Research on MMC-CLCC hybrid HVDC transmission system and its power flow reversal strategy

Abstract For the scenario of large-scale new energy transmission through DC and multiple DC landing points in load centers, a hybrid DC transmission system of MMC-CLCC is proposed. Firstly, the topological structure of the hybrid DC transmission system is designed. Considering the possible need for power flow reversal, the selection method of full-bridge and half-bridge sub-module ratios of MMC is designed, and the control strategy of the hybrid DC transmission system is proposed. Based on the negative voltage operation characteristics of the hybrid sub-module MMC, an online power flow reversal control strategy for the MMC-CLCC hybrid DC transmission system is proposed. Finally, the MMC-CLCC hybrid DC transmission system is built in PSCAD/EMTDC, and the power flow reversal strategies of the hybrid DC systems with different full-bridge sub-module ratios and different power flow reversal demands are simulated and verified. The results show that the proposed strategy can effectively achieve online power flow reversal, and avoid power interruption caused by the conventional power flow reversal strategy centered on the voltage polarity reversal after DC blocking, and the power flow reversal process is smooth and faster.