Overvoltage Suppression for Modular Multilevel Converter With Switch Open-Circuit Fault

Abstract

To enhance reliability and exploit advantages, modular multilevel converters (MMCs) are mostly desired with uninterrupted operational capability during submodule (SM) open-circuit fault. Once an SM fault is located, the closing signal will be delivered to the corresponding bypass switch. However, due to the response delay of mechanical switches, capacitor voltage of the SM with lower switch fault will continue increasing, thus resulting in severe overvoltage and threatening the safety of MMC. In view of this, this article proposes an arm current reshaping-based method to suppress capacitor overvoltage under SM failure. During the period of overvoltage suppression, all the arm currents are reshaped to decrease capacitor charging of the faulty SM and maintain the terminal currents of MMC. Besides, since the capacitor voltage variations are also reshaped by the reshaped arm current, common-mode voltage is injected to satisfy the requirements of arm voltages. Consequently, the degree of SM overvoltage can be alleviated by over 50% without extra hardware investment. Simulation and experimental results confirm the feasibility and validity of the proposed method.