Overview of SOGI-Based Single-Phase Phase-Locked Loops for Grid Synchronization Under Complex Grid Conditions

Synchronization is the key part to ensure the high performance of grid-connected systems.Phase-locked loop (PLL) is one of the most popular synchronizations due to its simple implementation and robustness under certain grid variations.Particularly, in single-phase applications, PLL based on second-order generalized integrator (SOGI-PLL) is widely used because of its simple structure, certain filtering ability and frequency adaptability.The sensitivity of SOGI-PLL to the dc offset and low-order harmonics has been studied a lot in the literature and many solutions have been proposed.However, as more and more power electronic devices are integrated into the power grid recently, the grid condition becomes more complex.As a result, the nonlinear loop coupling phenomenon of SOGI-PLL is more severe and obvious and thus the performance of SOGI-PLL is degraded a lot, especially under the conditions of grid frequency variations and weak grid.A popular method is to use the frequency-fixed SOGI-PLL (FFSOGI-PLL) while how to eliminate the estimation error under frequency variations remain an important task.Though some scattered methods have been proposed, a simple yet effective strategy is still missing.Besides, it has been shown that the system stability margin will be reduced due to the negative-resistance behavior of PLL.However, the models derived in the current documents cannot accurately reveal the instabilities caused by the standard SOGI-PLL for missing the dynamics of the frequency feedback loop.Moreover, the PLL parameters are usually designed according to the PLL bandwidth to guarantee system stability under a weak grid.How to optimize the parameters is still unclear.In view of this, this article further summarizes and reviews the existing achievements of single-phase SOGI-PLL, and points out the problems to be solved and the development direction to improve the SOGI-PLL under more complex and non-ideal grid conditions. INDEX TERMS Grid synchronization, single