A Neutral Point Potential Balancing Method for 3-level Power Converters in 2-stage 3-phase 4-wire Power Conversion System

Two-stage power conversion system (PCS) for energy storage systems has been considered in islanded operation mode. A three-level T-type three-leg three-phase four-wire topology (3LT 2 3L3P4W) is employed as AC/DC part and a three-level buck/boost converter is used as DC/DC interface. This study is mainly focused on balancing the neutral-point potential (NPP) of the PCS with asymmetrical load connection. An NPP low-frequency ripple model of 3LT 2 3L3P4 inverter is proposed considering various load conditions firstly, which includes detailed expression of low-frequency ripple components at fundamental frequency and fundamental frequency multipliers. Then, a novel NPP balancing method based on front-end three-level buck/boost converter under proportional resonant control is presented, in which the resonance points are set at fundamental frequency and triple fundamental frequency to suppress the dominant components of NPP ripple. Furthermore, a small-signal model of three-level buck/boost NPP control system is established and stability analysis is carried out. Simulation and experimental results have been provided to verify the accuracy of the NPP low-frequency ripple model and the effectiveness of the NPP balancing method.