An Adaptive Voltage-Balancing Method for High-Power Modular Multilevel Converters

The voltage balancing of the floating capacitors in modular multilevel converter (MMC) is of great importance for the safe operation. However, the existing voltage-balancing algorithms usually suffer from high switching frequency, leading to unnecessary switching losses and device costs. This paper proposes an adaptive capacitor voltage-balancing method for high-power MMCs with reduced switching frequency. The operational characteristics and switching process of the submodules (SMs) are analyzed in detail. And then the relationship between the switching frequency and the capacitor voltage deviation is derived. Based on the analysis, an adaptive method is proposed to make a trade-off between the switching losses and the balancing effect through a closed-loop control of the alternating number of the SMs. The calculation formulas of the switching frequency are also presented. PSCAD/EMTDC simulation and RT-LAB-based real-time control hardware in the loop (HIL) test results demonstrate the effectiveness of the proposed method under different operating conditions.