Four level nested neutral point-clamped (4L-NNPC) converter is a new type of multilevel converter with a simple topology

no series-connected power devices

and a common DC bus. It is very suitable for DC 10kV high-power density applications. However

in relevant research on this topology

the voltage fluctuation of the floating capacitor under existing voltage balancing modulation and control strategies has not been studied

and the mechanism of voltage balancing error has not been clarified. This paper takes the multiphase four-level nested neutral point-clamped H-bridge inverter for marine propulsion as the research object

and discusses the capacitor voltage under the current voltage balance control in both high-frequency and low-frequency conditions by taking carrier based three-level pulse width modulation (CB3L PWM) strategy as example. Aiming at the capacitor high-frequency voltage regulation error

a threshold selection method is proposed. Finally

a three-phase H-bridge 4L-NNPC simulation model and a 15kW-level 4L-NNPC prototype are built. The above theoretical analysis and threshold selection method are verified by simulation and experiment under extremely low-frequency and high-frequency output conditions

and the fluctuation of capacitor voltage has been reduced by 30% after optimization under high-frequency operating conditions. The simulation and experimental results verify the feasibility and correctness of the proposed modulation strategy.