A novel mitigation strategy of subsequent commutation failures in the hybrid cascaded LCC-MMC HVDC transmission system

The hybrid cascaded LCC-MMC HVDC system can maintain power transmission capacity by serially connecting the line-commutated converter and modular multi-level converter at the inverter side during commutation failures. Therefore, it is considered a feasible solution to mitigate commutation failures. However, under severe fault conditions, the LCC will still suffer from commutation failures because the reactive-power support of MMC is insufficient and inflexible. To handle this problem and enhance the stability of the hybrid system, this paper presents the commutation state factor to calculate the reactive-power consumption against commutation failures. Furthermore, the reactive-power distribution principle between different MMCs and LCC is investigated based on the voltage interaction factor and the reactive-power sensitive factor. Then, a novel flexible reactive-power coordination strategy is presented to mitigate the subsequent commutation failures. Compared with conven-tional methods, the proposed strategy can adequately utilize the reactive-power support ability of MMCs. And then LCC is capable of obtaining stronger immunity against commutation failures. Finally, simulation cases are conducted on the Baihetan-Jiangsu hybrid HVDC model using PSCAD/EMTDC software to verify the effective-ness of the proposed method.