A Novel Islanding Detection Method for Distributed PV System Based on PMUs

With the rapid development of distributed PV systems in distribution grids, unintentional islanding is one major concern which maybe cause safety hazards. The existing passive islanding detection methods are mainly based on the voltage and frequency deviations during islanding conditions. Passive methods have relatively large non-detection zone (NDZ) and susceptible to power system faults. Active islanding detection methods applied in inverters have relatively small non-detection zone, but they are usually harmful to power quality. This paper presents a new islanding detection method based on equivalent inter-connection line impedance, which is normally a small value. A measured impedance is calculated with the synchronized phasors from two mu PMUs which are installed in a utility substation and a PV system. Under the grid-connected condition, the measured impedance is no more than the equivalent line impedance. While under the islanding condition, the measured impedance is much larger than the equivalent line impedance. So, an islanding condition can be effectively detected by the comparison of the measured impedance with the equivalent line impedance. The non-detection zone of this proposed method is nearly zero. The detection time is normally less than 250ms. This method is applicable for PV systems connected to the power system with a distribution transformer. It is also applicable for a distribution grid with multiple distributed generation systems.