PWM control method for a four-level inverter

Several PWM techniques for three-level inverters have been introduced. However, multilevel inverters with more voltage levels have not been considered yet. The paper proposes a PWM control method for four-level inverters. The four-level inverter uses six power switches per leg, but the blocking voltage of each switch is equal to one third of the DC-link voltage. The control method is based on closed loop regulation of the line to line voltages. It uses independent hysteresis comparator controllers to regulate the direct and quadrature axis components of the three-phase output voltages. The vector selection is adjusted according to the input reference to improve the voltage generation. The DC capacitor voltage balance is considered, and a method to control the variations is introduced. Computer simulations and the result of an experiment are given to confirm the validity of the proposed control scheme