Transient characteristic of HVDC system during commutation failure,its effect on differential protection and countermeasures

The relation between equivalent power-frequency current and converter bus voltage during the commutation failure of DC system is obtained based on the CIGRE HVDC benchmark model. The criterion expression of current differential protection for AC-DC interconnected system is deduced and the differential protection connected to DC power system for transmission line is analyzed. The analytical results indicate that,the differential protection may reject to act when in-zone fault occurs in AC-DC interconnected operation. It is proposed to detect the internal fault based on the amplitude criterion. Simulative results by PSCAD / EMTDC validate the correctness of theoretical analysis and the effectiveness of proposed criterion.