DC Capacitor Voltage Control of MMC for HVDC using Negative Sequence Current

Abstract

Modular multilevel converters (MMC) have been studied as an ac-dc converter applied to high voltage dc power transmission (HVDC). In order for MMC to maintain the dc bus voltage, it is necessary to balance the dc capacitor voltage of each cell. This paper presents “phase arm balance control” and “upper and lower arm balance control” using negative sequence components of ac grid current and circulating current. The proposed method is verified by numerical simulation.