A Pre-Charging Control Scheme for a Single-Phase Modular Multilevel Converter

Abstract

Pre-charging control scheme for a modular multilevel converter (MMC) can be generally divided into two stages, viz., the uncontrolled rectifying and voltage boosting.Voltage boosting control scheme of three-phase MMC is mature. However, different from the three-phase MMC, single-phase has exclusive capacitor in the dc bus. As a result, voltage boosting stage of single-phase MMC has to take both the submodule capacitor and dc bus capacitor into consideration, and has become an obstacle for single-phase MMC. This paper presents a pre-charging scheme for a single-phase half-bridge sub-module (HBSM)-based MMC. The uncontrolled rectifying stage is also adopted. The sum and the difference of energy between the upper and lower arms are controlled to realize the voltage boosting of sub-modules. Moreover, the dc bus capacitors are charged to the rated voltage value by the rest energy flowing from the AC side. This control scheme can make the starting process rapidly, smoothly and stably, which is of great value for a single-phase MMC operating as a rectifier. A prototype of a single-phase MMC with 8 submodules per arm was built to demonstrate the effectiveness and validity of the proposed control scheme.