The metal-oxide (MO) arrester offers an alternative solution
to limit TOVs and can act alone or in conjunction with convertor control and
the switching of shunt capacitors and ac filters.
The basic idea behind the use of MO arresters to limit TOVs
at the convertor station is to exploit the high energy-absorption capability
offered by the MO equipment. Various solutions are possible. Two basic
approaches adopted in recent dc projects are discussed below:
a) MO arresters with an extremely low protective level are
used to limit the TOV to values of typically 1.4 pu and permanently connected
to the ac busbar.
To achieve this low protective level and because of the
given MO material characteristic, signiÞcant currents flow through the arrester
at normal operating voltages. The arrester, therefore, needs special cooling to
avoid overheating during continuous operation.
The use of such permanently connected MO arresters is
recommended where the initial two or three peaks of the TOV are higher than the
acceptable value and cannot be limited by other arresters installed in the
station.
b) Special MO arresters that are switched in by circuit
breakers in the case of high TOVs. When the TOV has been reduced either by
restarting the dc system or by switching out the shunt capacitors and ac
filters, the arrester is disconnected from the network to prevent overloading
caused by normal operating voltage.
This arrester can limit the TOV to values as low as 1.25 pu.
However, the closing time of the breaker must be considered with the result
that the over voltage is not limited until a few cycles after its occurrence.
In both solutions the possible fault contingencies must be
studied carefully to determine the maximum energy stress of the arrester. Lower
protective levels require higher energy capability of the MO equipment and
consequently lead to higher costs.
Experience shows that the optimum overall design could be in
the range of 1.25 - 1.4 pu.
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