Analysis and Arm Voltage Control of Isolated Modular Multilevel DC–DC Converter with Asymmetric Branch Impedance

Abstract

Isolated modular multilevel dc–dc converter (IMMDCC), which consists of two modular multilevel converters and a medium-frequency transformer, is attractive for medium-voltage applications because of the following features: the modular structure can handle higher voltage, and synthesizing square-wave voltages toward the alternating current (ac) link increases the dc voltage utilization ratio and achieves the zero-voltage switching operation. However, the asymmetry of arm impedance in practice inevitably leads to power imbalance and divergence of total capacitor voltages in the upper and lower arms of IMMDCC, thereby inducing large ripples in the common-mode arm current and adding a dc bias to the synthesized ac voltage. To compensate for the power imbalance and maintain the stable operation, the interarm phase-shift modulation (IAPSM) scheme and arm voltage-balancing control strategy based on this scheme are presented in this paper. Combining the IAPSM scheme with the intersubmodule phase-shift modulation scheme, deep rising and falling edges of the synthesized square-wave voltage in each phase leg are modified into staircase waveforms with 2 N + 1 steps, dv/dt in the ac-link is thus significantly reduced. Simulation and experimental results validate the theoretical analysis and control strategy.