A low switching frequency voltage balancing strategy of modular multilevel converter

The application of modular multilevel converter (MMC) to High Voltage Direct Current projects has to consider voltage balancing of the multiple floating capacitors. The common voltage balancing strategy, which based on the nearest level modulation (NLM) method, benefits from satisfied voltage balancing, but draws back the performance by its high switching frequency and high switching losses. This paper proposes a novel low switching frequency (NLSF) strategy to reduce the switching frequency by using the switching state of submodules (SMs) on the basis of good voltage balancing. A detailed 21-level back to back MMC-HVDC model has been built in PSCAD/EMTDC for demonstration. The simulation results show acceptable voltage balancing with dramatic reduction of the switching frequency by 61% in contrast to the common voltage balancing strategy.