The Control And Realization Of Three-Phase Four-Leg Inverter Under Unbalanced Loads

Compared with traditional three-phase three-leg inverter, the three-phase four-leg inverter can provide a path for zero sequence current, to deal with unbalanced loads. Under these conditions how to keep three phase output voltages symmetrical is the main focus of this paper.To facilitate the analysis of control system, the mathematic models of three-phase four-leg inverter and unbalanced loads in stationary and rotating coordinate frame systems have been established. The fourth leg provides a public access for three phase inductance current. So it greatly affects the switch's trigger action of other legs. The effects on mathematic models of three-phase four-leg inverter have also been discussed in this paper.Repetitive control scheme is widely used to reduce the voltage distortion with nonlinear and unbalanced loads. A PI-P double loop with repetitive control scheme in stationary coordinate frame system is adopted in this paper, which has been used to control the inverter under various unbalanced loads conditions. Simulation and experimental results all show that the system has good dynamic and static performance by use of this control strategy.According to the characteristics of resonant controller that tracking AC signals with non-static error by building the internal model of controlled signal in system, a new PIR-P double loop control scheme in rotating coordinate frame system is proposed. And a new controller design method based on the engineering design criteria is also proposed in the paper. Finally, simulation and experimental results verify the feasibility of this control method.