01-01-2013, 10:53 AM
Metal-Oxide Surge Arresters in High-Voltage Transmission and Distribution Systems
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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.
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
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.
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.
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)
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.
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.