Abstract
Islanding detection is critical to the safety of grid-tied inverters. This paper presents an impedance-based analysis of the active frequency drift (AFD) islanding detection method. To study this method, the output impedance of a grid-tied inverter is modeled. The $Z_{qq}$ of the inverter impedance features a negative incremental resistor, whose magnitude is equal to the dc impedance magnitude of the local load defined by the test standard. With a large value of feedforward gain $N$ in the AFD method, the phase of $Z_{qq}$ is shown to drop below 180°. Under an islanding condition, due to a lack of phase margin, the inverter system becomes unstable with a frequency drift away from its steady state. The islanding condition is then identified by a frequency detection unit. Experimental results verify the analysis.
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