Islanding Detection Method of Low Voltage Microgrid Based on Harmonic Impedance Measurement

Since the impedance characteristics of peer-to-peer low voltage microgrids are different from those of conventional microgrids and are resistive, P-U and Q-f droop control inverters are used. Current islanding detection methods are mainly applicable to inverters based on direct current control. By researching the limitations of passive islanding detection and active feedback islanding detection methods in droop control inverters, an islanding detection method based on harmonic impedance measurement is proposed. First, the alternative injected harmonics are obtained by frequency sweep analysis of harmonic impedance, according to which the injected harmonics are selected in combination with the background harmonics of the grid. It is measured whether the harmonic impedance value at the common coupling point exceeds the threshold value to determine whether islanding occurs. Second, an improved harmonic injection method is obtained by extracting the rated load current variation in real time, which effectively identifies the disturbance of load fluctuation and reduces the overall time of harmonic injection to improve power quality. Finally, the effectiveness of the proposed method is verified by simulation.