A Novel Cell-by-Cell Pre-charging Scheme of Modular Multilevel Converters

Inhalt:
This study presents a generalized novel MMC pre-charging scheme that is based on cell-bycell charging concept. In this proposal, the pre-charging is done in two steps. A first passive uncontrolled stage charges all cell capacitors to a minimum voltage required for cell switching activation is followed by a second active controlled stage, where blocking and bypassing of individual cells is employed to further charge the cell capacitors to their rated voltage in a cell-by-cell based manner. The proposed pre-charging scheme was simulated in Matlab/Simulink using a δ connected MMC with an auxiliary charging circuit, where calculation of limited peak charging current and dimensioning of auxiliary charging transformer were investigated. In this paper, two scenarios were analyzed; overlapping and non-overlapping cell-by-cell charging. It was shown how overlapping method provides a simple way to adjust charging current and charging time according to the overlapping voltage set point. Additionally, different numbers of converter cells were used to show that the overlapping cell-by-cell scheme can result in a significant decrease in charging time for higher number of cells compared to the simple non-overlapping case.