Research On Software Phase Locked Loop Under Faulty Grid

Increased energy demand and environmental problems have greatly promoted the development of renewable energy generation systems represented by solar and wind energy.Software phase locked loop(SPLL)is one of the important technologies of grid-connected renewable energy generation systems.The information of voltage phase,frequency and amplitude detected by SPLL is the basis of grid-connected control.Though the synchronous reference frame SPLL has good dynamic-state and steady-state characteristics under ideal grid condition,the large error of synchronous information detected by synchronous reference frame SPLL still exists under faulty grid conditions.Therefore,the SPLL technology under faulty grid conditions is further researched in this paper.Firstly,this paper studies the basic principle,mathematical model,dynamic and static performance of synchronous reference frame SPLL.Besides,the errors of synchronous reference frame SPLL under unbalanced voltage amplitude,harmonic distortion and DC bias conditions are analyzed in detail.Furthermore,the basic structure and working principle of dual second order generalized integrator SPLL and enhanced SPLL applied in unbalanced voltage amplitude condition are introduced.Secondly,in order to overcome the shortcomings of dual second order generalized integrator SPLL under conditions of harmonic distortion and DC bias,through cascading self-tuning filter,using integral feedback signal eliminate DC bias,and utilizing a modified frequency feedback structure,a modified dual second order generalized integrator SPLL is designed.To solve the problem that the grid synchronous information can’t be detected accurately by three-phase enhanced SPLL under harmonic distortion and DC bias conditions,the improved complex coefficient filter based on DC suppression is used to enhance the performance of harmonic elimination and DC suppression.At the same time,the low-pass filter is used to further reduce the influence of harmonics,and a modified three-phase enhanced SPLL is designed.Finally,the simulation models of synchronous reference frame SPLL,dual second order generalized integrator SPLL,modified dual second order generalized integrator SPLL,three-phase enhanced SPLL and modified three-phase enhanced SPLL are built in MATLAB/Simulink.And the simulation results of each phase-locked method under the grid fault conditions are compared and analyzed.On this basis,the experiment platform based on TMS320F2812 is built,the SPLL system program is designed and the experiment of SPLL under grid fault conditions is carried out.The correctness of the proposed modified method is verified by the simulation and experimental results.