Sequential Capacitors Charging Methods During Single Phase Modular Multilevel Converter Uncontrolled Start-Up Pre-Charging Phase

The pre-charging control scheme of the Modular Multilevel Converters is an important research subject. At the start-up of the MMC, all capacitors in each submodule of the MMC are required to be equally charged to a certain voltage value before transferring to the normal operation, otherwise a large inrush current may occur at the instant of the start-up threatening the safe operation of the IGBTs and the capacitors. Basically, to start-up an MMC, two phases need to be considered, Uncontrollable pre-charging phase and Controllable pre-charging phase, the Uncontrollable pre-charging phase, which is our main focus has been overlooked for a while. The conventional solution for limiting the huge inrush current during the aforementioned phase was using a limiting resistor and magnetic contactor arrangement as stated in plenty of papers for the last decade. In this paper, two new methods are introduced in order to pre-charge the capacitors, at the uncontrollable pre-charging phase to their nominal voltage without using the traditional solution. Detailed design algorithms of the proposed methods are introduced. Simulation results of the proposed methods are presented to show how employing these new methods would drastically limit the surge of the circulating current during the uncontrolled start-up phase.