Research on output stability of MMC-type multi-port solid-state transformer under asymmetric grid

In order to study the output stability characteristics of the Modular Multilevel Converter (MMC) type multi-port solid-state transformer under asymmetric grid conditions, the paper first models the input side and isolation side of the MMC-type multi-port solid-state transformer as a DC voltage source, and the output side as a three-phase grid-connected inverter. Then, based on the traditional synchronized reference frame phase-locked loop (SRF-PLL) and the dual second-order generalized integrator phase-locked loop (DSOGI-PLL), and using the harmonic linearization method, the sequence impedance model of the MMC-type multi-port solid-state transformer output stage under asymmetric grid is established. The study finds that the influence of asymmetric voltage on the inverter output impedance is relatively small, while asymmetric of three-phase grid impedance significantly affects the system stability. Furthermore, in the case of asymmetric grid voltage and grid impedance, the DSOGI-PLL can enhance the system stability. Finally, the paper conducts corresponding experimental validations, and the results confirm the correctness of the theoretical analysis in the paper.