Enhancement of Line-to-Line Voltage Support During Asymmetrical Microgrid Faults Using a Four-Leg Three-Level Inverter

Abstract

Recent research is focused not only to keep a three-phase inverter connected to the grid but also to support the voltage at the point of common coupling (PCC) during grid faults. The line-to-line PCC voltage can be enhanced by increasing the amplitude of positive and negative sequence current supplied to the grid for a given inverter rating. 3-leg inverters can supply only positive and negative sequence current to the grid. However, 4-leg inverters have the ability to supply zero-sequence current along with positive and negative sequence currents. This extra degree of freedom of control remains un-utilized during low voltage ride through operation. This paper presents a novel method to select the amplitude and phase angle of the zero-sequence current in such a way that the peak current amplitude of the 4-leg inverter remains minimum. Hence, it is possible to supply more positive and negative sequence currents to the grid. The proposed strategy can be applied to any asymmetrical grid fault condition. Experimental verification for two specific fault conditions has been carried out and results are presented.