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Modeling and Design of Islanding Detection Using Phase-Locked Loops in Three-Phase Grid-Interface Power Converters

Fig. 1. Modified PLL with small-signal feedforward for islanding detection
Islanding detection is critical to safety and power quality as well as the control mode of grid-interface power converters. This paper addresses islanding-detection methods on the basis of the grid synchronization instability mechanism. Although many modified phase-locked loop (PLL) methods have been proposed to achieve islanding detection, the PLL modeling and design are not clear in existing literature and a cut-and-trial process is usually required during the design phase. This paper proposes a systematic PLL modeling and design approach to evaluate different frequency-based islanding detection methods. Two different types of PLL-based islanding detection solutions are discussed, accounting for a majority of the existing methods. The first method is to modify the PLL to constantly move the stable equilibrium point. The second method is to modify the PLL small-signal characteristics to achieve a monotonic instability behavior under the islanded conditions. The design procedures of these methods are presented using the proposed PLL modeling approach. The impact of the modified PLLs on the converter's operation is also analyzed.

Fig.2 Modified PLL 2 Hz input step response with 1% 600 Hz grid harmonics
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