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The paper describes the addition of a fourth leg to the bridge of a three-phase inverter for the purpose of eliminating the common mode voltage to ground. An appropriate four-phase filter is used to eliminate common mode currents due to modulation. With a suitable modulation strategy and a three-phase LC filter on three legs of the inverter, sinusoidal output line-to-line voltages are obtained. A simple modification of the modulation strategy is implemented for the four-phase inverter to achieve a three-phase star output neutral to ground voltage which is equal to zero at all times. The modulation strategy thereby completely eliminates the common mode potential produced by traditional modulation techniques with traditional inverter topologies.
Julian et al. (Mon,) studied this question.