Metal-Oxide Surge Arresters in High-Voltage Transmission and Distribution Systems
Effective and reliable devices increasing system availability and reducing maintenance costs
Volker Hinrichsen, Siemens PTD, Berlin/Germany
Fundamentals
Surge arresters constitute an indispensable
aid to insulation coordination in electrical power
systems. Figure 1 makes this clear. There the
voltages which may appear in an electrical power
system are given in per-unit of the peak value of
the highest continuous line-to-earth voltage, depending on the duration of their appearance. The
voltage or overvoltage which can be reached
without the use of arresters, is a value of several
p.u. If instead, one considers the curve of the
withstand voltage of equipment insulation (here
equipment means electrical devices such as
5
Magnitude of (over-)voltage / p.u.
Abstract
Surge arresters protect equipment of transmission and distribution systems, worth several
magnitudes more than the arresters themselves,
from the effects of lightning and switching overvoltages. If properly designed and configured,
they are extremely reliable devices, able to offer
decades of service without causing any problems. This paper presents information about the
basic electrical characteristics and designs of
modern metal-oxide surge arresters. In addition
to the standard application – protection of power
transformers – examples are provided, in which
arresters help to reduce investment, repair and
maintenance costs. This benefit can be augmented when arresters are combined with other
equipment such as post insulators, disconnectors or earthing switches.
Possible voltages without arresters
Withstand voltage of equipment
4
3
2
1
Voltages limited by arresters
0
Lightning
overvoltages
(Microseconds)
Switching
overvoltages
(Milliseconds)
Temporary
overvoltages
(Seconds)
Time duration of (over-)voltage
Figure 1: Voltages and overvoltages in high-voltage
electrical power systems
power transformers) one notices that starting in
the range of switching overvoltages, and especially for lightning overvoltages, the equipment
insulation cannot withstand the occurring dielectric stresses. At this point, the arresters intervene. When in operation, it is certain that the
voltage that occurs at the terminal of the device while maintaining an adequate safety margin will stay below the withstand voltage. Arresters’
effect, therefore, involves lightning and switching
overvoltages.
Arresters installed today are all metal-oxide
(MO) arresters without gaps. The distinctive
feature of a MO-arrester is its extremely nonlinear voltage-current- or U-I-characteristic,
rendering unnecessary the disconnection of the
1200
1100
Peak value of voltage / kV
1000
Lightning impulse protection level = 823 kV
900
800
700
Peak value of rated voltage: 475 kV
600
500
400
Peak value of continuous operating voltage: 379 kV
300
Peak value of line-to-earth voltage: 343 kV
200
100
Leakage current: 100 µA
Nominal discharge current: 10 kA
0
10 -5
10 -4
10 -3
10 -2
10 -1
Highest voltage
of equipment
(Continuously)
1
10 1
10 2
10 3
10 4
10 5
Peak value of current / A
Figure 2: U-I-characteristic of a typical MO arrester in a system of Um = 420 kV
arrester from the line through serial spark-gaps,
as is found in the former gapped arresters with
SiC-resistors. In Figure 2, an example is shown
of the U-I-characteristic of a typical MO-arrester
connected between the conductor and the
ground in an effectively grounded 420-kVsystem. This arrester has a continuous operating
voltage (Uc) of 268 kV and a rated voltage (Ur) –
which characterizes the capability of the arrester
to deal with temporary overvoltages in the
system and can only be applied for a time period
of 10 seconds to 100 seconds – of 336 kV (these
values are r.m.s. values, while the U-Icharacteristic depicts them as peak values).
While a leakage current of about 100 µA flows
when the normal line-to-earth voltage is applied,
this arrester has a residual voltage of only 823
kV when a lightning impulse current of 10 kA –
the so-called nominal discharge current – is
impressed. This voltage is called the lightning impulse protection level of the arrester.
Equipment in the 420-kV-system normally has
a standard lightning impulse withstand level
(known as "BIL" in the IEEE standards) of 1425
kV. This (test voltage) value is not allowed to
ever be attained in practice. In accordance with
the IEC standards on insulation coordination [2]
[3], the highest occurring voltage in the case of a
non-self-restoring-insulation in operation should
stay below this value by a factor of 1.15, that is,
not exceed 1239 kV. Nevertheless, the lightning
impulse protection level of 823 kV offers more
than enough protection. It should, however, be
noted that this value represents a voltage across
the arrester terminals, caused by the flow of an
ideal standardized test current at the same level
as the arrester’s nominal discharge current.
Three significant causes can allow the voltage at
the terminals of the equipment to be protected to
take on a considerably higher value:
a) inductive voltage drops;
b) discharge currents higher than the nominal
discharge current;
c) separation effects by traveling wave processes between the terminals of the arrester
and of the equipment to be protected.
Especially the latter phenomenon has to be
taken into account when planning the optimal
location of an arrester (for detailed information
see [4]). It is the main reason for the limited protection zone of arresters, which is in the range of
5 m (Um = 24 kV, compensated neutral) to 60 m
(Um = 420 kV, effectively earthed). Arresters
should therefore be installed as close as possible
to the equipment to be protected. As a rule of
thumb, an arrester of a lightning impulse protection level equal to the standard lightning impulse
withstand voltage of the equipment to be protected, divided by 1.4, results in comfortable
safety margins if at the same time the aforementioned distances are not exceeded. Such an
arrester will act as an extremely reliable and
economical device for protecting precious
equipment like the big power transformer shown
in Figure 3.
Figure 3: Generator transformer (VEAG, Germany,
Um = 420 kV) protected by three surge arresters
Construction of modern MO surge arresters
During the past twenty years surge arrester
design and application has been dominated by
two major changes in technology. The first one,
introduction of the gapless metal-oxide arresters
in the late seventies and early eighties of the last
century, has considerably improved the protection characteristics and the reliability (reported
failure rates of metal-oxide arresters in transmission systems are close to zero), while at the
same time the construction, compared with that
of gapped SiC-arresters, has become less complicated and less prone to mechanical or dielectric defects. The next major step involved using
polymeric materials for the housings, starting in
the late eighties. For no other device within the
high-voltage transmission and distribution than
for surge arresters has the change to polymer
housings been so consistently carried out, and in
the distribution systems, for instance, porcelain
housed arresters are virtually no longer being
installed.
In the case of the conventional porcelain
housing, different properties – such as protection
from environmental impact and provision of sufficient creepage distance on the one hand and
mechanical strength on the other are united in a
single component. In an arrester with polymer
housing, however, these properties are apportioned to two different components. Mechanical
strength is almost always achieved with fiberglass-reinforced plastic (FRP) materials. In the
example shown in Figure 4, several rods serve
this purpose. They are strained in the aluminum
end fittings and enclose the MO-resistor stack.
This is how a mechanical high-strength unit out
of MO-resistors, end fittings and the FRP structure are created. This module is inserted in a
mold, in which silicone rubber is directly injected.
Thus it is possible to obtain a perfect bond of the
silicone rubber with the other components, voidfree and permanent. Similarly, in case of an arrester overload, which is an extremely rare event
but nevertheless has to be considered, a pressure buildup and the related risk of violent housing breakage has been avoided.
Flange with vent
Seal
Pressure relief diaphragm
Compression spring
Metal oxide resistors
Composite hollow insulator
(FRP tube / silicone rubber sheds)
Figure 5: Modern high-voltage arrester with housing
made of a composite hollow insulator
important being the possibility to design highvoltage arresters which are so mechanically
strong, that they can endure the strongest earthquakes intact and at the same time be used as a
post insulator in a substation (Figure 6).
Figure 4: Modern distribution arrester with
directly molded silicone rubber housing (left:
complete arrester, right: internal design)
An advantage of the applied silicone rubber in
this case, in comparison to cheaper materials,
are the excellent long-term properties. Another
advantage is the characteristic unique to silicone
rubber, hydrophobicity: even if the silicone surface is very dirty, water simply drips off. This
suppresses the formation of conductive layers
and advantageously affects the operational performance of the arrester in polluted conditions.
While this design principle constitutes the
most economical way to produce an arrester for
distribution or lower transmission voltages, it has
its technical limitations for higher system voltages. The possible length of the individual units
is limited to about one meter and the achievable
mechanical cantilever strength to values of about
5 kNm. Here another concept, as shown in Figure 5, has proven to be advantageous. In principle, this has the same design as a conventional
porcelain housed arrester. Indeed, essentially
only the porcelain insulator has been replaced
with a composite-hollow insulator. The composite-hollow insulator is made up of an FRPtube on which the silicone rubber sheds are directly molded on. This design principle offers
some considerable advantages for applications
up to the highest voltage levels, one of the most
Figure 6: Polymer housed arrester (Um = 550 kV)
during seismic testing on a shaking table
Another property is only found in this design:
in the case of an arrester overload it is certain
that with this construction a housing breakage
will never occur, not even any of the inner parts
are ejected. The tube will remain almost completely intact, and as a result it offers the best
possible safety for the whole switchgear.
It must be mentioned, however, that at these
system voltage levels the conventional porcelain
housed MO arresters, not explained here in further detail, are still pre-dominant.
Another principal design of arresters, metalenclosed gas-insulated arresters, which are of
great importance in protecting GIS, cannot be
dealt with here either.
Application examples
Protection of transformers is without a doubt
the most common application of surge arresters.
There are, however, many other fields where
arresters protect different kind of equipment from
the effect of overvoltages and thus help to improve power supply quality and reduce maintenance costs in the networks. It should always be
kept in mind here that in most cases the purchase price of the arresters is in the range of
only 1% or less of the equipment they protect.
Some more selected application examples are
given below. Partly they have become possible
only after introducing polymeric housings, which
allow arresters to operate even in areas of public
access (safety aspect!) and in any mounted position (such as horizontal or suspended).
- Protection of cables: cables are exposed to
lightning overvoltages if they are directly connected to overhead lines. This is the case particularly in distribution systems. Distribution overhead lines are normally not protected by shield
wires, so direct lightning strikes into the phase
conductors are quite common. Protection of the
cables directly at the cable terminations is
therefore a must. Figure 7 shows that this is,
however, not limited to distribution voltages.
breaking porcelain housing in case of an arrester
overload has prevented application of surge arresters on locomotives for a long time. Modern
breaking resistant designs of polymer housed
arresters have overcome this problem and offer
economical protection against damages to the
expensive locomotives (Figure 8).
Figure 8: Polymer housed arrester for protection of
electric locomotives (High speed train "ICE",
Deutsche Bahn, 15 kV/16.7 Hz)
- Protection of series capacitors: long a.c.
transmission lines require capacitive compensation of the line inductance. Series capacitor
banks installed along the line at distances of
several hundred kilometers are an effective
means of improving system stability and transmission capacity. To protect the series
capacitors from the effect of a.c. overvoltage in
case of a line fault current, arrester banks are
connected in parallel, which carry most parts of
the fault current while limiting the voltage across
the capacitors to undangerous values. By
connecting many (up to 100) MO-columns in
parallel, distributed to several housings (Figure
9), it is possible to increase the energy
Figure 9: Arrester bank for protection of a series
capacitor (Um = 550 kV, Eletronorte, Brazil)
Figure 7: 161-kV-tower with 6 cable terminations,
protected by suspended polymer housed surge
arresters (IEC, Israel)
- Protection of traction systems: damages of
electric locomotives frequently occur if no overvoltage protection is provided. The risk of a
absorption capability of the arrester bank to extreme values [5] [6]. Compared with other alternatives, this has turned out to be a good compromise between necessary investment and the
achievable degree of availability of the line.
- Protection of thyristors in HVDC applications: HVDC transmission lines and back-to-back
stations are gaining importance worldwide to exchange electric energy over long distances by
overhead lines or (sea-) cables, to interconnect
different grids or to supply concentrated load
centers. The main components of the converters
are thyristors, which have achieved a high degree of technical performance and reliability, but
are nevertheless expensive devices. Among the
different other locations within an HVDC converter station where surge arresters are indispensable [7] the valve protection arresters, which
protect the thyristors from overvoltages, play an
outstanding role. The maximum blocking voltage
of modern thyristors is about 8 kV, thus requiring
many of them to be connected in series in order
to handle transmission voltages up to ± 500 kV.
Arresters of extremely reduced protection levels
parallel to the valve towers allow the number of
thyristors in series to be reduced and thereby
help to considerably decrease the overall cost of
the converter (Figure 10).
Figure 10: Suspended valve tower with polymer
housed valve protection arresters (LADWP,
California/USA, ± 500 kV dc)
- Line arresters: distribution lines are normally
unshielded lines, thus making direct lightning
strikes with consequential power supply interruptions a comparatively likely event. But even
shielded transmission lines may be affected by
lightning, either by direct strikes to the line conductors in case of shielding failures, or by back
flashovers after a lightning strike into the tower or
the overhead ground wire. In an increasingly
competitive environment due to deregulation
there is growing interest in reducing outage time
and improving power supply quality. However,
later installation of (additional) overhead ground
wires, improvement of the footing resistance of
the towers in order to decrease the probability of
back flashovers, or other means, are not
possible in many cases for technical reasons,
and besides this, they are always very expensive. Here installation of surge arresters parallel
to the line insulators has turned out to be a costeffective alternative. There are many options for
optimizing the necessary investment against
power supply quality by protecting only part of
the towers, part of the phases, or by choosing
inexpensive arresters of low energy absorption
capability based on an arrester failure risk analysis [8] [9].
Improved benefit by combined arresters
As mentioned before, arresters – comparatively cheap devices within the electric power
supply system – are able to reliably protect
equipment worth several magnitudes more than
the arresters themselves. They can, however, be
used even more effectively when combined with
other devices. Two examples are given below.
- Arresters as post insulators: Normally surge
arresters are not used as post insulators. In the
very rare (but nevertheless possible) event of an
arrester failure the arrester housing may break
and completely lose its mechanical integrity. On
the other hand, achieving this additional function
is a favorable goal for economical as well as for
technical reasons. A post insulator, including a
pedestal and the required foundation work, can
easily create costs of several thousand Euros. A
simple calculation can show that drastic cost
reductions may be achieved when using arresters as post insulators. Saving space is another economical point. From a technical point of
view, omitting post insulators helps to reduce the
length of lines and bus bars in a substation,
which may improve the lightning overvoltage
protection of the equipment, as distances between the arrester and the equipment to be protected become shorter. The extremely low failure
rates of MO resistors over the past twenty years,
together with the availability of mechanical highstrength, breaking resistant polymer housed
arresters as depicted in Figure 5, have made
Figure 11: Polymer housed arresters used as post
insulators (EnBW, Germany, Um = 420 kV)
application of surge arresters as post insulators
possible [10]. An example is shown in Figure 11.
- Arresters integrated into disconnectors and
earthing switches: Arresters installed at the line
entrance of a substation can effectively protect
circuit breakers and instrument transformers
from the effects of nearby direct lightning strikes
into the overhead line conductors. While even for
new substations the additional space requirements are a serious problem, later installation of
surge arresters at the line entrance of already
existing substations is nearly impossible in most
cases. This problem can be overcome by integrating the arresters into the line side disconnectors, as shown in the left part of Figure 12.
The introduction of gapless MO arresters in the
late seventies and early eighties of the last century has remarkably improved the protection
characteristics, reliability and the ease of application in general. The use of polymer housings,
beginning in the late eighties, have contributed to
safety, thus opening new fields of application, for
instance in areas of public access, or where
there are extreme requirements on mechanical
strength. Countless applications exist in which
modern arresters help to reduce investment,
repair and maintenance costs in systems of
electric power supply. This benefit can be further
increased if arresters are combined with other
devices, such as post insulators, disconnectors
or earthing switches – solutions which have been
supported by the users' increasing confidence in
arrester performance and the possibilities offered
by modern arrester technology.
References
[1] IEC 60099-4, Edition 1.1, 1998-08
Surge arresters – Part 4: Metal-oxide surge arresters without
gaps for a.c. systems
[2] IEC 60071-1, Seventh edition, 1993-12
Insulation co-ordination – Part 1: Definitions, principles and
rules
[3] IEC 60071-2, Third edition, 1996-12
Insulation co-ordination – Part 2: Application guide
Figure 12: Arrester integrated in a 420-kV-disconnector (left) and in a 245-kV-earthing switch (right)
(RWE Net, Germany)
One column of a two-column rotating disconnector has been replaced by a surge arrester,
requiring only a modification of the arrester's
grading ring. This solution has been realized for
Um = 420 kV and 245 kV and is incidentally a
good example of an installation of surge arresters which has not only been the most economical but in fact virtually the only possible way
to solve existing problems with switchgear failure
caused by lightning overvoltage phenomena [11].
The picture on the right in Figure 12 shows a
similar arrangement of an earthing switch in
which the insulator column has been replaced by
an arrester.
Conclusion
Surge arresters protect equipment of transmission and distribution systems, worth several
magnitudes more than the arresters themselves,
from the effects of lightning and switching overvoltages. If properly designed and configured,
they are extremely reliable devices, offering decades of service without causing any problems.
[4] IEC 60099-5, First edition, 1996-02
Surge arresters – Part 5: Selection and application recommendations
[5] Renz, Hinrichsen
Quite at Home at –50 °C
EV Report 1/95, pp. 10 – 13
[6] Mainville, Riffon, Rollin, Hinrichsen
Pressure Relief Tests on Varistors for the Series Compensation Banks installed at the Montagnais Substation
IEEE/PES 1993 Summer Meeting, Vancouver/Canada, paper
93 SM 385-5 PWRD
[7] IEC 28/139/CDV 2001-02-09
IEC 60071-5: Insulation co-ordination – Part 5: Procedures
for HVDC Converter Stations
[8] CIGRÉ Working Group 33.11 Task Force 03
Application of Metal Oxide Surge Arresters to Overhead
Lines
Électra No. 186, October 1999, pp. 83 - 112
[9] Tarasiewicz, Rimmer, Morched
Transmission Line Arrester Energy, Cost, and Risk of Failure
Analysis for Partially Shielded Transmission Lines
IEEE Transactions on Power Delivery, Vol.15, No.3, July
2000, pp. 919 - 924
[10] Hinrichsen, Fien, Solbach, Priebe
Metal Oxide Surge Arresters with Composite Hollow Insulators for High-Voltage Systems
CIGRÉ conference Paris 1994, paper 33-203
[11] Hinrichsen, Göhler, Lipken, Breilmann
Economical Overvoltage Protection by Metal-Oxide Surge
Arresters Integrated in High-Voltage AIS Disconnectors –
Substation Integration, Design and Test Experience
CIGRÉ conference Paris 2000, paper 33-104