Optimizing Voltage Balancing Method of Power Electronic Transformer Based on MMC

In practical engineering applications, capacitor voltage sorting of Modular Multilevel Converter (MMC) in Power Electronic Transformer (PET) input stage is a huge engineering difficulty. To solve the problems of traditional voltage balancing sorting algorithm, such as high switching frequency, large amount of computation and large switching loss, an optimizing voltage balancing method is proposed to reduce time complexity and switching frequency. Firstly, Merge sort is used to select the appropriate elements as reference values of the randomized-select algorithm, and then the randomized-select algorithm is used for quick sorting. On this basis, the reordering factor is introduced. When the difference of capacitance voltage between sub-modules (SMs) is small, it avoids reordering and keeps trigger pulse unchanged; otherwise, it quickly reorders. Selective sorting of MMC controllers not only further reduces the computational complexity of the controllers, but also effectively reduces switching losses. Finally, the feasibility and validity of the proposed optimization method are verified with MATLAB simulation.