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Anti-Islanding Protection in Three-Phase Converters using Grid Synchronization Small-Signal Stability

Fig. 1. The proposed islanding detection method.
It is required in IEEE Std. 929-2000 and 1547-2003 that the distributed generation unit below 10 MW rating, such as solar inverters, has to effectively detect the islanding condition. CPES researchers present the islanding detection algorithms for three-phase grid-interface converters based on the grid synchronization small-signal stability. To study the grid synchronization behavior at the islanding condition, a low-frequency small-signal phase-locked loop (PLL) model is proposed by considering the converter interaction with the system impedances. Many of the frequency-related behaviors are explained and the crucial system parameters which impact the PLL stability are quantified by the proposed model as well. Based on the model, new islanding detection algorithms based on the small-signal stability theory are proposed to effectively detect the islanding event under the worst loading conditions. The proposed algorithms feature a small impact to the converter system operation as well as an easy implementation.

Fig. 2. Experimental test result showing the typical PLL output (left) and modified PLL output when islanding event occurs.