In renewable power generation systems

ensuring the synchronization of the inverter and the power grid is crucial for the stable operation of grid-connected inverters. Nowadays

the phase-locked loop (PLL) technology has become a widely used grid synchronization method because of its simple implementation and robustness under various grid conditions. Even though a lot of PLLs have been proposed

an overview and comparative analysis of multiple PLLs can be helpful for practical applications. In addition

the weak grid condition is a great challenge for the system. Therefore

this study first presents an overview of the existing PLLs together with their general structures and basic working principles. Depending on the implementation of the phase detector

the PLL can be divided into three categories: power-based PLL (pPLL)

orthogonal-signal-generator-based PLL (OSG-PLL) and adaptive-filter-based PLL (AF-PLL). Then

from the above classification

seven typical single-phase PLLs are selected for further study. Finally

some test results are given

and a comprehensive evaluation of the selected PLLs under different grid conditions is conducted.