This paper analyzes the reasons of subsequent commutation failure (SCF) of line commutated converter based high voltage direct current (LCC-HVDC). Firstly, according to the mechanism of commutation failure (CF), it can be found that the actual firing angle, DC current and AC voltage of inverter all have influence on CF. Secondly, based on the generation mechanism of firing pulse in LCC-HVDC, it is pointed out that the phase locked loop (PLL) dynamics can cause the deviation between the actual firing angle and the firing angle order from control system. Then, the transient characteristics of PLL after the first commutation failure (FCF) are analyzed. Compared with other factors that affect SCF, the PLL dynamics in fault recovery period is considered as the main reason leading to SCF. Finally, a method to suppress SCF is proposed, and the effectiveness of the proposed conclusion and method is verified through the simulation of CIGRE benchmark model in PSCAD/EMTDC.

LCC-HVDC; transient characteristics; subsequent commutation failure; PLL dynamics