Four-leg Inverter with Reduced Order Generalized Controller for Unbalanced Load Detection and Compensation

Abstract

Unbalanced load condition occurs in a three phase four wire microgrid system due to uneven distribution of single phase loads. This unbalanced loading causes negative and zero sequence currents in addition to the positive sequence current in the grid. The four leg inverter can be controlled to provide negative and zero sequence currents to address this issue. The positive and negative sequence currents are generally detected using phase-locked loop (PLL) and d-q transformations for controlling the inverter. However, these detection techniques cause delay and phase error. In this paper, an unbalanced current detection technique using reduced order generalized integrator (ROGI) is proposed. The proposed method accurately and rapidly extracts the positive, negative and zero sequences of the load currents and compensates the unbalance. In addition, an improved vector decomposition technique using two-dimensional voltage space vectors has been proposed to modulate the four-leg inverter. Thus, the modulation of four leg inverter is simplified and computational time of controller is reduced. The proposed method has been validated by simulation and experiments.