An improved voltage source inverter with 18-step output waveforms

A new voltage source inverter with 18-step output waveforms is proposed in this paper in order to reduce harmonic components involved in the output voltages. Even though a conventional 18-step inverter employs three output transformers, the proposed circuit is no necessity for them. It is based on the neutral-point-clamped inverter composed of 12 switching devices. By dividing the DC voltage source into four pieces and adding four auxiliary switching devices, the proposed circuit can output almost the same waveforms as a conventional 18-step inverter. In this paper, the circuit performances are described and output waveforms are discussed. Then, the optimum parameters are derived and effects on harmonic reduction can be clarified by theoretical and experimental results. In addition, methods for adjusting output voltages are investigated.