Phase-oriented control of a modular 3-Phase 3-level 4-leg inverter AC power source supplying floating or grounded loads

A control scheme for a high-performance three-phase AC power source is presented. The four-leg inverter output stage uses three bridge legs to generate the phase output voltages with reference to the neutral point potential, which is defined by the fourth bridge leg. The inverter is controlled using a phase-oriented control in order to achieve precise control of each output phase with unbalanced voltages and any kind of load. The neutral potential is controlled for maximum modulation range in case of floating load and controlled to zero for grounded load. The control scheme for the input PWM rectifier stage considers the control of the zero sequence voltage in order to prevent the appearance of circulating currents through the ground loop when the load star point is connected to ground. The control of the circulating currents also allows the safe parallel connection of two or more AC sources. Simulation results under balanced and unbalanced conditions demonstrate the performance of the AC source in terms of output voltage control and prevention of circulating ground currents.