Internal model principle based control of H∞ variable neutral-point in an inverter

A variable neutral point DC link of a 3-phase 4-wire DC-AC inverter is proposed in this paper to increase the utilization of the DC voltage. In order to decrease the control error which is mainly resulted from the 150Hz neutral voltage reference and the 50Hz unbalanced neutral line current, an internal model principle based control is introduced to eliminate the influence at these frequencies. A robust H^∞ controller is designed to maintain the stability of the control system and to reduce the influence of external uncertainties. Derivation of the variable neutral voltage reference, selection of frequency modes for the internal model, and H^∞ design process are introduced. Simulation and experimental results with a 30kW inverter are presented.