Abstract
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.
Highlights
Due to the increasing depletion of fossil fuels and the environmental problems caused by their large-scale usage, renewable energy power generation has received extensive attention all over the world
In order to solve the inherent double-frequency oscillation phenomenon in the power-based PLL (pPLL), scholars turn to the Phase-locked loop (PLL) based on the adaptive filter (AF-PLL) like enhanced PLL (EPLL) and the PLLs based on orthogonal signal generator (OSG-PLL)
The SOGI-PLL can be a high-performance single-phase PLL used in a wide range of applications
Summary
Due to the increasing depletion of fossil fuels and the environmental problems caused by their large-scale usage, renewable energy power generation has received extensive attention all over the world. The introduction of the filter greatly reduces the PLL bandwidth, resulting in a poor dynamic performance of this type of PLLs. In order to solve the inherent double-frequency oscillation phenomenon in the pPLL, scholars turn to the PLLs based on the adaptive filter (AF-PLL) like enhanced PLL (EPLL) and the PLLs based on orthogonal signal generator (OSG-PLL). The APF-PLL uses an all-pass filter to shift the phase of the fundamental signal by 90°, but it does not perform well in presence of the frequency changes and harmonics. As the increasing integration of renewable energies and power electronic devices into the power grid, some other non-ideal grid conditions like frequency variation and weak grid are more severe and these new issues can affect the performance of SOGI-PLL a lot as well.
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