Surge Arrester
Design Comparison
Tube and Cage Designs versus Wrap Design
Answers for energy.
Confusing Apples
with Oranges
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
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
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
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-molded, 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, tube-design
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 with
a directional pressure relief device.
The surge arrester, therefore, can
be pointed in a direction that will
minimize the risk of any damage to
the equipment close by.
The material used in the cage design
has a very low fire load and is selfextinguishing.
The wrap consists of flammable components (fiberglass with epoxy resin).
Surge arresters of this design will
continue to burn after an overload.
Siemens 3EQ tube design
Siemens 3EL and 3EK7 cage design
Wrap design (competition)
Among other applications, 3EQ tube
design surge arresters can replace station
post insulators.
The 3EL cage design offers high
performance in both transmission line
and substation applications.
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.
Siemens Power Transmission
and Distribution Inc.
Arrester Products
444 Highway 49 S.
Richland, MS 39218
Phone 601-932-9800
Fax
601-932-9911
Order No. E50001-U113-A307-V1-US00
Printed in USA
TH 263-080304 103133 PA 04080.3
www.usa.siemens.com/energy
www.siemens.com/arrester-download
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.