Midpoint Voltage Balanced Control for Three-Phase Four-Wire Inverter Based on Three-Phase Generalized Coordinate Transformation

The primary objective of this study is to implement three-phase generalized coordinate transformation on a three-phase four-wire inverter, each phase of which supplies an independent load, aiming for a simple control structure and enhanced control effect. In the condition where the three-phase four-wire inverter operates in an isolated manner, the three-phase unbalanced loads leads to current fluctuation in the neutral line. The fluctuation subsequently offsets midpoint voltage of the split capacitors. The issue can be addressed by increasing the capacitor size. However, it inversely impacts the power density of the PWM inverter due to the increased volume. To address these challenges, an effective approach involves continuously adjusting the amplitude ratios and phase differences of the inductor currents based on the amplitude ratios of the capacitor voltages. This adjustment not only ensures equal capacitor voltages amplitudes, but also regulates the zero-sequence current to zero, thereby achieving minimal the midpoint voltage fluctuation of the split capacitors, and allowing for a significantly reduced thickness of the neutral line and smaller sizes of the split capacitors. Finally, the efficacy of the proposed scheme is validated through simulations.