A harmonic current injection control scheme for active islanding detection of grid-connected inverters

Abstract

This paper presents a harmonic current injection control for active islanding detection of grid-connected inverters. A standardized islanding detection method called frequency feedback method with step reactive power injection is used in the inverter system for islanding detection. After an islanding event, this method measures the change of voltage harmonic distortion and injects reactive power to drive the frequency changing, however, in some case of islanding, the change of harmonic distortion is not large enough to meet the injection criteria, and there are the possibilities of non-detection. Especially in the case of three-phase system, because 3rd order harmonic distortion almost doesn't exist, the possibilities increase. To solve this problem, a 5th order harmonic current injection control based on d-q frame is proposed. The harmonic current is synchronized with the excitation current harmonic element of line frequency transformer. With this control, after an islanding event, voltage harmonic distortion increases, by measuring the increase, step reactive power injection function operates and the islanding can be detected according to the standardized method. The proposed harmonic current injection control and the islanding detection have been experimentally verified on a three-phase grid-connected PV inverter system.