Improved Voltage Source Inverter with 18-Step Output Waveforms

In order to reduce harmonic components involved in the output waveforms of voltage source inverters, it is an effective countermeasure to increase the level number or the step number of waveforms by means of multi-sequential or multi-parallel connections [1, 2J. This method, however, is forced to increase the size and the weight of the apparatus considerably, because it necessitates several output transformers and numerous static switching devices. For example, a conventional 18-step inverter must be employed using three three-phase output transformers and 18 switching devices.