Imbalance Mechanism of Capacitor Voltage and Control Strategy for Hybrid Modular Multilevel Converter with Negative Output

Abstract

In a flexible DC transmission system with a large-capacity overhead line, a hybrid modular multilevel converter (MMC) consisting of the half bridge submodule (HBSM) and the full bridge submodule(FBSM) has become a research hotspot due to its self-cleaning capability of DC short-circuit fault. Under the operating conditions of reducing the DC voltage, due to the different charging and discharging characteristics of the half bridge submodule and the full bridge submodule, a capacitor voltage imbalance may occur between the two submodules. This paper first studies the number configuration of half-bridge submodules and full-bridge sub-modules in modular multilevel converters. Then, the capacitor voltage imbalance mechanism of the hybrid MMC sub-module is analyzed in detail, and the optimal control strategy of the capacitor voltage balance is derived according to the configuration requirements of the two sub-modules. Finally, a hybrid MMC-based flexible DC transmission system is established in the PSCAD/EMTDC simulation environment, low DC voltage operation of the system are simulated to verify the correctness and effectiveness of the control strategy.