A low-frequency fluctuation suppressing method for a seven-level hybrid T-type nested neutral point clamped converter

Abstract

This paper proposed a low-frequency fluctuation suppressing method for a seven-level hybrid T-type nested neutral point clamped (7L-HTNNPC) converter that can be applied in medium-voltage and high-power powertrain systems. The 7L-HTNNPC converter is a combination of a 3L-ANPC unit and a four-level T-type neutral point clamped (4L-TNNPC) unit. The 3L-ANPC unit is modulated to switch at a low fundamental frequency while the remained 4L-TNNPC unit is operated at a high switching frequency. Compared to the existing modulation method, the proposed method has the advantage of resuppressing flying capacitor voltage fluctuations and reducing current harmonics with a low flying capacitance. Simulation results have been presented to verify the proposed low-frequency fluctuation suppressing method for the 7L-HTNNPC converter.