Effect of commutation failure on the distance protection and the countermeasures

Commutation failure (CF) may be caused by AC fault in AC/DC interconnected system, and it will affect the performance of distance protection. The transient characteristic of equivalent current of power-frequency of the dc-ac inverter is first analysed by introducing the concept of quadrant. Then, the effect of CF on distance protection is studied in detail with consideration of the connection mode of distance relay. The analysis results show that both the fault position and fault resistance contribute to the effect of CF on the performance of distance protection significantly. Furthermore, a novel countermeasure based on the amplitude of second harmonic is put forward, and a concept of 'successive trip' by using the change of measured reactance is also presented. Simulation results based on PSCAD/EMTDC prove the correctness of these conclusions.