Power Transmission and Distribution
Surge Arrester Design Comparison
Tube and Cage Designs versus Wrap Design
Confusing Apples with Oranges
Just as you cannot compare apples with
oranges without noting their distinct differences, surge arresters cannot be compared
simply by price, particularly when considering reliability issues of power systems.
A closer look at the different designs reveals distinct advantages in performance
and reliability provided by cage and tube
designs.
For instance, a directional pressure relief
in the Siemens 3EQ tube design surge arresters (up to 800 kV) allows you to control
the relief pressure in the direction of minimal damage. Or, the use of fibre glass reinforced plastic (FRP) rods in the 3EL cage
design (up to 300 kV) prevents interior
parts from being ejected. The table on the
right shows the advantages and disadvantages of the various designs.
For more information, feel free to contact
us at the address provided, or e-mail us at:
arrester@siemens.com
Siemens tube design
Directional pressure relief
Silicone Rubber sheds
Dry gas
Metal oxide varistor MOV
(glass collared)
FRP tube
FRP rods
Directional pressure relief
Siemens cage design
Silicone Rubber housing
Metal oxide varistor MOV
(glass collared)
FRP rods
Wrap design (competition)
Metal cap
Metal disk
EPDM based polymer or
Silicone Rubber housing
Metal oxide varistors MOV
(mullite collared)
FRP wrap
2
Siemens 3EQ tube design
Siemens 3EL and
3EK7 cage design
Wrap design (competition)
Design
Silicone is directly molded onto
the FRP tube. This tube ensures
the high mechanical strength
of this surge arresters. At the
ends, the flanges are fixed with
a special sealing system. The
flanges are equipped with a directional pressure relief device.
Inside the FRP tube the MOV
stack is supported by FRP rods.
This stack is clamped between
the flanges.
Silicone is directly molded
onto MOV blocks and FRP rods,
ensuring a total enclosure
for all components. The MOV
blocks are clamped between
the metal end fittings with
pre-stressed FRP rods.
Wrap design surge arresters have MOV
blocks wrapped with fiberglass ribbons
impregnated with epoxy resin to create a
stack of MOV blocks. This MOV stack is
then inserted into a pre-modled, EPDM
based alloy or Silicone Rubber housing to
create a module. Multiple modules are
screwed together for higher ratings. The
space between the module and housing
is filled with grease or some form of dry
interface material. The two ends are then
covered with steel caps.
Respective
mechanical
characteristics
The very high mechanical
strength permits the use of this
design in all areas where highest mechanical strenght is
needed e.g. seismic activities,
wind loads. In addition, this
design provides a very high
degree of safety in the event of
an overload, because no parts
are ejected and the housing
does not break. Furthermore,
the housing retains 75% of its
mechanical strength after an
overload and can be used as a
post insulator support.
A further advantage is that, even
with very large housings, tubedesign arresters can be mounted
at any vertical plane angle
throughout the 360º range.
The cage design is notable for
its high mechanical strength in
conjunction with minimal use
of materials, low pressure
release in the event of an overload, and light weight. This
design is particularly suitable
for most standard applications
and for line arresters.
The cantilever moment for this design
is about half that of the cage design and
much lower in comparison to the tube
design. The stacks of MOV blocks have no
reinforcement and are simply held together
with fiberglass wrap. Also, the higher rated
units with long modules are joined only by
a stud. This design does not provide enough
mechanical strength, permits very high
deflection under operating loads, and cannot withstand 0.5 g of ground acceleration
as required by IEEE standard 693. Wrap
design arresters will relieve pressure through
their housings in any direction. They are
not suitable for applications where utilities
need the pressure to be relieved in a specific
preferred direction away from other equipment in the station.
Performance
under moisture
and pollution
The flanges are combined with
an excellent sealing system to
prevent moisture ingress and
partial discharges. This sealing
system has been proven in field
service for over 35 years.
Very long units can be produced;
this improves their ability to
perform well despite pollution.
The Silicone Rubber provides
hydrophobicity (water repellency) throughout the arrester’s
service life.
Silicone is directly molded onto
MOV blocks and fiberglass
rods, ensuring the total enclosure of all components to prevent partial discharges or moisture ingress. The Silicone
Rubber provides hydrophobicity (water repellency) throughout the arrester’s service life.
When pushed over the wrap, the insulation
material may trap air, resulting in a high risk
of failure due to moisture ingress and partial
discharges. There is also a high risk of material aging if no Silicone Rubber is used. Other
materials are not always stable when exposed
to UV radiation and ozone.
Most common wrap designs use EPDM based
rubber alloys containing only an insignificant
amount of silicone oil. In contrast to Silicone
Rubbers (with a silicone content of at least
80%), the EPDM based alloys or polymer
blends cannot provide or retain the hydrophobicity of the surge arresters. Their performance with respect to tracking, flash-over
and pollution is extremly poor.
Additional
differences
The tube design is equipped
The material used in the cage
with a directional pressure
design has a very low fire load
relief device. The surge arrester, and is self-extinguishing.
therefore, can be pointed in a
direction that will minimize the
risk of any damage to the equipment close by.
The wrap consists of flammable components (fiberglass with epoxy resin). Surge
arresters of this design will continue to
burn after an overload.
3
Siemens 3EQ tube design
Among other applications, 3EQ tube
design surge arresters can replace station
post insulators.
Siemens 3EL and 3EK7 cage design
The 3EL cage design offers high performance
in both transmission line and substation
applications.
Wrap design (competition)
In wrap designs, the EPDM housing is
pushed over the epoxy resin wrap and can
easily be removed, as shown at left. These
designs have a high risk of failure due to
moisture ingress. Their mechanical strength
is very low – a reliability issue.
Please contact us at:
Phone: +49 30 / 386 33 222
E-mail: arrester@siemens.de
Siemens AG
Power Transmission
and Distribution
High Voltage Division (PTD H51)
Nonnendammallee 104
13629 Berlin
Germany
E-mail: arrester@siemens.de
www.siemens.com/arrester
The information in this document contains general
descriptions of the technical options available,
which do not always have to be present in
individual cases. The required features should
therefore be specified in each individual case at
the time of closing the contract.
Subject to change without prior notice.
Order No. E50001-U113-A307-X-7600
Printed in Germany
Dispo 30000
TH 263-040488 101129 PA 10042.
If you have any questions about
Power Transmission and Distribution,
our Customer Support Center is available around the clock.
Phone: +49 180 / 524 70 00 (charges depending
Fax:
+49 180 / 524 24 71 on provider)
E-mail: support@ptd.siemens.de
www.siemens.com/ptd-support