Research On Single-phase Phase-locked Loop Under Harmonic Distorted Power Grid Condition

In recent years,the rapid development of science and industrial technology has brought convenience to people’s lives,but it has also been accompanied by serious global energy crisis,climate warming and other problems.Vigorously developing new source of energy and promoting energy reform are expected to become an important means to solve the global energy crisis.In the new energy grid-connected system,the performance of the grid synchronization unit will directly affect the stability of the entire grid-connected system.Power grid synchronization technology can be divided into many types.This paper mainly studies power grid synchronization technology based on single-phase PLL.First of all,based on the analysis of the basic principles and mathematical models of the single-phase PLL,the dynamic and steady-state performance of the PLL are studied in detail,and it is emphasized that the single-phase PLL based on synchronous reference frame will produce phase error under harmonic distortion power grid,and cannot achieve precise and rapid phase-locking.APF-PLL is a widely used grid synchronization method in grid-connected power converters.However,the performance of APF-PLL degrades under distorted grid conditions and the estimated phase and frequency will contain low-frequency oscillation terms,which is not conducive to practical application.To deal with this problem,this paper proposes two advanced APF-PLLs,which have good ability to suppress harmonics and dc offset interference without sacrificing the dynamic response of the system.In the first one,a band-pass filter is used as the prefilter of APF-PLL to process the input voltage signal,and in the second one,the APF-PLL input signal is processed by using a harmonic decoupling network and multiple prefilters.In addition,their model derivation and parameter design procedure are described in detail.The simulation and experimental results presented in this paper finally verify the superiority and effectiveness of the two proposed methods.In order to further improve the dynamic performance of the PLL,using SDFT as a prefilter,this paper proposes an improved single-phase synchronous reference frame PLL with a fixed sampling frequency,which can accurately and quickly obtain the grid parameters under distorted grid conditions.Most importantly,the paper theoretically analyzes phase and amplitude errors generated by SDFT for the first time and a quantitative compensation method,which is easy to implement.Finally,the proposed SDFT-PLL is compared with other PLLs through simulation and experiments.The experimental results show the effectiveness and practicability of the proposed SDFTPLL.In harmonic distortion power grid,SDFT-PLL is a widely used method for designing advanced PLL.The SDFT-based PLL,nevertheless,has a limited ability in achieving fast response(a response time about one cycle of the fundamental frequency).To deal with this problem,HCDFT-PLL is proposed,enabling a fast and accurate synchronisation even in distorted grid.In this study,the fundamental voltage of the PLL input signal is obtained by a HCDFT filter.Then,phase,frequency and amplitude can be detected by the improved synchronous reference frame PLL.Besides,in order to solve the phase error by HCDFT when the frequency changes,a phase compensator is used.Finally,the experimental results show that HCDFT-PLL can accurately and quickly obtain phase information under grid voltage disturbances,and the response time is about half a fundamental frequency cycle.