Manufacturing and Application
of Cage Design High Voltage Metal Oxide Surge Arresters
K. Steinfeld, B. Kruska and W. Welsch
Siemens Surge Arresters and Limiters, Berlin, Germany
Abstract: Metal oxide surge arresters protect electric
power systems from overvoltages. The conventional
design of high voltage metal oxide surge arresters
includes a stack of metal oxide non-linear resistors
and a housing, either a porcelain hollow core insulator
or a polymer hollow compound insulator (tube
design). However, the technological progress in the
polymer industry makes designs possible which are
simpler and more cost effective. As an alternative to
the above mentioned tube design, the active part of the
arrester may be directly covered by silicone rubber.
The cage design introduced here consists of a stack of
metal oxide elements which are kept in between the
aluminium end fittings under high mechanical
pressure by pre-stressed fibre reinforced plastic rods.
The silicone rubber sheds are then directly moulded
onto the construction. Due to their cost effectiveness,
cage design surge arresters are advantageous over tube
design arresters for standard mechanical requirements.
1.
Introduction
Electric power systems are exposed to overvoltages of
different origin which might endanger the equipment
such as transformers, instrument transformers or
circuit breakers, only to name a few. Fig. 1
summarises the different kinds of overvoltages as a
function of their duration in comparison to the highest
voltage for equipment.
Lightning strokes into the electric power system or
its vicinity lead to lightning overvoltages in the range
of microseconds, switching action within the system
cause switching overvoltages in the range of
milliseconds and certain operating conditions due to
load flow control cause temporary overvoltages which
can last for several seconds.
As can be seen from Figure 1, the equipment is
designed to withstand the highest voltage for
equipment as well as temporary overvoltages.
However, the insulation of the equipment is not
capable of withstanding lightning and switching
overvoltages. Without countermeasures, occurrences
of these overvoltages in the system can lead to
breakdown of the equipment insulation and its failure.
In order to protect electric power system
equipment from lightning and switching overvoltages,
surge arresters are used within the system as a tool for
insulation co-ordination.
Figure 1: Overvoltages in high voltage electric power
systems compared to the highest voltage for equipment
The purpose of using a surge arrester is to always
limit the voltage across the terminals of the equipment
to be protected below its insulation withstand voltage.
This is achieved by connecting elements with an
extremely non linear voltage current characteristic
(varistor) in parallel to the terminals of the equipment.
So called metal oxide (MO) surge arresters containing
ceramic MO elements mainly made from zinc oxide
(ZnO) and bismuth oxide are used nowadays [1]. Due
to the high non linearity of the material there is no
need for series spark gaps any more as they were used
in silicon carbide (SiC) surge arresters.
2.
Design of High Voltage Surge Arresters
In the past twenty years there were two major changes
in the technology of surge arresters. Firstly, the
gapped SiC arrester technology was replaced by the
gapless metal oxide (MO) arrester technology in the
late seventies to early eighties. As a consequence, the
protection characteristic was improved, the reliability
was dramatically increased to failure rates close to
zero and the design became much simpler.
In the late eighties to early nineties, polymeric
housings were introduced using fibre reinforced
plastic (FRP) tubes with sheds made from silicone
rubber or ethylene propylene diene copolymer
(EPDM) [2]. Since then, porcelain housings have
almost been fully replaced by polymer housings in the
medium voltage distribution systems for new
installations and polymer technology is increasingly
used in high voltage power systems even up to
800 kV. The success of polymer housings lies in the
1
versatility of the possible designs and properties
which allow a wide range of arresters with respect to
mechanical properties, short circuit behaviour and
costs.
An MO high voltage surge arrester basically consists
of a stack of cylindrical MO elements kept together by
a supporting structure and a housing. The general
purpose of the housing is to
· protect the MO elements from environmental
impacts such as humidity and pollution as well as
damages due to transport,
· carry external forces, e.g. by conductor wires,
wind or earthquake,
· control the pressure relief behaviour in case of
electrically overloading the arrester,
· provide a dielectric strength (withstand voltage)
above the protection level of the arrester and to
· keep the stack of MO elements together by
maintaining a certain pressure within the stack.
There are different approaches to categorise the
various designs of MO surge arresters. Basically,
designs with and without internal gas volume can be
distinguished. The former includes both porcelain and
polymer tube design and the latter includes cage and
wrapped design.
In the following, the different above mentioned
designs of MO surge arresters are explained and
compared to each other.
2.1.
Tube Design MO High Voltage Surge
Arresters
As indicated in Figure 2 and Figure 3, a tube design
MO surge arrester includes a hollow core housing
either of porcelain or polymer. A stack of MO
elements forms the electrically active part and flanges
including sealing and pressure relief system form the
top and bottom of the arrester. The housing is the
mechanically supporting part which carries the
mechanical forces.
The design of porcelain and polymer tube design
surge arresters is almost identical, the only difference
is the housing which in case of the polymer housing
consists of an FRP tube with polymer sheds directly
moulded onto it. Usually, silicone rubber is used as
shed material rather than EPDM due to its excellent
chemical resistance and hydrophobicity [3]. The
mechanical resistance of the FRP tube material is
significantly better than of porcelain resulting in
higher cantilever strength and headload of the housing
with a much lower weight at the same time.
Furthermore, the mechanical properties may be
adjusted according to the customer’s needs by varying
the parameters of the FRP tube such as wall thickness
or fibre angle. Thus, polymer tube design arresters are
used where there are highest mechanical requirements,
particularly for stations in areas with seismic activity.
2
Figure 2: Tube design MO surge arrester with porcelain
housing (flange section only)
Figure 3: Tube design MO surge arrester with polymer
housing (flange section only)
A very important part of tube design MO surge
arresters with respect to safety and reliability is the
sealing and pressure relief system. The sealing system
must be designed to prevent ingress of moisture for
the whole lifetime. This can only be achieved by an
appropriate but simple construction and a very careful
choice and combination of the materials used. In case
of an arrester failure and internal short circuit, the
pressure relief system must release the pressure inside
the housing which is caused by the arc heat before the
housing is violently destroyed by the pressure shock
wave. Furthermore, the pressure relief forces the arc
out of the housing to prevent from further pressure
built-up and burning of the internal parts and housing.
In this context, it must be pointed out that failure of
an arrester is a very rare event. Gapped SiC surge
arresters failed quite frequently and it is generally
recommended to replace them by modern MO surge
arresters [4], the failure rates of which are by far lower
and appear to be comparable with other equipment
such as transformers. In most cases failures were
caused by deficiency of the sealing system due to
transportation damage or stressing the arrester above
its specification, e.g. by direct lightning strokes with
extremely high currents or by voltage transfer.
Steinfeld, K., Kruska, B. & Welsch, W.
A porcelain housing is designed to withstand the
internal pressure and arc heat and remain
mechanically undamaged during flow of short circuit
current and pressure relief. However, occasionally the
arrester housing will collapse due to thermal
mechanical stresses within the housing which are
caused by the arc heat on the surface of the porcelain.
As shown in Figure 4, the effect is that the arrester
falls into pieces non-violently neither endangering the
surrounding substation equipment nor personnel in its
vicinity. Although such a result is considered
successfully passed in a short circuit test according to
IEC 60099-4 [5], it implies that porcelain arresters
must not be used as supporting insulator but a post
insulator requiring additional space must be installed.
As opposed to porcelain, the FRP tube material
does not show a secondary thermal break due to its
thermal-mechanical properties. Even after short circuit
and pressure relief the remaining mechanical strength
of the FRP tube is at least 75% of the initial value.
Figure 5 shows a tube design polymer surge arrester
after a short circuit test.
Only thermal decomposition of the silicone rubber
sheds occurred leaving silicone dioxide (sand) on the
surface, but there was no burning and the arrester
housing has kept its full mechanical strength. Thus,
tube design polymer housing surge arresters may be
used in a double function as arrester and post insulator
saving space in the substation [6].
2.2.
Cage Design MO High Voltage Surge
Arresters
Instead of using a mechanically supporting
porcelain or FRP tube to accommodate the stack of
MO elements, the MO elements themselves can be
used as mechanically supporting part. As shown in
Figure 6, this can be achieved by clamping them in
between the end fittings using a cage of FRP rods,
which contributes to the name of this design. The
silicone rubber insulation is then moulded directly
onto the MO elements without any internal gas
volume left.
Figure 6: Cage design surge arrester with polymer sheds
directly moulded onto MO elements and FRP rods
Figure 4: Secondary thermal break of a porcelain arrester
after short circuit test
Figure 5: Tube design polymer surge arrester before (left)
and after (right) short circuit test
Due to the high compressive strength of the MO
elements (about 500 MPa) and the high tensile
strength of the FRP rods (700 to 1000 MPa) arresters
with high mechanical resistance can be manufactured
with this design principle. To obtain a sufficient
mechanical resistance in terms of cantilever strength
or headload a pre-stress of about 100 kN typically is
applied which is however far from utilising the above
mentioned strength of either MO elements or FRP
rods.
There are some specific advantages of cage design
surge arresters over arresters with other design
principles:
Cage design surge arresters can be manufactured
more cost effective as compared to tube design surge
arresters due to the simple construction and the use of
a comparatively little amount of material.
Manufacturing and application of cage design high voltage metal oxide surge arresters
3
As opposed to tube design surge arresters there is
no pressure relief and thus no sealing system needed.
The MO elements are safely kept in place by the
cage together with the high compressive force and
they are embedded into the silicone rubber. Thus, the
active part is almost perfectly protected from
mechanical impact resulting in a high transport safety.
Furthermore, cage design surge arresters are of
comparatively low weight and thus easy to transport,
handle and install.
Since there is a direct contact of the MO elements
with the polymer material, heat produced by the MO
elements is more easily dissipated through the housing
into the environment as compared to tube design surge
arrester. This increases thermal stability and allows to
utilise more the MO elements with respect to electrical
stress.
If silicone rubber is used as the housing material,
the only combustible material is the 30% epoxi resin
fraction of the FRP rods, which results in a high safety
considering the burning behaviour.
When there is light, there must be shadow. Of
course, cage design surge arresters have their limits
particularly if compared to tube design surge arresters:
Among arresters without internal gas volume, cage
design surge arresters provide the highest mechanical
resistance, in terms of static cantilever strength
2,8 kNm is a typical value. In comparison, tube design
surge arresters with a static cantilever strength of as
much as 50 kNm are available. Thus, cage design
surge arresters can not be used if standard mechanical
requirements are exceeded. As a consequence of the
mechanical properties, cage design surge arresters do
have a comparatively large, clearly visible deflection
when stressed with the specified headloads.
The mechanical performance of cage design surge
arresters relies on the internal pre-stress and thus on an
intact cage: Since rods may be broken in case of
failure after overloading the arrester, there is a certain
risk of a collapse of the arrester. Consequently, as
opposed to tube design surge arresters, cage design
surge arresters can not be used as post insulators.
2.3.
Wrapped Design MO Surge Arresters
Another possible surge arrester design which is
shown in Figure 7 is the so-called wrapped design.
Hereby, a fibre glass cloth impregnated with epoxi
resin is wrapped around the stack of MO elements
including the end fittings. After polymerisation of the
epoxi resin, the housing material which forms sheds
and insulation is moulded directly onto the wrap.
Alternatively, pre-manufactured sheds may be slipped
over the wrap. This comparatively simple design
allows to manufacture cheap arresters particularly for
medium voltage systems where mainly costs are
decisive and reliability plays a less important role as
compared to high voltage systems.
4
Figure 7: Wrapped design MO surge arrester with polymer
sheds directly moulded onto FRP layer
Consequently, tube design porcelain arresters have
almost totally disappeared from the medium voltage
distribution systems for new installations and were
replaced by polymer surge arrester either of cage or
wrapped design.
The FRP cloth forms a mechanically stable
enclosure around the MO elements. In case of an
arrester failure, the pressure built-up by the arc
burning inside this enclosure is limited by its
mechanical resistance. If the wrap thickness is chosen
too large, a dangerous pressure built-up may occur,
violently shattering the housing, whereas a thin wrap
results in low mechanical resistance towards external
forces. Thus, the thickness has to be balanced between
safe pressure relief behaviour and mechanical
resistance. As indicated in Table 1, this compromise
usually results in a lower mechanical strength as
compared to the tube or cage design. Other measures
against violent shattering such as providing pressure
relief slots along the FRP wrap weaken the
mechanical resistance, too.
Occasionally, medium voltage surge arresters are
arranged in series and parallel connection to form a
high voltage surge arrester. By connecting in series,
the required rated voltage is achieved and connecting
in parallel at the same time results in the necessary
energy absorption capability and mechanical stability.
Since low cost medium voltage surge arresters can be
used for this arrangement, cost effective high voltage
surge arresters can be produced this way. However,
this design bears certain limitations and risks:
The mechanical resistance is very low resulting in
only little cantilever strength and headload.
Without careful adjustment of the residual voltages
of the arresters in parallel, the current distribution will
be non-uniform. This may lead to electrical and
thermal overloading of the arrester with the lowest
residual voltage even if the total current is within the
specifications.
The series connection of medium voltage surge
arresters includes conductive end fittings or flanges in
Steinfeld, K., Kruska, B. & Welsch, W.
between the insulating housing material. In case of
pollution layers on single arrester units which might
occur e.g. on the lower units in the morning hours due
to dew close to the ground, surface currents occur
flowing over the housing via the metallic parts. This
results in a significant field distortion which might
cause thermal runaway and failure. This phenomenon
generally is a concern for multi unit surge arrester and
is the reason why the unit length should be as long as
possible.
Figure 8 shows an example for a wrapped design
medium voltage surge arrester after failure and short
circuit. As can be seen, a collapse of the housing must
be taken into consideration when applying this type of
surge arresters to high voltage power systems.
Figure 8: Medium voltage wrapped design MO surge
arrester after short circuit
3.
Manufacturing of Cage Design
MO Surge Arresters
As explained in chapter 2.2, the mechanical resistance
of cage design MO surge arresters results from prestressing both the FRP rods and the MO elements with
considerable force. Two techniques to achieve a
sufficient pressure on the MO elements are possible
depending on the mechanical requirements:
· For lower mechanical resistance of the arrester a
lower pre-stress is required. In this case a spring
can be used on top or bottom of the stack of MO
elements.
· For higher mechanical resistance of the arrester a
higher pre-stress is required. Here, the FRP rods
are stretched for some millimetres e.g. using a
hydraulic machine. The gap occurring within the
stack of MO elements is then filled e.g. by spacers
or a threaded bolt. Thus, the cage of FRP rods
itself acts as a spring keeping the mechanical
force.
The self-supporting, pre-stressed arrangement of MO
elements, end fittings and cage of FRP rods, the socalled active part, is then carefully cleaned by using a
solvent. To ensure a sufficient bonding of the different
materials of the active part (glass surface of the MO
elements, aluminium and FRP) to the silicon rubber, it
is treated with a primer. The active part is then prewarmed in an oven in order to achieve the necessary
thermal conditions for the silicone rubber moulding
and polymerisation process as well as a short process
time. After inserting the primered, pre-warmed active
part into the mould, the housing is produced by
silicone rubber injection moulding. The final step of
the manufacturing process is to clean the arrester from
flash.
4.
Application of Cage Design
MO Surge Arresters
There are basically two typical applications for cage
design surge arresters, the use as a station surge
arrester in regular upright installation and the
suspended installation.
The mechanical limits of cage design surge
arresters mentioned in chapter 2.2 do restrict their use
as station surge arresters to a system voltage (highest
voltage for equipment, Um) of about 300 kV. Above
this voltage, due to the required length of the arrester
there is only very little permissible headload left
which will not be sufficient in many cases. In
addition, there will be a significant deflection at
specified headload which might lead to insufficient
clearance between phases. Above 300 kV, only the
tube design seems to safely provide the necessary
mechanical resistance.
These restrictions do not apply to suspended
installation of a cage design surge arrester either in
substations or in the power line where the arrester can
be used as a so-called transmission line surge arrester
(TLSA) [7]. In this case there is little or no cantilever
force acting on the arrester and the cage design can be
used up to the highest system voltage levels.
The application of a cage design surge arrester as
TLSA can be seen in Figure 9.
Figure 9: Cage design surge arrester used as transmission
line surge arrester (TLSA) in a high voltage overhead power line
Manufacturing and application of cage design high voltage metal oxide surge arresters
5
5.
Conclusion
There are different design alternatives for high
voltage MO surge arresters with polymer housing
which include tube design (either porcelain or polymer
housing) and cage design. The wrapped design is
restricted to medium voltage applications due to its
limited mechanical resistance. Arrangements of cage
or wrapped design medium voltage surge arresters in
parallel and series connection to form a high voltage
surge arrester are the exception rather than the rule,
since this solution provides only limited mechanical,
electrical and pollution performance. Both tube design
and cage design MO surge arresters provide safe
pressure relief behaviour, but there is a significant
difference in mechanical resistance in terms of
cantilever strength, headload and deflection. Typical
values for the different designs regarding an arrester
for 170 kV system voltage as an example are
summarised in Table 1.
Table 1:
Typical mechanical properties of different high
voltage polymer surge arrester designs
Design
Property
System voltage
in kV
Static cantilever
strength in kNm
Length
in m
Headload
in kN
Deflection
in mm
6.
References
[1]
Einzinger R.,
“Metal Oxide Varistors”,
Annual Review Materials Science, 1987.17, pp. 299-321
[2]
V. Hinrichsen, Fien H., Solbach H.B., Priebe J.,
“Metal Oxide Surge Arresters with Composite Hollow
Insulators for High-Voltage Systems”,
CIGRÉ 1994 Session, 28 August - 3 September,
Paris, France, paper 33-203
[3]
Gubanski M.,
“Wettability of Naturally Aged Silicone and EPDM
Composite Insulators”,
IEEE Transactions on Power Delivery,
Vol. 5, No. 3, July 1990, pp. 1527-1535
[4]
M. Darveniza, D.R. Mercer, R.M. Watson,
“An Assessment of the Reliability of In-Service Gapped
Silicon-Carbide Distribution Surge Arresters”,
IEEE Transactions on Power Delivery,
Vol. 11, No. 4, October 1996, pp. 1789-1797
[5]
IEC 60099-4, Edition 1.2, 2001-12, Surge Arresters,
Part 4: MO Surge Arresters Without Gaps for AC Systems
[6]
K. Steinfeld, R. Göhler,
“Rating and Design of Metal-Oxide Surge Arresters for High
Voltage AC Systems”,
PowerCon 2002, Kunming, China October 13-17, 2002
[7]
E. Tarasiewicz, F. Rimmer, A. 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
Tube
design
170
Cage
design
170
Wrap’d
design
170
14,7
2,8
0,8
1,76
1,77
1,56
Dr. Kai Steinfeld,
Director R&D
8,4
1,6
0,5
44
173
n.a.
Siemens PTD H 42,
Nonnendammallee 104,
D-13629 Berlin
Cage design high voltage polymer surge arresters
are appropriate for system voltages below 300 kV and
standard mechanical requirements if regularly
installed in upright position. However, if installed
suspended, e.g. as TLSA, cage design surge arresters
may be used up to the highest system voltages.
Cage design surge arresters should not be applied
when a mechanical resistance is essential even after
short circuit, e.g. if the surge arresters serves a dual
function as arrester and post insulator to carry a
busbar. In this case, tube design surge arresters are the
only alternative.
Above 300 kV system voltage or in case of highest
mechanical stresses, cage design surge arresters may
not provide the necessary mechanical resistance any
more. Thus, for system voltages above 300 kV, tube
design surge arresters appear to be more appropriate.
Author adress:
kai.steinfeld@siemens.com
Bernd Kruska
R&D engineer
Siemens PTD H 42,
Nonnendammallee 104,
D-13629 Berlin
bernd.kruska@siemens.com
Wolfgang Welsch
Director Sales
Siemens PTD H 43,
Nonnendammallee 104,
D-13629 Berlin
wolfgang.welsch@siemens.com
6
Steinfeld, K., Kruska, B. & Welsch, W.