Decreasing the Commutation Failure Frequency in HVDC Transmission Systems

In this paper we show how a fairly large proportion of those commutation failures that are due to single-phased short circuits to earth can be avoided. In a control circuit based on a digital signal processor (DSP) it is possible, with instantaneous results, to switch from a normal control strategy with equidistant firing pulses to a strategy that takes into consideration the potentially dangerous voltage changes on the supply lines. If the supply voltages are monitored continuously, it is possible to calculate the necessity of advancing the firing pulses to avoid commutation failures. In the paper such a calculation technique is described. The monitoring times may be approximately 50 sec apart corresponding to a distance of 0.016 radians or about 1 .