A Control Strategy for Modular Multilevel Converters Based on Arm Current Control Under Grid-Side Open-Circuit Fault

To further improve the power supply sustainability of the ac/dc hybrid distribution networks, this article proposes a control strategy for the modular multilevel converter (MMC) under grid-side open-circuit fault, which is based on independent arm current control. Firstly, the behaviors of the MMC after the faults are analyzed, including single-phase and two-phase open-circuit faults. For single-phase open-circuit fault, the proposed strategy can be achieved by reconfiguring the amplitude and phase of the arm currents, thereby eliminating the zero-sequence current. For two-phase open-circuit fault, both the arm currents and voltages need to be reconfigured. After that, a method for detecting and locating specific faults is provided, which can be implemented by detecting the amplitude of the zero-sequence current. Besides, the postfault characteristics of the MMC under single-phase open-circuit condition are given, including submodule (SM) capacitor voltage ripple, circulating current, arm rms current and arm peak current. According to it, long-term postfault operation of the MMC can be guaranteed with 50% of the rated power. Additionally, stable dc-bus voltage and balanced SM capacitor voltage are satisfied even though one phase is disconnected. Moreover, with the proposed control strategy, although the MMC is able to operate for a short period of time in emergency situations when two phases are disconnected, it is not supposed to transmit power due to the inevitable large current on neutral conductors. Finally, the feasibility and effectiveness of the proposed control strategy is validated by simulation and experimental results.