Analysis of the Mechanism and Control of the Unbalanced Operation of Three-Phase Four-Wire Inverters

In this paper, a solution is proposed to the problem of the unequal phase imbalance of output voltage caused by a three-phase, four-wire, split capacitor inverter when the load is unbalanced. First, the triple-loop control strategy was used to solve the unequal amplitude problem. This method used the feedforward + feedback composite control strategy on the inductor current inner-loop and voltage mid-loop to decrease the disturbance of the power and load. And the Root Mean Square (RMS) of voltage on the outer-loop completed the control of amplitude for the three-phase voltage. Second, to solve the imbalanced phase problem, the imbalance operation mechanism of the three-phase four-wire inverter was analyzed. It is known from the analysis that the phase imbalance is related to the DC-side splitting capacitance. The function relations between the DC-side capacitance and phase angle between each phase was simulated by MATLAB. But, it was too complicated to calculate the magnitude of the capacitance value through the functional relationship. In order to simplify the design of the DC-side splitting capacitor, the relations among the imbalanced current, the voltage fluctuations of the DC-side capacitor and the harmonics of load voltage were analyzed. In addition, by following the requirement of the national standard about the harmonics of load voltage, a DC-side capacitor design was mentioned to decrease the influence of imbalanced phase. Finally, simulation and experimental results show that the three-phase load voltage is stable, the THD value is less than 3%, and three-phase voltage unbalance is less than 2%, thus verifying the effectiveness of the proposed DC-side split capacitor design and control strategy.