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, signi√ěcant 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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