Stability Control Method Of LCL Type Grid-tied Inverter Considering The Influence Of Phase-locked Loop

Scattered distributed generations need to be connected to the public grid through a large number of transformers and long transmission lines,which leads to the increase of the equivalent impedance of the public grid,and the grid is increasingly showing weak grid characteristics.With the gradual increase of the grid impedance,the coupling phenomenon between the phase-locked loop and grid impedance is intensified,which may cause harmonic current and even cause system instability.Based on the impedance model,this paper analyzes the instability mechanism of the weak grid system caused by the phase-locked loop,and proposes control methods to improve the system stability.The main research contents of this paper are as follows:(1)This paper summarizes the stability analysis methods of grid-tied inverter,investigates the influence of phase-locked loop on grid-tied system stability and the improvement methods in weak grid.The output impedance model of a single-phase LCL type grid-tied inverter with PLL is established.Based on the impedance stability criterion,the influence of PLL bandwidth,structure and different grid impedance on the stability of grid-tied inverter in weak grid are analyzed in detail.The simulation verifies the correctness of the modeling and grid-tied inverter stability analysis process.(2)Although the series low-pass filter in the phase-locked loop can improve the stability of the system,it will lead to the phase lag of the grid-tied current.Based on the equivalent transformation and characteristic analysis,this paper proposes an impedance reshaping method based on current second-order correction.A multi-objective constraint function of fundamental amplitude and phase constraints,weak grid stability constraint and impedance intersection constraints is established to optimize the design of secondorder correction link parameters.The proposed method improves weak grid stability of grid-tied inverter,eliminates the phase lag of the grid-tied current caused by low-pass filter,and achieves unity power factor grid-tied in weak grid.(3)For grid-tied inverter with grid voltage feedforward,the negative damping characteristic of grid voltage feedforward and phase-locked loop are considered respectively,and a phase-locked loop structure with bandpass filter is proposed based on the retrofit of phase-locked loop.And the damping coefficient of the bandpass filter is designed based on the system stability margin and power factor.For the limitation of the phase increase of the band-pass filter,an adaptive grid voltage control method with proportional and differential feedforward is proposed.The phase margin under different impedance is used to adaptively adjust the coefficients of the grid voltage feedforward term.With the method,the low-frequency amplitude of the inverter output impedance is improved,the impedance remolding frequency band is extended and the system stability in weak grid is improved.(4)Matlab/Simulink is used to build the simulation model of a single-phase gridtied system.Both of the impedance remodeling method based on current second-order correction and the grid-tied inverter stabilization method based on adaptive composite impedance correction are verified.The experiment platform of grid-tied inverter based on Star Sim real-time simulation system with TI DSP TMS320F28335 as the control core is established to realize the impedance remodeling method based on current second-order correction.And the feasibility of the proposed control strategy is verified.