Eliminating Common Mode Conducted Emissions in Three-Phase Four-Leg Inverters

This paper presents a control method to eliminate the common mode voltage created by a three-phase, four-leg voltage source inverter. Wide band gap devices create an opportunity to use pulse density modulation instead of pulse width modulation, with two inverter poles switching high and two low at all times to eliminate common mode voltage without increasing losses. In this way the common mode voltage is always zero to the precision that the inverter legs can be simultaneously transitioned. The novel control strategy also produces balanced output voltage to meet the power quality requirements in MIL-STD-1399 when the load is unbalanced. Elimination of the common mode voltage removes the need for a large common mode choke to suppress circulating currents and to meet the conducted emission limits in MIL-STD-461G. A state space model is derived, used to compute the inverter reference voltages and to simulate the proposed control system. Experimental measurements on a laboratory prototype validate the model and verify the functionality of the novel control method.