Improved current order control strategy for effective mitigation of commutation failure in HVDC system

Commutation failure problem will become more severe when the inverter of the LCC-HVDC system connects to a weak AC system. Suppressing the occurrence of commutation failure as well as improving the fault recovery characteristics of the systems are the urgent requirements for a practical HVDC project in China. In this paper, a novel additional control strategy is proposed to reduce the probability of the commutation failure and to increase the recovery speed of DC system, especially when connected to weak AC systems. In the proposed control strategy, the DC current variation signal is adopted to increase both the extinction angle and the rate of the DC current. Finally, the results show that the proposed control strategy is very effective in reducing the number of commutation failures, and even would enhance recovery speed of the corresponding DC system.