Research On Three-phase Phase-Locked Loop Under Unbalanced Power Grid

Due to the popularity of distributed generation systems,how to improve the performance of grid-connected converters has become a hot research topic.Whether the grid-connected converter can accurately obtain the phase-frequency information of the grid voltage is critical when the grid voltage signal is asymmetric,including harmonics or phase-frequency mutation.At present,the most widely used grid-connected synchronization technology is the phase-locked loop technology.Whether the phase-locked loop technology can quickly and accurately obtain the amplitude and phase of grid voltage is the key to the stability and reliability of the whole grid-connected converter system.Therefore,the design of a phase-locked loop that can adapt to various power grid faults is the focus of current research.Firstly,this thesis studies the basic working principle of PLL.Through detailed analysis of three components of PLL: phase discriminator(PD),loop filter(LPF),voltage controlled oscillator(VCO),the phase model of PLL is obtained.By analyzing the phase model,it is found that PLL is a negative feedback control system,and most of them are second-order PLL.Then the vector diagram and control structure of the phase-locked loop(SRF-PLL)based on single synchronous rotating coordinate system are analyzed,and its small signal model is obtained.The principle and structure of the phase-locked loop(DSOGI-PLL)based on double second-order generalized integrator are studied,and an adaptive filter adapting to the change of frequency is constructed to solve the problem of its inability to lock phase quickly,and an improved double-second-order adaptive filter is designed.The structure block diagram of generalized integrator phase-locked loop(DSOGI-FLL-PLL);the vector diagram of three-phase unbalanced voltage in double synchronous rotating coordinate system is analyzed,and the decoupling network structure is deduced.The structure block diagram and small signal model of double decoupled synchronous rotating coordinate system phase-locked loop(DDSRF-PLL)are obtained.To improve the harmonic suppression ability of DDSRF-PLL,an improved phase-locked loop based on DDSRF-PLL is proposed.Loop: A post-transform filter is introduced into DDSRF-PLL to make the phase-frequency information pass through a low-pass filter before output,so as to improve the phase-locked performance,and its structure block diagram is designed.Finally,the above four PLLs are simulated by using Matlab/Simulink under two-phase voltage drop,harmonic and DC bias conditions.By comparing the simulation results,it is concluded that SRF-PLL can't lock phase in the fault grid environment;DSOGI-FLL-PLL and improved DDSRF-PLL can adapt to harmonic disturbance and accurately track the phase-frequency information of voltage in the voltage environment with low-order harmonics;and improved DDSRF-PLL can eliminate the influence of DC bias and achieve fast precision in the case of voltage with DC bias.Figure [43] Table [6] Reference [57]...