A Proposal of Control Method for Regulating Capacitor Voltages of Neutral Point Clamped Modular Multilevel Converter

Inhalt:
Modular Multilevel Converter (MMC) has been in practical use for high voltage applications. Neutral Point Clamped Modular Multilevel Converter (NPC-MMC) is a hybrid circuit topology which consists of sub-module(SM)s and self-turn off switching devices of which withstand voltage is higher than half of the DC voltage. NPC-MMC has half number of SM compared with MMC, so there is a possibility that NPC-MMC is smaller than MMC. However, there has been no study that tried to control NPC-MMC. This paper proposes a control method of regulating SM capacitor voltages of a three-phase NPC-MMC. In this control method, a circulating current is used in order to balance SM capacitor voltages like MMC, however, the control method is characterized by superposing DC, fundamental AC and 3rd harmonic on the circulating current. Experimental results prove that the control method effectively regulates SM capacitor voltages in four-quadrant operations.