An Improved Voltage Ripple Control Algorithm for Modular Multilevel Converter Based Variable Speed Drive

Modular Multilevel Converter (MMC) is one of the most promising topologies for high power- medium voltage application. However, the use of MMC in variable speed drive applications is still limited. This is due to the fact that the fluctuation of the submodules capacitor voltage is large at low speed constant torque operation. This paper proposes an improved balancing approach based on Space Vector Pulse Width Modulation (SVPWM). The proposed method uses only one SVPWM, which not only simplifies the calculation but also reduces the submodules capacitor fluctuation and the circulating current in the MMC, thereby allowing the operation of MMC - based variable speed drive over the entire operating speed range and improve the converter efficiency. The closed loop operation of the MMC - based variable speed drive is verified through extensive simulations.