Fault-Tolerant Independent Pole Control Scheme of Zigzag MMC for Bipolar MVDC Distribution Systems

This paper presents a fault-tolerant independent pole control scheme for a zigzag modular multilevel converter (MMC) targeting bipolar medium-voltage direct-current (MVDC) distribution systems. An MMC which adopts a zigzag transformer as an AC grid interface transformer, namely zigzag MMC, has inherent bipole operation capability with unbalanced loads of positive and negative poles on the DC side. Nevertheless, this topology has not been thoroughly investigated for its ability to maintain independent control of each pole in the case of a fault in the other. With a conventional MMC control which relies on normal operation of both poles, a fault in a single pole is likely to impact the overall operation of the converter. To tackle this problem, the proposed control scheme separates the arm current controllers and energy balancing controllers within upper and lower arms individually. Additi…