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

Islanding detection is critical to safety and power quality as well as the control mode of grid-interface power converters. This paper addresses the 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 the existing literature and a cut-and-trail 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 solution 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 to the converter´s operation is also analyzed.