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.