H∞ current control strategy for the neutral point of a three-phase inverter

In this paper, an H^∞ current controller is proposed to maintain a balanced neutral point for a three-phase four-wire inverter, which can be used in microgrid applications. The neutral-point circuit consists of a conventional neutral leg and a split DC link. The neutral point is balanced with respect to the two DC source terminals (as required, e.g., in neutral-point-clamped three-level converters) even when the neutral current is large so that the inverter can be connected to an unbalanced load and/or utility grid. The controller, designed by using the H^∞ control techniques, can eliminate the current flowing through the split capacitors. This leads to very small deviations of the neutral point from the mid-point of the DC source, in spite of the possibly large neutral current. Experimental results are presented to demonstrate the excellent performance of the proposed control scheme.