Common-mode Voltage Suppression for Hybrid 4-leg Three-level FC-NPC Inverter

Abstract

Neutral Point Clamped (NPC) three-level inverter has been applied in industry as a mature technology, but suffering from the neutral point voltage (NPV) fluctuation problem especially for medium voltage vector charging. This paper deals with the common-mode voltage (CMV) issue of a hybrid inverter, which consists of a classic NPC inverter connected to a flying capacitor (FC) leg. The FC leg has the capability to actively balance the NPV in each switching cycle, and the NPC inverter can maintain NP voltage balance without sacrificing degrees of freedom. This paper introduces the modulation-based method of CMV elimination applied to the hybrid inverter, which can immensely reduce the CMV and leakage current. Simulation and experimental results are provided to validate that the hybrid inverter has better ability of NPV balance than the regular NPC inverter and the FC-leg can obtain much smaller NPV ripple. CMV attenuation can be achieved by applying common-mode elimination PWM in hybrid inverter.