A simplified 24-step inverter without output transformers

In this paper, we propose an improved multi-level voltage source inverter whose output harmonic components can be effectively reduced regardless of simple construction. It is based on the neutral-point-clamped (NPC) inverter. The input DC source is divided into six sections and an auxiliary circuit composed of six switching devices and four clamping diodes are added into the conventional NPC inverter. By controlling the auxiliary circuit appropriately, quasi 24-step voltage waveforms can be outputted from the proposed inverter though it has no output transformers. In this paper, the circuit performances are described and the output voltage waveforms are derived theoretically. The circuit constants and the control parameters for decreasing the output harmonics are discussed. It is made clear that the proposed inverter can be performed with the same output distortions approximately as the conventional 24-step inverter with 24 switching devices and the heavy output transformers. Moreover, in order to adjust the output voltages, the step width control and the PWM control are investigated theoretically and experimentally.