Abstract: A 2-stage three-level power conversion system（PCS）was studied，in which the 3-level T-type 3-leg

3-phase 4-wire（3LT23L3P4W）topology was adopted as AC/DC part while the 3-level Buck/Boost topology was

utilized as DC/DC interface. To address the neutral point potential（NPP）unbalance of the PCS，firstly，with the

asymmetry extent and power factor of AC current accounted，a model of the NPP low-frequency ripple in

3LT23L3P4W was established. Secondly，an NPP balancing method based on front-end 3-level Buck/Boost under

quasi-PR control was proposed，in which resonance points were set at fundamental frequency and triple fundamental frequency to suppress the main ripple components. Finally，a 30 kW PCS simulation model was built in Matlab/Simulink. Simulation results verify the accuracy of the NPP low-frequency ripple model，and show that the proposed NPP balancing method can effectively suppress the NPP low-frequency ripple under different operating conditions.

     Key words: power conversion system（PCS）；3-level T-type；3-level Buck/Boost；neutral point potential

（NPP）balance