Modulation and voltage balancing control of dual five-level ANPC inverter for ship electric propulsion systems

Abstract

Open-end winding motors are used extensively in ship electric propulsion systems, in which medium-voltage high-power inverters are a critical component. To increase the system voltage and power density, a dual five-level active neutral-point clamped (ANPC) inverter is proposed herein to drive medium-voltage open-end winding motors for ship electric propulsion. Each phase of this inverter comprises two five-level ANPC bridges and all the phases are powered by a common direct-current link. A hybrid modulation method is proposed to control this inverter. The series-connected switches in all the five-level ANPC bridges are operated at the fundamental frequency, and the other switches are controlled with a phase-shifted pulse-width modulation (PWM), which can achieve a natural balance between the neutral-point voltage and flying capacitor voltages in a carrier period. A closed-loop capacitor voltage balancing method based on adjusting the duty ratios of the PWM signals is proposed. The neutral-point voltage and flying capacitor voltages can be controlled independently and balanced without affecting the output phase voltage. Simulation and experimental results are presented to demonstrate the validity of this method.