way that the arrester can withstand all power frequency voltages and also temporary overvoltages without being overloaded
in any possible situation. This means that T × Uc must be
always higher than the maximum possible temporary
overvoltages UTOV in the system.
NOTE: Ferromagnetic resonances are the exception. They can
become so high and exist for so long that they may not be taken
into consideration by the dimensioning of the continuous
voltage if the arrester should still be able to fulfill its protection
function in a meaningful way.
If ferromagnetic resonances appear, then this generally means
that the arrester is overloaded. The system user should take
the necessary measures to avoid ferromagnetic resonances.
An MO surge arrester can fulfill its function of protection
properly if the lightning impulse protection level Upl lies clearly
below the lightning impulse withstanding voltage (LIWV) of the
electrical equipment to be protected, the safety factor Ks is also
to be taken into consideration.
The choice of the continuous operating voltage Uc is described
in detail in section 6. The following paragraphs briefly deal
with the necessary energy handling capability and the protection characteristic of MO surge arresters in medium-voltage
systems.
03 Typical values of the lightning impulse withstanding voltage (LIWV) and the lightning impulse protection level Upl = 4 p.u.
21a Six-arrester arrangement with Uc ≥ 1.05 × Us
21b Neptune design. A1, A2, A3 and A4 are
four similar arresters, each with Uc ≥ 0.661 × Us
Um in kV rms
LIWV in kV pv
3.6
7.2
12
17.5
24
36
40
60
75
95
125
170
Upl in kV pv
11.8
23.5
39.2
57.2
78.4
117.6
LIWV/Upl
3.39
2.55
1.91
1.66
1.59
1.45
38
| M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S
The point is to set the voltage-current characteristic of the
arrester in a way that both requirements are met. Figure 23
shows in a simplified way the principle of insulation coordination. The lightning impulse withstand voltage (withstand voltage
of the insulation) is relatively high compared to the system voltage, as can be seen in Figure 23. This automatically results in a
large distance between the maximum admissible voltage at the
electrical equipment to be protected and the lightning impulse
protection level; see also Table 3.
NOTE: The acronym “BIL,” which is often used for “basic
lightning impulse insulation level,” is exclusively to be found
in the US standards (IEEE/ANSI standards). It is similar to the
“lightning impulse withstand voltage” (LIWV) as used in the IEC
definition.
As mentioned above, it makes sense to choose a continuous
operating voltage Uc higher than was calculated (10 percent).
As a rule, there is enough distance between the maximum
admissible voltage at the electrical equipment and the protection level of the arrester.
7.2 Selection of nominal discharge current,
charge and energy
M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S |
transfer rating Qrs and the thermal charge transfer rating Qth.
The thermal energy rating Wth results from the energy that may
be stored in a loaded transmission line, or in other energy
sources like capacitor banks etc. Therefore, it is necessary to
know the possible energy stores in a system, such as cables,
capacitors or capacitor banks and inductivities.
Figure 24 shows a statistical evaluation of all the measured
lightning currents. The curve of the mean value shows the
probability of the occurred lightning current peak values.
The probability of reaching or exceeding 20 kA is 80 percent,
whereas lightning currents with peak values of over 100 kA are
very rare. The specified lightning currents and the high current
impulses are derived from these lightning current statistics.
Assuming that in medium-voltage systems a lightning current
diverts in the case of a far distance direct stroke, and that
flash-overs between phases and at insulators will occur, one
can get a nominal discharge current of In = 5 kA. A wave shape
of approximately 8/20 μs results for the lightning current if a
flashover occurred at one of the insulators. The worst case to
be considered is a direct stroke in a phase wire in front of a
substation without an insulator between the point of stroke and
the substation. In this case it can be assumed that a lightning
current of e.g. 20 kA diverts in both directions of the line and
half of the lightning current (10 kA) travels undamped into the
substation.
The lightning current parameters are taken from lightning
statistics. The expected magnitude and probability of lightning
discharge currents are correlated to the repetitive charge
U
p.u.
unprotected,
endangered area
9
8
lightning overvoltages
7
6
LIWV
KS
5
Upl
4
3
T x UC
Requirements of equipment, related to Um
2
UC
System preconditions, related to US
Design parameters of MO arresters
1 p.u. = US x √2 / √3
23 Comparison of the possible occurring voltages in a typical medium
voltage system, the withstand voltages of the electrical equipment and
the parameters of the MO surge arrester. The lightning overvoltages are
decisive in medium-voltage systems. That is why are shown only the
parameters for the lightning overvoltages.
UTOV
1
7.3 Protection level
P[%]
99.0
The switching impulse protective level Ups is decisive for the
coordination of the insulation in transmission systems of higher
system voltages. It is less important in the medium-voltage systems discussed here. Of prime importance here is the lightning
impulse protection level Upl and, if necessary, the protection
level at steep current impulse, such as when vacuum breakers
are in the system.
90.0
70.0
50.0
20.0
10.0
2.0
0.2
0
I [kA]
0
10.0
100.0
24 Statistical evaluation of lightning measurements all over
the world. Described is the probability of occurrence above the
lightning current’s peak values (adapted from Cigré survey).
The nominal discharge current can be chosen according to
the thunderstorm activity in a region or the expected threat
of lightning to a substation. In this way, the requirements for
the arresters can be clearly specified together with the repetitive
charge transfer rating Qrs and the thermal charge transfer rating
Qth or the thermal energy rating Wth. MO surge arresters with
In = 10 kA and classification DH are used in applications in
medium-voltage systems.
Higher nominal discharge currents (In = 20 kA) and higher
classifications like SL, SM and SH are chosen in special cases
in medium-voltage systems, such as:
• Regions with extreme thunderstorm activities and the danger
of direct lightning strikes
• Overhead lines at concrete poles or wooden poles and cross
arms that are not earthed
• Arresters placed at locations where people are often to be
found (for instance in electrical traction systems)
• Lines that demand exceptional high safety standards for the
working process
• For protection of motors, generators and cables
• Arc furnace protection
• Capacitors and capacitor banks
• Very long cables
• Rotating machines
10
UL-E
39
Generally speaking, the protection level should be as low as
possible to ensure optimal protection. As previously emphasized, the operational safety of the arrester in the system is
always to be preferred to the complete exploitation of the
protection level. These opposing requirements are mainly
non-critical in the medium-voltage systems, see Figure 23
and Table 3.
The protection ratio Upl/Uc is fundamentally important.
The smaller the ratio, the lower the protection level with the
same Uc, and the better the protection. If a very low protection
level is technically absolutely necessary in a specific case, it is
possible to choose an arrester with a better protection ratio.
As a rule, this is an arrester with a higher energy rating, because
these arresters have MO resistors with a larger diameter as an
active part. The choice of an MO surge arrester with the same
Uc but a higher energy rating offers better protection in the
system, although the operational safety stays the same, and it
also provides a higher energy handling capability. Moreover, an
MO surge arrester with a lower protection level always provides
a larger protection distance.
Therefore, the choice of arrester or the comparison of different
products should also take into consideration the protection ratio
Upl/Uc in addition to the nominal discharge current and the
charge or energy handling capability.
In this context, the temporary overload capability of an MO
surge arrester with temporary overvoltages should also be
observed. A high resistance towards temporary overvoltages
generally means that the voltage-current characteristic of an
MO surge arrester was set so high that all power frequent overvoltages that occur do not fundamentally exceed the knee point
of the U-I characteristic. However, this means that the residual
voltage of an MO surge arrester lies correspondingly high,
which causes an unfavorably high protection level.
40
| M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S
7.4 Selection of arrester housing
Silicone or EPDM are almost exclusively used today as housing
material for medium-voltage arresters. Silicone is preferred due
to its excellent behavior, especially in regard to pollution. The
choice of the housing for MO surge arresters in medium-voltage
systems is not critical. The flashover distance of the arrester
housing and the creepage distance along the surface of the
housing are to be taken in account.
The minimum flashover distance is determined by the required
withstand values of the test voltages which have to be applied
in the relevant withstand tests, the lightning voltage impulse test
and the AC withstand test with power frequency for one minute
performed with an empty housing or with the MO resistors
replaced by insulating material.
The height of the test voltage to be applied is related to the
protection characteristic of the MO surge arrester. The test
voltage during the test with lightning voltage impulse must be
1.3 times the residual voltage of the arrester at In. The housings
for 10 kA- and 20 kA station class arresters intended for use in
systems with Us ≤ 245 kV, must withstand for one minute an AC
voltage test with a peak value of the testing voltage 1.06 times
the switching protection level. Housings of distribution class
arresters must withstand a power-frequency voltage with a peak
value equal to the lightning impulse protection level multiplied
by 0.88 for a duration of one minute.
M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S |
is determined by the voltage-current characteristic curve of the
active part, and the arrester naturally protects its own housing
against overvoltages. The real provable withstand values of the
housing are generally higher than the demanded minimum
values corresponding to IEC, especially with arresters for lower
voltage levels.
The behavior of the external insulation under pollution and
applied operating AC voltage is important and determines the
creepage distance. The pollution classes and the corresponding
reference unified specific creepage distances (RUSCD) are
specified in IEC 60507: 2013 and IEC/ TS 60815-1: 2008, see
Table 4. IEC/TS 60815-3: 2008 refers to polymer insulators for
AC systems. For the purpose of standardization, five classes
of pollution characterizing the site severity are qualitatively
defined.
It is possible, however, to specify the reductions of the creepage distances for synthetic materials that have a regenerative
hydrophobicity, such as silicone, towards ceramic insulations.
These reductions (shown in Table 4) are based on general
recommendations given in IEC 60815-3, results from tests and
field experience.
NOTE: The creepage distance for a MO surge arrester is
sometimes specified in relation to the continuous operating
voltage Uc. Therefore, it is important to carefully consider the
voltage to which the creepage requirements are related.
The resulting values for the arrester housings are, as a rule,
lower than the insulation values for insulation of devices and
installations. This is proper because the voltage at the arrester
04 Correlation of pollution class and creepage distance
Minimum recommended specific
creepage distance in mm/kV*
Possible reduction of the creepage
distance with silicone insulation
a – Very light
22.0
30%
b – Light
27.8
30%
Pollution class
c – Medium
34.7
20%
d – Heavy
43.3
No reduction recommended
e – Very heavy
53.7
No reduction
* The shortest specific creepage distance for insulators between phase and earth.
41
08 Protective distance of
MO surge arresters
The place of installation of an MO surge arrester is, besides the correct choice, critical
for an optimized protection of the equipment. The MO surge arrester must be as close
as possible to the equipment to provide best protection.
8.1 General considerations
The higher its lightning impulse withstand voltage (LIWV) lies
above the residual voltage of the arrester at nominal discharge
current In, the better the equipment is protected against lightning overvoltages.
Modern MO surge arresters with a residual voltage of
Ures ≤ 3.33 x Uc at In maintain a value of Upl ≤ 4 p.u., even in
systems with high-ohmic earthed or insulated transformer
neutrals. The Upl is the lightning impulse protection level of
the arrester. A summary of the typical values are given in
Table 3.
It should be noted that the specified residual voltages Ures from
the data sheets apply for the terminals of the arrester, which
means they are valid only for the place where the arrester is
installed. The voltage at the devices that are to be protected is
always higher than the voltage that is directly at the arrester terminals in view of the reflections of the overvoltages at the end
of lines. Further, inductive voltages drops along the connections
from line to the arrester terminal and the earth conductor have
to be considered. Therefore, the overvoltage protection no
longer exists if the arrester is placed too far from the device to
be protected. The protective distance L is understood to be the
maximum distance between the arrester and the equipment at
which the latter is still sufficiently protected.
8.2 Traveling waves
Voltage and current impulses with a rise time shorter than
the travel time of an electromagnetic wave along the line, travel
along the line as traveling waves. This means that, disregarding
damping, the current and voltage impulse travels along the line
without changing its form. Therefore, it is in another place at a
later time.
Current and voltage are connected to one another because of
the surge impedance of the line. The surge impedance results
from the inductivity and capacitance per unit length of the line,
disregarding the ohmic resistance per unit length and the
conductivity of the insulation.
Z=
√L’
√C’
L’ = Inductivity per unit length in H/km
C’ = Capacitance per unit length in F/km
Only the voltage impulses are important when analyzing the
overvoltages.
When a voltage traveling wave on a line reaches a point of
discontinuity, i.e. a change in the surge impedance, part of the
voltage is “reflected” backward and part is transmitted forward.
This means that voltage decreases and voltage increases
appear on the connections of the overhead lines to the cable,
and at the end of the line. Especially at the end of the line, such
as at open connections or transformers, reflections appear that
lead to a doubling of the voltage. The height of the voltage for
each moment and for each place on the line is the sum of the
respective present values of all voltage waves.
In the moment an MO surge arrester is limiting the voltage
by conducting the charge to earth, it is very low-ohmic and
can be considered a short- circuit. The consequence is that
the travelling voltage wave is reflected back and forth between
a very high impedance (positive reflection) and a short-circuit
(negative reflection). Further, in the moment the MO surge
arrester limits the voltage, all overvoltage is reflected negatively.
Thus, the MO surge arrester protects in both directions. To simplify matters, a funnel-shaped voltage increase results from the
arrester, as can be seen in Figure 25.
Considering that the voltage at the end of an open line (e.g.
transformer) can reach a maximum of twice the residual voltage
Ures of the arrester, it can clearly be seen that the distance between the point of installation of the arrester and the equipment
to be protected should be as short as possible to ensure good
protection.
As seen in Figure 25, an MO surge arrester installed in position
X A1 will not protect the transformer, because the voltage at the
transformer will be higher than the withstand voltage (LIWV) of
the transformer insulation. If the arrester is installed at position
X A2, the voltage at the transformer is well below the LIWV and
provides very good protection.
| M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S
42
8.3 Protective distance
On the overhead line in Figure 26 an overvoltage U travels as a
traveling wave with speed v towards the line end E. At point E is
the equipment to be protected. For the following analysis, it is
considered that the equipment to be protected is highohmic
(transformer, open circuit breaker). When the traveling wave
reaches E, it is positively reflected and the voltage increases to
2 x U. The function of arrester A is to prevent unacceptably high
voltage values at the equipment to be protected. Under the simplified assumption that the front, with wave steepness S, of the
incoming overvoltage wave is time-constant, the following
relationship applies for the maximum value UE:
UE = Ures +
2 × S × (a + b)
v = 300 m/μs
v
A protection factor Ks is recommended between the LIWV of
the equipment and the maximum lightning overvoltage that
occurs (see also Figure 23). This protection factor takes into
consideration, among other things, any ageing of the insulation
and the statistical uncertainties in defining the lightning impulse
withstanding voltage of the equipment. For outdoor insulation a
safety factor of Ks = 1.15 is recommended (IEC 60071).
This results in:
LIWV
≥ UE = Ures +
Ks
2×S×L
v
L=a+b
The required equation for the protection distance is:
L=
v
2×S
X
v
2×S
- Ures
It should be mentioned that the given approximation for L is
valid in the strict sense only for b = 0, in practice, however, it
gives sufficiently precise values.
The steepness S of the incoming overvoltage wave must be
known in order to determine the protective distance as it is
above described. A general value for the steepness S cannot be
M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S |
given, because it depends on various parameters and statistics.
Values between S = 800 kV/μs and S = 1,550 kV/μs are to be
expected in medium-voltage systems, depending on the pole
construction and insulators used. As a rule of thumb, a steepness of S = 1,000 kV/μs can be used for rough calculations.
43
UT
U2
It is certainly to be assumed that the arrester and the equipment
to be protected are connected to the same earthing system.
The connections on the high-voltage side and the earth side
must be short and straight. Especially connection b should be
as short as possible. In this way, it makes sense to lead the
overhead line first to the arrester, and from there directly to the
bushing of the transformer, for example.
LIWV
U2
Ures
8.4 Induced voltages
As mentioned above, the connections between the arrester terminals and the equipment to be protected must be as short and
straight as possible. This is because inductive voltages appear
at each conductor due to the self-inductivity during the flowing
of an impulse current. These induced voltages are considerable
during high rate of changes di/dt, such as when lightning currents occur. The induced voltage is calculated as:
XA1
ZL = 450 Ω
XA2
XT
ZT = > ∞
Ui = L × di/dt
For example: an approximate inductive voltage of Ui = 1.2 kV
per meter connection line results from an inductivity of L = 1 μH
for a straight wire of 1 meter length and a lightning current of
10 kA peak value of the wave shape 8/20 μs.
25 Voltage increase from the point of
arrester installation in both directions.
The specified residual voltages, which are to be found in the
data sheets, are always the voltages between the arrester terminals only. Especially in case of steep current impulses (e.g. rise
time of 1 μs) the induced voltages have to be considered in a
protection concept.
U
v
S
a
UE
E
b
A
26 Assumption for the calculation of the
voltage at the end of a line and for the
determination of the protective distance L.
Ures
44
| M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S
09 Equipment protection
To reach an optimized protection each equipment needs special attention and must be
treated separately.
M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S |
45
It is always advisable to install arresters
on both sides of all the transformers,
particularly in regions with high
thunderstorm activity.
side are capacitive transmitted to the low-voltage side here as
well. Thus, in principle arresters should also be installed on the
low-voltage side of the medium-voltage transformers.
Figure 28 shows in principle the resistive and capacitive
coupling from the medium-voltage to the low-voltage side of
a distribution transformer.
Overhead line
9.1 General considerations
9.2 Protection of transformers
To reach an appropriate overvoltage protection in medium-voltage systems, it is necessary to find the best compromise
between the costs and the benefits of the protection devices
to be used. An ideal technical-economic balance is to be striven
for. Overvoltage protection that is accurately applied reduces:
• Outages of lines and substations
• Interruptions of critical manufacturing processes that demand
high voltage stability
• Costs due to interruptions in the energy supply
• Costs for the replacement and repair of electrical equipment
• Ageing of the insulation (e.g. cables)
• Maintenance work
Generally, all transformers that are directly linked to lightning-endangered lines have to be equipped with arresters
between phase and earth.
The aim of overvoltage protection is to guarantee an uninterrupted supply of electrical energy with high voltage stability to
the greatest degree possible.
Therefore, the costs for a set of surge arresters are not the
primarily consideration, but the costs that may arise on a
long-term basis if adequate overvoltage protection is not used.
In fact, all electrical equipment and installations in high-voltage
and medium-voltage systems need overvoltage protection.
In particular, the following equipment needs protection:
• Transformers
• Cables and cable sheaths
• Capacitors and capacitor banks
• Overhead lines
• Rotating machines (motors and generators)
• Power electronics
• Coils and line traps
• Traction equipment (rolling stock and power supply)
AC and DC
It is sometime insufficient to install only one arrester per line in
the substation, considering the limited protective distance of the
arresters and the spatial distance between the equipment in the
substation. If the various pieces of equipment are installed too
far from one another, it is necessary to consider where to place
an additional arrester.
Some typical cases are described in the following paragraphs.
As described in section 6, the occurring power frequency
voltages in a power system depend on the system voltage Us
and the handling of the neutral of the transformers in the system. It is obvious that the continuous operating voltage Uc
of the MO surge arresters, which have to protect the transformers and the neutrals of the transformers, have to be chosen to
be equal to or higher than the calculated values in section 6.
It was explained that all MO surge arresters have a limited
protective distance, which has to be considered when choosing
the place of installation. At distribution levels (Us ≤ 52 kV),
arresters can often be located very close to the equipment to
be protected, e.g. the transformers. In this case, and where
possible, the earth terminal of the arrester and the equipment,
in this case the transformer, should be bonded with a very
short, straight conductor. Figure 27 gives hints for good and
poor connection principles.
T
T
C
1
C
2
Poor connection
The connection leads are too long and the
transformer and the MO surge arrester do not
have the same earthing point.
3
Very good connection
The MO surge arrester is earthed directly at
the transformer tank. The loop is very short.
In this way the inductance is kept to a
minimum.
Good connection
Common earth of MO surge arrester and
transformer. The connection leads are
much shorter.
27 Examples of good and poor connection
principles for MO surge arresters in distribution
systems.
MV
Low earth resistance is essential, and it should be as low as
possible in order to limit the earth potential rise at the earth
terminal, and hence mitigate safety hazards and flashover on
the low-voltage side of the transformer. A value for earth
resistance of 10 Ω or less is considered to be sufficient.
If a transformer connects a high-voltage system with a medium-voltage system and only the line on the high-voltage side is
lightning-endangered, it is necessary to install an arrester on
the medium-voltage side as well. Transient overvoltages can
be transmitted up to 40 percent capacitive from the primary
(high-voltage side) to the medium-voltage side. That is why it
is also necessary to install an arrester on the medium-voltage
side, even though the medium-voltage side is not directly
endangered by lightning. The situation is similar with transformers that connect a medium-voltage system to a low-voltage system. The high-frequency overvoltages from the medium-voltage
T
C
LV
C
0.4 Ures
i
Ures
RE
28 Coupling of a lightning overvoltage
through a medium-voltage transformer.
ΔU
ΔU
46
| M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S
9.3 Protection of cables
Disruptive breakdowns in cable insulations lead to grave
damage and require expensive repairs. Flashovers along
the cable bushings can damage them and lead to the same
consequences as insulation breakdowns. It is well known that
repeated overvoltage stresses negatively influence the ageing
behavior of the cable insulation, which means that the service
life of the cable is shortened. Cables must therefore be treated
like station equipment and protected against transient
overvoltages (e.g. lightning) with arresters. The arresters are
to be placed directly next to the cable bushings here as well.
The junction lines should be as short as possible. It must be
noted that the earth connection of the arrester is directly attached to the cable sheath.
Longer cables require arrester protection at both ends. For
short cable sections, protection on one side can be sufficient.
This is possible because the protection range of an arrester at
one end of the cable can still offer sufficient protection at the
other end. A cable that connects an overhead line with a substation is often only endangered by lightning on the side of the
overhead line. Therefore, the arrester must be installed at the
junction between the overhead line and the cable.
9.4 Cable sheath protection
The cable sheath of a single-conductor cable in high-voltage
systems is earthed on one side only for thermal reasons. This
procedure is increasingly used in medium-voltage cable systems as well to avoid additional losses in the cable sheath. If the
cable sheath stays open on one side, the sheath can take up to
50 percent of an incoming overvoltage on the inner conductor
on the non-earthed side. The sheath insulation is not able to
cope with this overvoltage stress. Breakdowns between the
sheath and the earth can occur, which damage the external
insulation of the sheath.
That is why it is necessary to protect the cable sheath against
overvoltages on the unearthed side with an arrester. The voltage
induced along the cable sheath in case of a short-circuit is
decisive for the continuous operating voltage Uc. The induced
voltage is dependent on the way the cable is installed and can
at most amount to 0.3 kV per kA of short-circuit current and
kilometer of cable length. The continuous voltage to be chosen
for the arrester which protect the cable sheath results from:
UC ≥
Ui
T
× IK × LK in kV
IK = Maximum 50 Hz short-circuit current per phase in kA
LK = Length of the unearthed cable section in km
With Ui = 0.3 kV and T = 1.343 for a maximum fault clearing
time of t < 3 s (from TOV curve for the MWK) of the short-circuit
current, the result is: 0.3
UC ≥
0.3
1.343
× IK × LK = 0.22 × IK × LK in kV
9.5 Arresters in metal-enclosed
medium-voltage substations (cubicles)
It is often necessary to install arresters in a metal-enclosed
medium-voltage substation. If a cable connects the substation
with a lightning-endangered line, an arrester with a nominal
current of In = 10 kA should be installed at the cable bushing.
The conditions are different if the arresters must limit switching
overvoltages instead of lightning overvoltages. The former could
occur during switching if the inductive current is interrupted
before it reaches its natural zero crossing. In addition, vacuum
breakers can produce high and very steep overvoltages.
9.6 Generator connected to a
lightning-endangered MV line
If a loaded generator is suddenly disconnected from the system
(load rejection), its terminal voltage increases until the voltage
regulator readjusts the generator voltage after a few seconds.
The relationship between this temporary overvoltage and the
normal operating voltage is called the load rejection factor δ L.
This factor can reach a value of up to 1.5. In the worst case,
the arrester could be charged with a temporary overvoltage
of UTOV = δ L x Us, which must be taken into account when
choosing Uc.
UC ≥
according to TOV curve
High-voltage motors can be over-stressed by multiple restrikes
resulting from being switched off during the run-up. This is
especially critical when the cut-off current is less than 600 A.
In order to protect these motors, it is necessary to install surge
arresters directly at the engine terminals or alternatively at the
circuit breaker. The dimensioning of Uc is to be carried out
according to the recommendations in section 6.
If the capacitors are connected in a star, then they are
discharged by the arrester parallel to bank between conductor
and earth. During the discharge up to the voltage of √2 x Uc,
the arresters are loaded in terms of power with:
WC =
SK
ω
× [ 3 - (Uc/Us)2 ]
SK = 3-phase reactive power of the capacitor battery
Wc = The discharge energy taken up by the arrester
It is necessary to use an arrester with a residual voltage Ures as
low as possible because of the insulation of the motors, which
is generally sensitive to overvoltages, especially if it is aged.
That is why arresters should be chosen with an especially favorable Upl/Uc ratio. Under certain circumstances, it is possible to
use the lowest allowable arrester limit of Uc. However, in no case
is Uc allowed to be lower than Us/√3.
Typical arresters used for the protection of electrical motors are
MWK, or MWD for indoor applications.
9.8 Arresters parallel to a capacitor bank
Normally, no overvoltage occurs when a capacitor bank is
switched off. The circuit breaker interrupts the current in the
natural zero crossing, and the voltage in the capacitors to
earth reaches a maximum of 1.5 p.u. As a result of the network
voltage varying at the power frequency, a voltage across the
open circuit breaker of 2.5 p.u. is caused.
A high-frequency transient effect takes place between the
capacitor voltage and the operating voltage if the breaker
re-strikes. During this process, the capacitor is charged with a
higher voltage. This overvoltage at the capacitor between the
conductor and the earth reaches a maximum of 3 p.u.
Assuming that the arrester must carry out this process three
times without any cool down phase, it follows with Uc ≥ Us that
Wc 6 × SK
WC
UC
≥
6 × SK
ω × Us
The thermal energy rating Wth of the arrester with Uc must thus
be adjusted to the reactive power of the battery. The maximum
admissible reactive power values of the parallel capacitor
battery for different arrester types can be found in Table 5.
If the reactive power of the parallel capacitor bank for a
certain arrester type exceeds the limiting values from Table 5,
an arrester with higher thermal energy rating must be selected.
For systems that are not operated with a standard voltage,
the limiting values for SK are found in the column with the lower
standard voltage. If the reactive power is very large, arresters
connected parallel are to be chosen. In this case, the manufacturer has to be informed in order to take the necessary measures to guarantee a sufficient current distribution between the
parallel arresters. The manufacturer should also be consulted
when arresters with Uc < Us are being used.
T
With the help of an example the Uc of an arrester for the generator protection should be determined. With Us = 24 kV, load
rejection factor δL = 1.5 and t = 10 s results for the type
POLIM-H..N
1.5 × 24 kV
1.291
= 27.8 kV
Ui = Induced voltage occurring along the cable sheath in kV
T = Resistance of the arrester against temporary overvoltages
9.7 Protection of motors
47
δL × Us
The duration t of UTOV determines T and lies in a range of 3 to 10
seconds. The high operational safety requirements for generators make it advisable to use arresters with low residual voltage
Ures and high energy handling capability W’. That is why the
arresters of the type POLIM-H..N are recommended for generator protection.
UC ≥
M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S |
In this way, the type POLIM-H 28 N can be chosen for this case
(worst case scenario considered).
Generators as important equipment need special attention in
overvoltage protection. Therefore, it is especially important to
place the arresters close to the generator terminals.
05 Arrester parallel to a capacitor bank. Maximum admissible reactive power SK of the capacitor battery for the indicated
arrester type. Three discharges of the battery are allowed without a cool down phase for the arrester. W/Uc: The arrester
energy absorption capability in relation to Uc.
Arrester type
Uc ≥ Us
Wth /Uc in kJ/kV
Us in kV
POLIM-D
POLIM-K
POLIM-I
MWK/MWD
POLIM-S
POLIM-H
3.6*
5.6
6.25
10.0
15.0
SK in MVA r
SK in MVA r
SK in MVA r
SK in MVA r
SK in MVA r
3.6
0.68
1.06
1.18
1.88
2.83
7.2
1.37
2.09
2.38
3.74
5.69
12
2.28
3.48
3.96
6.24
9.48
17.5
3.32
5.07
5.77
9.10
13.8
24
4.56
6.96
7.92
12.48
18.96
36
6.84
10.44
11.88
18.72
28.44
* Equivalent to Q th = 1.1 C
48
| M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S
M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S |
9.9 Line traps (parallel protection)
9.10 Line arresters
Line traps are air-core coils that are installed in high-voltage
lines. Their inductivity L is in the range of mH. If no measures
are taken, the lightning current in the conductor must flow
through the line trap. Even relatively small current rates of rise of
several kA/μs would produce overvoltages on the line amounting to several million volts and would lead to a flashover. Arresters are connected to the line trap to prevent this. These arresters take over the lightning currents and limit the overvoltage to
its residual voltage Ures.
Line arresters are arresters that are installed parallel to insulators on poles along an overhead line. The reason for the use
of line arresters is the necessity to avoid short interruptions
or outages of the overhead lines due to lightning overvoltages
or the necessity to reduce the frequency of their occurrence.
As a rule, the line arresters are installed in connection with an
earthed shielding wire. In this application the arresters are
called NGLAs (non-gapped line arresters). Line arresters are
used in regions with high thunderstorm activity and a very poor
earthing situation.
When an earth fault to earth or a short-circuit occurs in a
high-voltage system, the fault current IK flows through the
conductor. This power frequency current would overload the
arrester. Uc must therefore be selected so that the short-circuit
current flows through the line trap. It induces a temporary
overvoltage of UTOV = ω x L x IK, which determines Uc at the
line trap.
UC ≥
UTOV
T
=
ω × L × IK
T
The continuous voltage Uc for MO surge arresters that are used
as line arresters is to be determined in exactly the same manner
as those used for the protection of substations or transformers.
Since the line arresters should protect especially against the
effects of lightning strokes, it is necessary to dimension them
according to the lightning parameters of the respective region
(probability, current steepness, charge, footing resistance,
a.s.o). As a rule, the line arresters are equipped with disconnectors, so that an arrester that is overloaded can disconnect itself
from the system and no earth fault appears.
IK = Maximum fault current through the line trap.
L = Inductance of the line trap.
It may be assumed T = 1.343 for the duration of short-circuit
current of t < 3 s (from TOV curve for the MWK).
A special usage of line arresters is MO surge arresters with
an external series gap. These “EGLAs” (externally gapped line
arresters) are to be found in several countries. Figure 29 shows
in principle the arrangement of an EGLA. The challenge is the
coordination of the spark-gap in series with the MO surge
arrester and the spark gap parallel to the insulator to be
protected, and also the residual voltage of the MO surge
arrester used. In IEC 60099-8 and 60099-5 application
principles and test procedures are given in detail.
Earth wire
EGLA
NGLA
Phase wire
Insulator with arcing horn
Insulator with arcing horn
Tower
Tower
RE
MO arrester parallel to an
insulator in an overhead line.
These so-called NGLAs
(Non Gapped Line Arresters)
are installed as desired
with or without disconnector.
29 Possible execution of line
arresters (description in principle).
RE,M
MO arrester with an external
spark gap in series parallel to
an insulator in an overhead line
(EGLA = Externally Gapped
Line Arrester).
RE,M
Continuous improvement
of the products and process
technology require specific
test equipment to perform
development tests in house.
49
50
| M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S
10 MO surge arresters
in parallel connection
There are two reasons to connect arresters in parallel: to increase the energy handling
capability, or optimize protection using MO surge arresters with different U-I characteristics
and energy ratings.
10.1 General considerations
Arresters are generally considered as single devices, i.e. they
fulfill their task in the place in which they are installed according
to their specified data, independent of other nearby devices.
That is why it is possible in principle to install different kinds of
arresters close to one another on a phase wire in the system.
However, it is necessary to take into consideration that according to different ways of functioning, some arresters may
become useless, while others may become overstressed,
such as in cases when arresters with spark-gaps and without
spark-gaps are installed in parallel, or when MO surge arresters
with different voltage-current characteristics are used in
parallel.
Deliberate parallel connections of MO surge arresters are made
if the energy absorption needs to be increased, the residual
voltage should be reduced, or the energy absorption and the
residual voltage should be deliberately dimensioned in a
different way.
10.2 Parallel connections to increase
the energy handling capability
Two or more MO surge arresters can be connected in parallel
in order to increase the energy handling capability if during an
application the energy occurring cannot be handled by a single
MO surge arrester. The requirement for equal current sharing,
and consequently even energy sharing, between the arresters
is the fact that the arresters have to have almost identical voltage current characteristics. In view of the extreme non-linearity
of MO resistors, small differences in the residual voltage in the
area of switching current impulses bring big differences in
current. With a nonlinearity coefficient of α ≈ 30 in the region
of switching current impulses on the voltage- current
characteristic, a difference of 5 percent in the residual voltage
would lead to a current sharing ratio of 1:4 between the surge
arresters. Therefore, it is absolutely necessary to perform a current sharing measurement on all MO surge arresters that are to
be intended to work in parallel. The manufacturer must be informed when the order is made if the user intends to intentionally connect more MO surge arresters in parallel. It is also to be
noted that the arresters
M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S |
time, the voltage is limited as much as possible. Thus, the
arrester in the station has to discharge only a small part of the
current, and at the same time protects the transformer against
overvoltages due to reflections.
In practice, this principle can be used by choosing two MO
surge arresters of the same type, such as MWK (class SL): the
arrester in the station has a continuous operating voltage Uc of
about 10 percent higher than the arrester outside on the pole.
The same result is reached if two MO surge arresters with the
same continuous operating voltage Uc but of different types are
installed, such as a MWK (class SL) on the pole and a POLIM-D
(class DH) in front of the transformer. Taking into consideration
the smaller cross-section of the MO resistors of the POLIM-D
compared to the MWK, its residual voltage characteristic lies
automatically higher than the one of the MWK.
In English-speaking countries, the arrester on the pole is
described as a “riser pole” arrester. This is not a type description for an arrester, but specifies the installation place, which is
the place where the cable rises up on the pole and where it is
connected with the overhead line.
Figure 30 shows an example. The MO surge arrester on the
pole directly at the cable bushing is, for example, an MWK 20
with Upl = 61.4 kV, and the arrester in the station is, for example,
a POLIM-D 20 with Upl = 70 kV. This coordination of residual
voltage and the energy handling capability makes it possible
that the larger amount of the current is discharged against the
earth on the exterior of the station. In case of an unfavorable
ground situation or in extremely lightning-endangered regions,
the installation of an earth wire for some span width in front of
the station is recommended.
are to be installed close to one another and are to be connected
together with short connections of low inductance. If this is not
taken into consideration, then separation effects may appear,
which lead to uneven current sharing and consequently to an
overstress of one of the arresters.
The parallel connection of MO surge arresters has, besides the
sharing of the current over more arresters, the positive effect of
a better (i.e. lower) protection level. This is because the current
density per arrester becomes lower in view of current sharing,
and consequently a lower residual voltage occurs.
It is to be strongly emphasized that it is always better to use a
MO surge arrester with a larger MO resistor diameter than to
connect more MO surge arresters in parallel with smaller MO
resistor diameters.
Earth wire
Phase wire
Cable bushing
Tower
Cable
Tower
Substation
10.3 Coordination of parallel-connected
MO surge arresters
In some cases, it is necessary or advantageous to use two
arresters in an installation separated from one another in space,
but electrically parallel on the same line. This is, for instance,
the situation when in view of the distances in a substation, one
of the arresters is installed at the entrance of the station and
another arrester is placed directly in front of the transformer,
at a certain distance. In such a case, two arresters of the same
type and with the same continuous voltage may be used.
In case of an incoming voltage, both arresters will discharge a
part of the current towards the earth and will provide very good
overvoltage protection. However, it is not to be assumed that
the energy occurred will be uniformly shared.
MO surge arresters of different types or of the same type with
different characteristics that are matched to one another are
used deliberately if uneven sharing of the energy absorption is
intended. This is the case, for example, in stations in which the
transformer is connected through a cable to the overhead line,
see Figure 30. An arrester is installed on the pole at the junction
of the overhead line to the cable, and this arrester has a higher
energy absorption capability and a lower residual voltage characteristic than the arrester in the station in front of the transformer. The effect of this is that the largest part of the energy is
absorbed by the arrester outside on the pole, and at the same
RE,M
30 Arrangement of two MO surge arresters
to protect a station with cable entry.
RE,M
51
RE,S
52
| M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S
11 Accessories
To meet worldwide all different installation and performance requirements a large number
of accessories is available.
M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S |
53
12 Monitoring of
MO surge arresters
The performance of modern MO surge arresters does not change under normal system
conditions over the whole life time, assuming correct application. Monitoring of events,
like surges due to lightning and/or switching, give valuable information about activities
in sub-stations.
11.1 Spark prevention unit
The spark prevention unit (SPU) is a device to avoid wildfire hazards caused by thermally overloaded surge arresters. The SPU
is installed in the earth connection of a medium-voltage arrester,
as shown in Figure 31.
The SPU monitors the load and thermal behavior of the surge
arrester and interrupts the current in case of overload. Comparing to other solutions, e.g. arc rotators, the concept of the SPU
prevents the spark production instead of controlling it. In this
way, violent arrester failures and related arcing, sparking or
emission of hot particles do not occur. The SPU is approved
for application with classes DH and SL arresters up to 36 kV
continuous operating voltage and that includes a trip indication
clearly visible from ground level.
The earth connection must be flexible, and it is necessary to
have sufficient distance beneath the arrester, so that the disconnected earth connection can hang freely and the applied
operating voltage that occurs at the foot of the arrester does
not lead to spark-over.
The purpose of disconnectors is to prevent overstressed
arresters from leading to a permanent short-circuit resulting
in the system switching off. It is thus possible to continue to
supply consumers with electrical energy. This is surely an
advantage in inaccessible areas or if the overstressed arrester
cannot be quickly replaced. The disadvantage is that there is no
overvoltage protection as long as the arrester is disconnected.
That is why it is important to replace arresters that are out of
order and were disconnected from the system as quickly as
possible.
If high-voltage fuses are installed in the same current path as
the disconnectors, the response characteristics of both protection devices have to be matched to one another. The disconnector has to respond in time before the fuse or at the same time
as it. This concept prevents the switching on an existing
short-circuit when a new fuse is installed.
11.3 Indicators
Indicators are devices that clearly indicate an overstressed arrester, i.e. a short-circuited arrester. Such devices are installed
either on the overvoltage side or on the earth side directly at
the arrester. In the event of an overstress, the short-circuit is
permanent and the system is switched off, but the damaged
arrester can clearly be detected and in this way can be quickly
replaced. Indicators are used in lines or stations with arresters
that cannot easily be visually controlled.
31 Medium-voltage arrester of class SL with spark prevention unit.
Left: SPU installed below surge arrester.
Right: SPU tripped after overload of surge arrester.
11.2 Disconnectors
Disconnectors are used for automatically disconnecting a
surge arrester that has been overstressed. Disconnectors are
generally placed on the earth side directly under the arrester.
11.4 Brackets, ground plates and
clamping devices
A variety of different installation material like brackets (hangers),
ground plates and clamping devices is available.
Different methods of diagnosis and indicators were discussed
and developed in the past for the condition monitoring of MO
surge arresters.
Surge counters can be installed if there is interest in monitoring
the occurrences of the discharges of an arrester in the system.
These surge counters count all discharges above the threshold
value of the surge counter. Some modern products classify the
current surges according their height. A milli-ampere-meter
(mA-meter) can be installed if the continuously flowing leakage
current of an MO surge arrester is to be monitored.
If monitoring devices are used, for example, for measuring the
continuous current that flows through an arrester, it is important
to watch the current tendency. The momentary value cannot
provide enough information about the condition of an arrester.
For this, it is necessary to make the first measurement directly
after the arrester installation and to record the conditions during
the measurement (voltage, ambient temperature, pollution of the
arrester housing, etc.)
Thermal vision cameras can be used to detect the temperature
of MO surge arresters. Here again, the absolute temperature is
not really important, but for instance differences in temperature
of arresters in the same sub-station, or a steady increasing
temperature over time.
Besides monitoring the MO surge arresters, the number and
magnitude of counted surges gives valuable information about
the events in a sub-station. It can also provide helpful statistics
for the performance, potential malfunction and stresses seen by
the system.
In high-voltage applications, especially for GIS arresters
monitoring devices like mA meters and surge counters are
frequently installed.
54
| M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S
M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S |
13 Overload and failure analysis
14 Summary and developments
The analysis of failed or overstressed MO surge arresters can give information about the
reason of the failure. However, the information received from failed arresters is rather vague,
because of secondary effects due to arcing.
Ongoing research and development in material, design and production technology
of MO surge arresters ensure reliable performance under very different and
specific applications.
The reliability of modern MO surge arresters is very high.
The probability of high-voltage arresters breaking down is
virtually zero. With medium-voltage arresters, it is approximately
0.1 percent throughout the world; however, there are considerable differences regionally.
Modern MO surge arresters with direct silicone molding are
to be found in a large number of varieties, covering every necessity. In recent decades, they have proved to be very reliable as
protection elements in the system.
The sealing system was the weak point in some older products
with porcelain housings. Humidity was able to enter the housing
after years of operation due to corrosion of the metal parts or
due to deterioration of the sealing rings, which eventually led
to the arrester breaking down.
For modern MO surge arresters direct-sealed with silicone,
there are only a few reasons for overstress. These include:
extreme lightning strokes in the line directly at the arrester
or unexpected high power-frequency overvoltages because
of earth failures, ferromagnetic resonances, or a short-circuit
between two systems with different system voltages.
As a rule, the MO surge arrester builds a permanent earth or
short-circuit in case of an overstress.
If an arrester breaks down in the system, it is, in principle possible to get an idea of the cause of the failure from the failure
mode. However, the information received from overstressed
arresters is rather vague, because it is generally not possible to
differentiate between the cause of the failure and the secondary
effects due to the arc. If an overload case is to be examined, the
following information should be available:
• All the lightning strikes that occurred close to the arrester
before the breakdown and, if possible, also the height of
the lightning current
• All the circuit breaker operations before the affected line
broke down
• The existing voltage at the arrester terminals before the
breakdown and, if possible, a recording of the voltages
• Any earth faults at other points in the affected system
• A line diagram of the line or the installation with the position
of the arrester before the breakdown
• Counting data of the surge counter, if any
• Ambient conditions at the time of the breakdown
If an arrester breaks down in a phase and it is replaced, the
other two arresters in the other phases should be also replaced,
or they should at least be examined to determine if they have
also been damaged. It is thus recommended that all three arresters be sent to the manufacturer for examination.
It bears mentioning that an MO surge arrester fulfils its
protection function even in a case of overloading. The voltage
decreases towards zero due to the fact that an earth or
short-circuit is produced, and in this way, the devices
connected in parallel to the arrester are protected against
excessively high voltages.
The protection that takes place in an overload case is deliberately used in some special cases as the last possibility to
protect very important and expensive electrical equipment.
If the aim is to overstress an MO surge arrester at a predetermined point – such as the exterior of a building – this arrester is
dimensioned to be deliberately weaker, from the voltage point of
view, than the other arresters in the installation. These so-called
“victim” or “sacrificing” arresters can be seen as an electrically
predetermined breaking point in the system.
They protect electrical equipment that is much more expensive
than themselves, and thereby guarantee high reliability and a
good energy supply. They act as insurance against breakdowns
in regard to high overvoltages. Integrated solutions are in
discussion, and corresponding installations, devices and concepts are being developed for systems that are becoming more
complicated. At the same time, the available space is shrinking.
This means that a device has to perform more functions under
certain circumstances. Correspondingly, an arrester could
perform, in addition to the function of overvoltage protection,
the function of a support insulator as well.
55
Therefore, it is necessary to continue developing and optimizing
MO surge arresters and all other electrical equipment. At the
same time, it is necessary to continuously revise the standards
and the application guidelines, because the requirements and
the tests are
changing, as well as the field experience is growing.
The application of MO surge arresters, MO resistors or MO
material in general (e.g. microvaristors) is increasing due to
new applications in DC systems, DC circuit breakers, and
power electronics.
Questions about lightning and overvoltage protection are
dealt with in different committees and working groups in
IEC (standardization and application recommendations),
Cigré and CIRED (field experience and trends).
56
| M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S
Acronyms/Abbreviations
M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S |
57
Literature
Consult the following literature for further information of the fundamentals of
MO surge arresters and specific applications:
AIS
Air insulated substation
ANSI
American National Standards Institute
BIL
Basic lightning insulation level (peak value).
Similar to the LIVW according to IEC. The term BIL is
used exclusively in US standards.
CENELEC
European Committee for Electrotechnical
Standardization
Cigré
International Council on Large Electric Systems
CIRED
International Conference on Electricity
Distribution
DH
Distribution High (arrester class)
DL
Distribution Low (arrester class)
DM
Distribution Medium (arrester class)
GIS
Gas insulated substation
IEC
International Electrotechnical Commission
MO
Metal-oxide
p.u.
Per unit, 1 p.u. = √2 ×Us /√3
RUSCD
Reference unified specific creepage distance
SC A3
Study committee A3 of Cigré, responsible for high-voltage Equipment.
SH
Station High (arrester class)
SiC
Silicon carbide
SIWV
Standard rated switching impulse withstand
voltage of an equipment or insulation
configuration (generally given in kV)
SL
Station Low (arrester class)
SM
Station Medium (arrester class)
SPU
Spark Prevention Unit
TC 37
Technical Committee 37 in IEC, responsible for surge arresters
U ps
Arrester switching protective level SIPL, i.e. the maximum
residual voltage of the arrester for the switching impulse
discharge current specified for its class.
Ur
Rated voltage of an arrester, i.e. maximum
permissible r.m.s. value of power-frequency voltage between its terminals at which it is designed to operate correctly under TOV conditions
(t = 10 s).
U ref
Reference voltage of an arrester, i.e. the
peak value of power-frequency voltage divided by √2
which is obtained when the reference current flows
through the arrester.
U res
Residual voltage of an arrester, i.e. the peak
value of voltage that appears between the
terminals of an arrester during the passage of
discharge current.
Us
Highest voltage of a system, i.e. highest value
of the phase-to-phase operating voltage (r.m.s. value) that
occurs under normal operating conditions in the system.
Varistor
Variable resistor
ZnO
Zinc-oxide
Cigré TB 60: Metal Oxide Arresters in AC Systems,
April 1991
by WG 06 of SC 33
Cigré TB 287: Protection of MV and LV Networks
against Lightning. Part 1: Common Topics
by CIGRE-CIRED JWG C4.402, 2006
Cigré TB 440: Use of Surge Arresters for Lightning
Protection of Transmission Lines
by CIGRE WG C4.301, 2010
ISBN: 978-2-85873-128-2
Cigré TB 441: Protection of MV and LV Networks
against Lightning. Part 2: Lightning Protection of
Medium Voltage Networks
by Cigré WG C4.4.02, 2010
ISBN. 978-2-85873-129-9
Cigré TB 455: Aspects for the Application of
Composite Insulators to High Voltage (≥ 72 kV)
Apparatus
by CIGRE WG A3.21, 2011
ISBN: 978-2-85873-144-2
Cigré TB 544: MO Surge Arresters Stresses and
Test Procedures
by CIGRE WG A3.17, 2013
ISBN: 978-2-85873-239-5
Cigré TB 549: Lightning Parameters for Engineering
Applications
by CIGRE WG C4.407, 2013
ISBN: 978-2-85873-244-9
TOV
Temporary overvoltage with power frequency of limited
time duration
Cigré TB 550: Protection of MV and LV Networks
against Lightning. Part 3: Lightning Protection of
Low Voltage Networks
by CIGRE WG C4.408, 2013
ISBN: 978-2-85873-245-6
Uc
Continuous operating voltage of an arrester,
i.e. the designated permissible r.m.s. value of power-frequency voltage that may be applied continuously between the arrester terminals.
Cigré TB 696: MO Surge Arresters – Metal Oxide
Resistors and Surge Arresters for Emerging System
Conditions
by CIGRE WG A3.25, 2017
ISBN: 978-2-85873-399-6
Lightning current impulse
8/20 current impulse with rise time of 8 μs and time to
half-value of 20 μs.
Um
Highest voltage for equipment, i.e. highest value of the
phase-to-phase voltage (r.m.s. value) for which the equipment is designed in respect of
its insulation.
Hinrichsen, Reinhard, Richter
(on behalf of Cigré WG A3.17)
Energy Handling Capability of High-Voltage
Metal-Oxide Surge Arresters – Part 1: A Critical
Review of the Standards
LIWV
Standard rated lightning impulse withstand voltage of an
equipment or insulation configuration (generally given in
kV).
Un
Nominal voltage of a system, i.e. a suitable
approximate value of voltage used to identify
a system.
Cigré SC A3 Technical Colloquium, Rio de Janeiro,
September 12/12, 2007
MCOV
Maximum continuous operating voltage (= Uc). Defined
and used in US standards.
U pl
Arrester lightning impulse protective level LIPL, i.e. the
maximum residual voltage of the arrester at the nominal
discharge current In.
IEEE
Institute of Electrical and Electronics Engineers
In
Nominal discharge current of an arrester, i.e. the peak
value of lightning current impulse which is used to classify an arrester.
k
Earth fault factor, k × Us /√3 is the maximum
voltage between phase and earth in case of an earth fault
Reinhard, Hinrichsen, Richter, Greuter
(on behalf of Cigré A3.17)
Energy Handling Capability of High-Voltage Metal-Oxide
Surge Arresters – Part 2: Results of a Research Test
Program
Cigré Session 2008, Paris, Report A3-309
Richter, Schmidt, Kannus, Lahti, Hinrichsen,
Neumann, Petrusch, Steinfeld
Long Term Performance of Polymer Housed MO surge
arresters
Cigré Session 2004, Paris, Report A3-110
Greuter, F., Perkins, R., Rossinelli,M., Schmückle, F.:
The metal-oxide resistor – at the heart of modern surge
arresters; Hitachi Energy Technik 1/89
W. Heiss, G. Balzer, O. Schmitt, B. Richter:
Surge Arresters for Cable Sheath Preventing Power
Losses in M.V. Systems.
CIRED 2001, Amsterdam, 18.-21. June 2001
M. Darveniza, L.R. Tumma, B. Richter, D.A. Roby:
Multipulse Lightning Currents and Metal-Oxide Arresters.
IEEE/PES Summer Meeting, 96 SM 398 PWRD, 1996.
W. Schmidt, J. Meppelink, B. Richter, K. Feser, L.
Kehl, D. Qiu:
Behavior of MO-Surge Arrester Blocks to Fast Transients.
IEEE Transactions on Power Delivery, Vol. 4, No 1,
January 1989.
Richter, B., Krause, C. and Meppelink, J.:
Measurement of the U-I characteristic of MO resistors
at current impulses of different wave shapes and peak
values.
Fifth Int. Sym. on High Voltage Engineering, Paper 82.03,
Braunschweig (Germany), 1987
W. Schmidt, B. Richter, G. Schett:
Metal oxide surge arresters for gas-insulated substations
(GIS) – Design requirements and applications;
CIGRE Paris Session 1992, Report 33-203
L. Gebhardt, B. Richter:
Surge arrester application of MV-Capacitor banks to
mitigate problems of switching restrike;
19 th International Conference on Electricity Distribution
(CIRED), Paper 0639, Vienna, 21-24 May 2007.
B. Richter
New Test Requirements for Distribution Arresters;
32nd International Conference on Lightning Protection
(ICLP), Shanghai, China
13th – 17th Oct. 2014
T. Christen, L. Donzel, and F. Greuter:
“Nonlinear resistive electric field grading part 1: Theory
and simulation,” IEEE Electr. Insul. Mag., vol.26, no. 6, pp.
48-60, Nov./Dec. 2010.
L. Donzel, F. Greuter, and T. Christen:
“Nonlinear resistive electric field grading part 2: Materials
and applications,” IEEE Electr. Insul. Mag., vol.27, no.2,
pp.18-29, March/April 2011.
IEC standards relevant for MO surge arresters
The selection describes the current state of the most
important IEC standards on MO surge arresters and
associated topics.
IEC 60099-4, Edition 3.0, 2014-06
Surge arresters – Part 4: Metal-oxide surge arresters
without gaps for a.c. systems
IEC 60099-5, Edition 3.0, 2018-01
Surge arresters – Part 5: Selection and application
recommendations
IEC 60099-6, Edition 1.0, 2002-08
Surge arresters – Part 6: Surge arresters containing both
series and parallel gapped structures – Rated 52 kV and
less
IEC 60099-8, Edition 1.0, 2011-01
Surge arresters – Part 8: Metal-oxide surge arresters with
external series gap (EGLA) for overhead transmission and
distribution lines of a.c. systems above 1 kV
IEC 60099-9, Edition 1.0, 2014-06
Surge arresters – Part 9: Metal-oxide surge arresters
without gaps for HVDC converter stations
IEC 60071-1, Edition 8.1, 2011-03
Insulation coordination – Part 1: Definitions, principles
and rules
IEC 60071-2, Edition 3.0, 1996-12
Insulation coordination – Part 2: Application guide
IEC 60060-1, Edition 3.0, 2010-09
High-voltage test techniques. Part 1: General definitions
and test requirements
IEC 60507, Edition 3.0, 2013-12
Artificial pollution tests on high-voltage ceramic and glass
insulators to be used on a.c. systems
IEC/TS 60815-1, Edition 1.0, 2008-10
Selection and dimensioning of high-voltage insulators
intended for use in polluted conditions – Part 1:
Definitions, information and general principles
IEC/TS 60815-2, Edition 1.0, 2008-10
Selection and dimensioning of high-voltage insulators
intended for use in polluted conditions – Part 2: Ceramic
and glass insulators for a.c. systems
IEC/TS 60815-3, Edition 1.0, 2008-10
Selection and dimensioning of high-voltage insulators
intended for use in polluted conditions – Part 3: Polymer
insulators for a.c. systems
IEC 60038, Edition 7.0, 2009-06
IEC standard voltages
US standards relevant to MO surge
arresters
IEEE C62.11 – 2012
IEEE Standard for Metal-Oxide Surge Arresters for
AC Power Circuits (> 1 kV)
IEEE C62-22 – 2009
IEEE Guide for the Application of Metal-Oxide Surge
Arresters for Alternating Current systems
58
| M E TA L- OX I D E S U R G E A R R E S T E R S I N M E D I U M -V O LTAG E S Y S T E M S A P P L I C AT I O N G U I D E L I N E S
Additional information
We reserve the right to make technical
changes or modify the contents of this
document without prior notice. With regard
to purchase orders, the agreed particulars
shall prevail. Hitachi Energy Ltd does not
accept any responsibility whatsoever for
potential errors or possible lack of information in this document.
We reserve all rights in this document
and in the subject matter and illustrations
contained therein. Any reproduction, disclosure to third parties or utilization of its
contents – in whole or in parts – is forbidden without prior written consent of
Hitachi Energy Ltd.
Hitachi Energy Switzerland Ltd.
PGHV, Division Surge Arresters
Jurastrasse 45
CH-5430 Wettingen /Switzerland
Tel. +41 58585 2911
Fax +41 58585 5570
Email: sales.sa@ch.abb.com
hitachienergy.com/offering/product-and-system/surge-arresters
© 2022 Hitachi Energy. All rights reserved.
1HC0075561 EN2 AC 31-05-2022