A novel method to mitigate commutation failures in HVDC systems

In this paper we show a method to mitigate commutation failures in HVDC systems due to voltage dips from AC systems. The abc-αβ transformation is used for three-phase fault detection, and the zero-sequence voltage, which is obtained by the addition of the three-phase voltages, is used for single-phase fault detection. Both detection methods are based on instantaneous values, which ensure fast reaction of the control system when AC faults occur. After detecting the fault, an additional angle is deducted from the firing order at the inverter station, which in practice enlarges the commutation margin. The test results show that this method is very effective in reducing the risk of commutation failures.