An improved high-frequency common-mode voltage injection method in modular multilevel converter in motor drive application

This paper discusses the application of Modular Multilevel Converter (MMC) in motor drive. In the startup and low-speed operation, large capacitor voltage ripples appear in MMC. This paper proposes an improved high-frequency common-mode voltage injection method. In the improved method, the amplitude of injected voltage is varying according to the instantaneous value of the output voltage, and it can take full use of the arm voltage margin. This will lead to further reduction in the circulating current and higher efficiency. Both three-phase and single-phase cases are discussed, and comparisons are made between the existing control method and the improved one. This method can be more effective in the single-phase system since there is no interaction of three-phase output voltages. MMC simulation model built in PSCAD/EMTDC verifies the validity.