A New Capacitor Voltage Balancing Method for Trapezoidal Operation of Modular Multilevel DC-DC Converters

In this paper, a new submodule (SM) capacitor voltage balancing algorithm is proposed for the trapezoidal operation of a Modular Multilevel DC-DC converter (MMC-DC-DC) for HVDC/MVDC. Trapezoidal wave-shaping leads to SM switching at fundamental frequency, making the balancing process more challenging. In the proposed method, balancing is achieved by controlling the equivalent duty-cycle of each SM. This control is made possible by the exploitation of the ramps in the trapezoidal voltage waveforms, that allow to impose an uneven power flow in the different SMs. The proposed balancing has been validated in PLECS simulation for a 10MW, 20kV 20kV MMC-DC-DC converter.