An optimized Closed-loop Precharging Strategy for Modular Multilevel Converters Based on Power Balancing Control under start or fault restart conditions

Abstract

Modular multilevel converter (MMC)-based high-voltage DC (HVDC) systems have found rapid growth in power grids. To reduce the inrush current, it is necessary to precharge the submodule (SM) capacitors to the rated voltage before normal operation. In this paper, an optimized pre-charging strategy for MMC is proposed based on power balancing . First, the charging power is obtained by a designed closed-loop control of the SM capacitors. Then the total power released by the AC or DC side could be calculated by power conservation and feedback by controlling the charging current. At the same time, the power difference between the upper and lower bridge arm of each phase and the internal voltage balancing of bridge arms are also considered by the proposed bridge arm and SM voltage algorithms. By comparing with conventional closed-loop pre-charging strategies, the pre-charging speed is greatly enhanced as the achievement of synchronous charging of upper and lower bridge arm SMs. Moreover, as the same voltage and current double closed-loop control applied in both MMC pre-charging and normal state, the state transition time could be reduced. Finally, a series of simulations and prototype experiments are conducted to verify the feasibility of the proposed pre-charging strategy.