Startup strategy of VSC-HVDC system based on modular multilevel converter

Based on modular multilevel converter (MMC) for high voltage direct current (HVDC) system, the startup strategy is researched. The equivalent charging circuits and physical models for uncontrolled and controlled charging process of MMC-HVDC system are derived and the charging current stresses are analyzed. In traditional methods, series resistors in the arm or in the dc lines are used to limit the surge current when the MMC station unlocked at controlled charge step from dc side. In this paper, new startup method without extra source is proposed. With this method, charging current of MMC from dc side at controlled stage is minimized. Experimental results testify the effectiveness of this strategy.