A Redundancy Strategy for Modular Multilevel Converter Based on Carrier Phase Shift Modulation

In the existing redundancy strategy for modular multilevel converter (MMC) under carrier phase shifting sinusoidal pulse width modulation (CPS-SPWM), the capacitor voltages of redundant submodule should be charged from zero which is slow for the system recovery. To solve this problem, an online redundancy strategy for MMC under CPS-SPWM is proposed in this paper. Firstly, to maintain the reference value of all capacitor voltages, N normal sub-modules of each arm are selected to be normally triggered through a proposed method with certain time intervals for N+1 level MMC system, and the rest of submodules are bypassed. When some submodules have problems to quit the circuit, the remaining submodules can still be operated by using the proposed method without recharge the redundant submodule. Secondly, based on the redundant MMC system, the packet pre-charging strategy is put forward, in which all the capacitor voltages of submodules can be pre-charged to the rated value with low complexity control system through regulating the number of charged submodules. Finally, simulations in PSCAD/EMTDC show the effectiveness and practicability of the proposed strategy.