Passive Islanding Detection Approach Based on Tracking the Frequency-Dependent Impedance Change

This paper presents a passive islanding detection approach based on measurements of the frequency-dependent impedance at the point of common coupling. The approach exploits the presence of harmonics injected by the electric power system (EPS) and harmonics injected by the distributed generator (DG). Models of impedance are developed to characterize the interconnection topology among the DG, the EPS, and the local load. The interconnection topology changes when islanding occurs resulting in a frequency-dependent change of the impedance as seen by the DG. Islanding is detected when certain changes in the impedance magnitude are detected at certain harmonics. The approach is new: 1) an impedance-based analytic model is derived from the interconnection topology and this may be used as a basis for feature extraction and threshold selection; 2) a passive algorithm is proposed as an enhancement to the under/overfrequency (UF/OF) method; 3) a grid impedance parameter space is derived to characterize the detection zone; and 4) a local load parameter space is used to compare the nondetection zone (NDZ) of the proposed method with that of the UF/OF method without the enhancement. The proposed approach is shown to reduce the NDZ when combined with the UF/OF method.