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A Novel Anti-Islanding Detection Algorithm for Three-Phase Distributed Generation Systems

Fig. 1. Grid-interface converter under study.
This paper presents a novel anti-islanding (AI) detection algorithm for three-phase (3φ) grid-interface converters, as illustrated in Fig. 1, in distributed generation systems. The synchronous reference frame PLL is widely used, as shown in Fig. 2. According to the analysis, the PLL output frequency can be directly observed to detect the islanding event, since the output will run out of the steady state (60Hz).

However, the PLL will be stable at 60Hz after the islanding event if the local load is the RLC load with 60 Hz resonant frequency. In order to effectively detect the islanding condition under this load, the SRF PLL is modified to be unstable when islanding occurs. As shown in Fig. 3, an additional feedback loop is used to destablize the PLL under islanding conditions.

As shown in Fig. 4, when islanding occurs, the PLL output will be unstable using the proposed method. The proposed algorithm has a small impact on the converter system operation and can be implemented easily.

Fig. 2. Synchronous reference frame PLL.
Fig. 3. Proposed PLL for anti-islanding detection.
Fig. 4.The proposed PLL output when islanding occurs: ω2 (purple) [6 Hz/DIV], one-phase voltage va (maroon) [100V/DIV].
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