A Comprehensive Sub-module Capacitor Voltage Control Strategy of Isolated Modular Multilevel DC Converter

In order to balance the sub-module capacitor voltages of isolated modular multilevel DC converter (IMMDC) under asymmetric parameters and light load conditions, this paper proposed a comprehensive sub-module capacitor voltage control strategy for IMMDC, including two layers, i.e., arm-balancing control and inner-arm balancing control. A general mathematical model of IMMDC was established considering the arm parameters asymmetry, which reveals the mechanism that the asymmetry of the arm parameters leads to the imbalance of the capacitor voltages and the generation of the ac circulating currents under bidirectional power transmission. Subsequently, an arm-balancing control method based on pulse width modulation (PWM) combined with phase-shift control was proposed to balance the sub-module capacitor voltages and to suppress the ac circulating currents. Besides, considering that the conventional inner-arm balancing control method may be invalid under different operation modes and voltage gains, an operation-mode adaptation inner-arm sub-module capacitor voltage balancing control method was also proposed to balance the inner-arm sub-module capacitor voltages. Finally, the effectiveness and feasibility of the proposed comprehensive sub-module capacitor voltage control strategy were validated by the simulation and experiment. © 2021 Chin. Soc. for Elec. Eng.