Optimized modulation method for the modular multilevel converter with redundant sub-modules under arm-asymmetric operating conditions

摘要

Considering the synthetical effects of redundant submodules and arm-asymmetric operating conditions within the modular multilevel converter (MMC), a modified mathematical model is established in this paper, and the modified switching function and the mechanism of circulating current are analyzed. On this basis, an optimized modulation strategy is proposed to minimize the fundamental component of the inner circulating current. Unlike previous strategies, the sub-modules in healthy arms need not be bypassed when SM faults occur and the degree of asymmetry will be reduced. In the proposed strategy, the system can operate well under arm-asymmetric conditions and the redundant sub-modules can be fully utilized. A model of two-terminal MMC-HVDC system is built in PSCAD/EMTDC, and the simulation result proves the validity and the feasibility of the proposed strategy.