An Islanding Detection Method Using Synchronized Small-AC-Signal Injection for Grid-Forming Inverters in Microgrids

Islanding detection is of great importance for the microgrid in order to change the control objectives and to avoid unsynchronized reclosure. Active islanding detection methods based on impedance measurement have very small nondetection zone in the single-inverter system, but require the coordination of the perturbation current injected by each inverter in the microgrid consisting of multiple inverters. The conventional solution by injecting a harmonic current synchronized with the fundamental output voltage is affected by different winding connections in grid-side transformers of inverters and degrades the voltage quality in stand-alone mode. This article proposes an islanding detection method based on impedance measurement for the microgrid comprised of grid-forming inverters with seamless transfer capabilities. The proposed method achieves synchronized injection of small-ac-signal (SACS) current using the SACS current droop control and the virtual impedance. The synchronized SACS current injection in the proposed method is independent of fundamental components and is not influenced by different winding connections in grid-side transformers of inverters. Moreover, the SACS current limiting control is introduced to mitigate the impact of injected SACS current on the grid current quality. In addition, the control scheme is peer-to-peer and enables plug-and-play functionality. The effectiveness and advantages of the proposed method are validated through simulation and experimental results.