Characterization of Transient Overvoltage of Sending-end AC System When Commutation Failure Caused by Asymmetrical Fault

This paper theoretically analyzes the dynamic process of the transient overvoltage on the rectifier ac system when commutation failure is triggered by different types of asymmetrical faults. Firstly, by analyzing the angular difference between two neighboring commutated-phase line voltages of the 12-pulsation rectifier after asymmetrical fault, the periodic fluctuation law of DC turn-off angle of single-phase grounding fault, two phase short circuit fault and two-phase grounding fault are proposed. Secondly, the transient reactive power characteristics of the rectifier during commutation failure due to asymmetrical fault are analyzed in detail, revealing that the asymmetrical fault causes the DC system to periodically enter the constant current control in advance, which ultimately results in the peak transient overvoltage showing a periodic time distribution characteristic. The peak transient overvoltage due to the single-phase grounding fault is obtained to be present at 1/4 and 3/4 cycle, and the peak transient overvoltage due to the two-phase inter-phase failure or the two phase grounding fault is obtained to be present at the 1/2 and 1 cycle. Finally, based on the PSCAD platform's CIGRE-HVDC benchmark model, the correctness of the content of this paper is verified by simulation, which effectively guides the formulation of overvoltage anticipation in system operation.