A Commutation Failure Suppression Strategy for UHVDC Hierarchical Connection Systems

In the UHVDC Hierarchical connection system, when the AC system on the inverter side fails, the commutation failure of the high and low end converters may occur at the same time due to the delay in starting the commutation failure prevention control of the non-faulty layer. Therefore, a coordinated control strategy of high and low side converters based on the commutation time and voltage integral area is proposed. Taking advantage of the fact that the commutation failure prevention control of the faulty layer can respond to faults more quickly, the proposed strategy introduces the output of the commutation failure prevention control of the faulty layer into the non-faulty layer, so that the start time of the commutation failure prevention control of the non-faulty layer is advanced, increase the turn-off angle of the non-faulty layer converter; at the same time, the voltage dependent current order limiter control is modified to reduce the DC current during the fault process. A simulation model was built in PSCAD/EMTDC to verify the proposed strategy under different working conditions. The results show that this strategy can significantly improve the sensitivity of non-faulty layer converters to faults and reduce the risk of simultaneous commutation failures.