A Modulation Strategy to Operate Five-Level HC Inverter at Wide Power Factors With Enhanced Neutral Point Voltage Balancing Capability

Clamped multilevel inverters suffer significantly from neutral point (NP) voltage unbalancing in low power factor applications, which will lead to severe device overvoltage problems and affect its output performance. The five-level hybrid clamped (HC) inverter is a state-of-the-art topology with additional redundant switching states, which makes it possible to suppress the NP low-frequency voltage ripples better. Therefore, a novel and simple modulation for five-level HC inverter with enhanced NP voltage balancing capability is proposed. Based on the influence of NP currents on the dc-link capacitor voltages, the different switching states combinations with capacitor voltages naturally balancing constraints are analyzed comprehensively, and the relationship between the duty ratio of average NP currents and the reference modulation voltage is designed as a bi-triangular function to minimize the currents flowing out of the NPs. Compared with traditional modulation, the equivalent adjustment ability of NP currents is greatly enhanced by the zero-sequence voltage injection, and the low-frequency voltage ripples of DC-link capacitors are suppressed significantly at whole power factors. Moreover, the active balancing methods are explored to achieve independent and decoupling control of all capacitor voltages. Simulations and experiments verify the viability of the modulation strategy and the capacitor voltage balancing method.