Research On Control Strategy Of LCL Grid-connected Converter Considering Weak Grid Conditions

In recent years,with the advancement of industrial technology,the country has developed renewable energy on a large scale.At the same time,power electronic technology has been rapidly developed,and distributed power generation systems have been widely adopted.At the end of the power grid,far away from the load center,the power grid exhibits weak grid characteristics,which are characterized by a large grid impedance value,a low system short-circuit capacity,and a large number of background harmonics.As a key component of a distributed power generation system based on renewable energy,grid-connected converters are essential for safe and stable operation.When considering weak grid conditions,grid-connected converter systems are prone to resonance and seriously affect the grid-connected current.Waveform quality.This article takes the LCL three-level grid-connected converter as the research object,on the basis of completing the interface filter selection,establishing the main circuit mathematical model,resonance suppression,and phase-locked loop on the current control system,considering the weak grid conditions The stability of the grid-connected converter was studied in depth,and an improved current control strategy was proposed.This article first compares the advantages and disadvantages of commonly used L-type filters and LCL-type filters used in grid-connected converters,analyzes the working principle of LCL-type grid-connected converters,and establishes LCLtype grid-connected converter The mathematical model of the main circuit in the static abc coordinate system and the rotating dq coordinate system;for the resonance spike problem of the LCL interface filter of the grid-connected converter,the traditional passive damping and active damping resonance suppression strategies are compared and analyzed.The defects of the traditional damping strategy under weak power grid conditions are analyzed theoretically and simulated for the traditional proportional integral,proportional resonance and vector resonance controllers.It is pointed out that the traditional proportional integral and proportional resonance controllers are applied to grid-connected converters under weak grid conditions Deficiencies.On this basis,an improved active damping resonance suppression strategy combining traditional active damping strategy and VPI control is proposed to weaken the effect of grid impedance on the active damping resonance resonance peak.Finally,a simulation model of the LCL gridconnected converter system was built,and the accuracy and implementability of the improved resonance suppression strategy were verified through simulation experiments.Then,in view of the problem that the grid-connected converter current control system will be coupled with the phase-locked loop,a grid-connected converter current control system model considering weak grid conditions is established,and on this basis,the grid-connected converter is established The small-signal model indicates that the phase-locked loop introduces a disturbance signal into the current control system through the modulation voltage Um and the grid current feedback I2,so that the control performance of the original current control system cannot reach the ideal state,resulting in a decrease in system stability.It compares the advantages and disadvantages of the commonly used system stability analysis methods,and combines the traditional transfer function method and the Nyquist criterion to determine the stability criterion of two system stability analysis methods to stabilize the grid-connected converter system considering weak grid conditions Analysis and detailed analysis of the impact of different nature of grid impedance on the converter system.According to the small-signal model of the converter considering the disturbance of the phase-locked loop,the impedance equivalent circuit of the grid-connected converter system considering the weak grid conditions is given.According to the Nyquist stability criterion,a method based on the equivalent Impedance converter stability criterion.Finally,because the disturbance of the phase-locked loop is one of the important reasons that affect the current control accuracy of the LCL grid-connected converter under weak grid conditions,this paper analyzes the traditional current control strategy considering the phase-locked loop disturbance compensation and points out its shortcomings.An improved current control strategy for phase-loop small-signal disturbance compensation was carried out.Simulation experiments were conducted for grid impedances with different inductance values.The accuracy of the proposed current control strategy for the LCL grid-connected converter was verified through experiments.