A Startup Method for Flying-Capacitor Modular Multilevel Converter (FC-MMC) With Effective Damping of LC Oscillations

The flying-capacitor modular multilevel converter (FC-MMC) overcomes the low-/zero-speed operation issue of conventional MMC in medium-voltage motor drive application. However, the introduced flying capacitor (FC) in each phase leg brings great difficulties for circuit startup: 1) precharging of FC to nominal voltage is necessary for normal operation; and 2) the oscillation between FC and inner buffer inductors need to be mitigated in the precharging process. In this paper, a startup method with simple control algorithm is proposed for solving the above two issues. The proposed method operates the switches of submodules (SMs) to make the FC connected in parallel with SM capacitors for charging up. Meanwhile, the oscillation caused by the equivalent LC circuit is effectively mitigated by properly implementing the damping modes. The suppressed oscillation prevents the FC from any negative voltage, which guarantees the safety of FC, especially when electrolytic capacitor is utilized. The validation of the proposal is proved by simulations and experiments.