A Current Distortion Compensation Scheme for Four-Switch Inverters

The four-switch inverter, having a lower number of insulated gate bipolar transistors (IGBTs), has been studied for the possibility of reducing the inverter cost. But it has a limited performance in the low-frequency region, because the balance among the phase currents collapses due to the fluctuation of the center tap voltage of the dc-link capacitors. This problem could be solved if the dc-link capacitance is infinitely large, but it is a costly solution. In this paper, this problem is looked at front the perspective of source impedance and the voltage variation caused by the current flow through the capacitors. The source impedance of the center tap is large compared with other normal IGBT arms. This causes an asymmetry among the three voltage sources, resulting in phase current distortion and unbalance. Second, the capacitor voltage change caused by current flow is another source of current distortion and unbalance. The voltage errors are derived, and based on them, a compensation method is proposed. Effectiveness of the proposed method is demonstrated by the simulation and experimental results.