A Capacitor Voltage Fluctuation Suppression Method Employing Compound Mutual Inductance Charge Channel for Modular Multilevel Converter Operating at Low Frequency

The voltage fluctuation problem would be produced in the submodule capacitor when the modular multilevel converter (MMC) is operating at the low-frequency stage, which will seriously affect the low speed and starting up performance of the variable speed motor load system. A capacitor voltage fluctuation suppression method is proposed in this article by employing the compound mutual inductance charge channel. The compound mutual inductance charge channel is formed by the interarm charge channels, the intra-arm charge channels and the interphase charge channels. The interarm charge channel constructed between the bridge arms can suppress the reversed-phase voltage fluctuation components in the capacitor, while the in-phase voltage fluctuation components and the circulating current components can be reduced by the interphase charge channel. In addition, the voltage balancing of the submodule capacitors can be achieved by the intra-arm charge channels, so that the complex voltage balancing algorithm, as well as the related sensors, are not required and can be removed. Therefore, the proposed MMC would have good static and dynamic performance even under the low frequency operating conditions. The experimental and simulation results verify the effectiveness of the theoretical analyses.