Controlling Common-mode Voltages in Multilevel Inverters

This paper proposes a novel method based on pulse width modulation techniques to reduce and control the common-mode voltage in three-phase multilevel inverters. Besides controlling the common-mode voltage, this method is capable of controlling capacitors voltages and load currents with low switching losses and harmonic contents. In fact, due to the existence of different pulse patterns and the possibility of choosing the optimum patterns based on the most important criteria, the controllability of this method has risen properly. Furthermore, the controller randomly selects one of the optimum pulse patterns, which has another advantage of spreading the spectrum contents of the output voltage for low electromagnetic interferences and mechanical vibration. This method can be applied to a three-phase, three-level inverter with the flying capacitor topology. Another advantage of this method is that the technique can be applied to more voltage levels without significantly changing the control algorithm. The simulation results of a five-level inverter in this paper indicate that the proposed technique can be used to implement a multilevel inverter.