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. Additional power reference coupling is introduced to indirectly inject circulating currents when both poles are healthy like the conventional MMC. When a single pole fault occurs, a seamless energy-balancing mode transition can be achieved by modifying inputs to current reference calculators. An AC-side single line-to-ground (SLG) fault ride through (FRT) strategy can also be integrated without compromising the independence of both poles. A point-to-point zigzag-MMC based bipolar MVDC distribution system is tested by controller hardware-in-the-loop (CHIL) test setup. The test results under single pole-to-ground (PtG) fault and AC SLG fault validate the effectiveness of the proposed control scheme. Moreover, experimental results using a downscaled MMC prototype and a zigzag transformer further demonstrate the performance of the proposed control scheme under abrupt single-pole blocking and AC SLG fault conditions.