Circulating current suppression with improved DC‐link power quality for modular multilevel converter

Abstract

Despite having more advantages, a modular multilevel converter (MMC) has a circulating current as one of its main drawbacks that augments arm current greatly. However, circulating current suppression (CCS) of the conventional MMC will deteriorate the power quality of direct current (DC)-link voltage. This study proposes the CCS by applying the three-winding transformer and a DC-alternating current CCS inverter. The mathematical models of the proposed MMC and circulating current are analysed in detail. The parameter setting of the three-winding transformer is also discussed and is set to have an equivalent arm inductance of the conventional MMC to make comparisons. The comparing results validate that the proposed MMC can exhibit much better performance under steady and dynamic conditions. The significant DC-link voltage ripple that exists in the conventional MMC does not appear. The mean-square-error values of DC-link voltage have more than 90 per cent improvement under steady and dynamic conditions. Due to this contribution, the proposed MMC can provide better DC-link power quality.