An Improved Capacitor Voltage Balanced Method of Modular Multi-level Converter

Modular Multi-level Converter (MMC) has been widely used in the field of flexible DC transmission due to its modularity, low switching loss, low harmonic distortion rate, etc. The balance of the sub-module capacitor voltage in MMC control system is critical and it is hardly guaranteed by the traditional Carrier Phase Shift Modulation (CPSM) scheme. Therefore, an improved capacitor voltage balanced scheme is proposed in this paper. By analyzing the composition of the sub-module (SM) DC capacitor voltage variation, it is found that the bridge arm current has a great influence on the voltage balance of SMs. Thus, the bridge arm current is employed in the CPSM scheme. This improved voltage balanced method can effectively increase the transient settling speed of the SM voltage balance without introducing extra switching loss and large calculation cost. The simulation results show that the proposed capacitor voltage balanced method can reduce the transient settling time by more than 75% when comparing with the traditional scheme, which verifies the effectiveness and superiority of proposed method.