Parallel PI Voltage - H∞ Current Controller for the Neutral Point of a Three-Phase Inverter

For applications in renewable energy and distributed generation, there is often a need to have a neutral line to provide a current path for unbalanced loads. This can be achieved by using a neutral-point circuit that consists of a conventional neutral leg and a split dc link. In this paper, an H^∞ current controller is proposed to force the current flowing through the split dc link to be nearly zero so that the neutral-point voltage is stable, and then, a voltage control loop is added to eliminate the imbalance of the voltage so that the neutral-point voltage is balanced with respect to the dc terminals. The combination of these two controllers, connected in parallel, is able to maintain the neutral point at the midpoint of the dc bus even when the neutral current is large. Hence, the inverter can be connected to unbalanced loads and/or the utility grid. Experimental results are presented to demonstrate the excellent performance of the proposed control scheme. The proposed strategy can also be applied to neutral-point-clamped three-level converters.