Voltage Equalization Strategy for the Input Stage of MMC-Type Power Electronic Transformers Based on Dynamic Average of Capacitor Voltage

Traditional voltage-balancing strategies for modular multilevel converter (MMC)-based power-electronic transformers often encounter significant challenges, including high computational complexity in sorting algorithms, elevated submodule switching frequencies, and frequent power device operations. To mitigate these issues, this paper proposes an advanced voltage-balancing strategy for the input stage of MMC-based power-electronic transformers, utilizing the dynamic average value of submodule capacitor voltages. The proposed approach integrates the average voltage variation per cycle as a constraint within an optimized bubble sorting algorithm, thereby minimizing unnecessary switching state transitions. Furthermore, a predefined maximum voltage deviation threshold is introduced to ensure that the capacitor voltage uniformity complies with stringent engineering requirements. The efficacy of the proposed strategy is rigorously validated through a comprehensive Matlab / Simulink simulation model of an MMC-based power-electronic transformer. Experimental results demonstrate that the proposed method achieves a 21% reduction in switching frequency while maintaining capacitor voltage deviations within 5%, significantly enhancing system efficiency and operational reliability compared to conventional approaches.