Analysis Of Influence Of Controller Parameters On The Impedance And Stability Of AC-DC-AC System

The limitations of traditional fossil energy have prompted the application and development of various new energy generation,which makes the application of AC-DC-AC system more and more extensive.However,with the wide application of AC-DC-AC system,the problem of stability and safe operation has become more and more significant.Therefore,the research on the safe and stable operation of AC-DC-AC system has become the focus of many researchers.The factors that influence the stability of system provides theoretical support for guiding system stability control methods and improving system stability,and has important research significance.In this paper,the intersection of two three-phase grid-connected converter interconnection system as the research object,because the equivalent impedance analysis method of the system rectifier side is similar to the inverter side of the system The establishment of equivalent impedance model and the influence of controller parameters on the system equivalent impedance and stability are analyzed.The main work of the dissertation is as follows:First,this paper simplifies the processing of the AC-DC-AC system,and analyzes the small-disturbance modeling of the inverter's phase-locked loop.Due to the existence of a phase-locked loop in a three-phase system,the system exhibits a dual disturbance frequency output characteristic with a single disturbance frequency input.The harmonic linearization method was applied again.The complex impedance of the rectifier and the inverter was modeled respectively.The impedance of the rectifier port on the AC side and the port admittance model on the DC side of the rectifier were obtained.The admittance model of the port on the DC side of the inverter was inversed.Transformer AC side port impedance model.Based on the established impedance model of the rectifier and inverter ports,a complex impedance impedance matrix model of the system is established.Finally,the equivalent impedance model of the AC side of the inverter in the complex frequency domain is obtained.By injecting perturbations at the PCC point and measuring the corresponding frequency grid-connected current perturbation output,the harmonic impedances of the rectifier and the inverter at various frequencies are obtained.The impedance points obtained by the frequency sweep and the analytical curves corresponding to the impedance model are plotted in the same logarithmic coordinate system for comparative analysis,and the correctness of the model is verified.Secondly,the paper analyzes the cut-off frequency of the rectifier current inner loop and voltage outer loop,the cut-off frequency of inverter phase-locked loop and current inner loop to the system impedance.By plotting the planar Bode diagram and multidimensional Bode diagram of the system impedance,a comparative analysis of the single control parameters is carried out to obtain the results of the influence of the controller parameters on the system impedance: 1.Rectifier current inner loop,voltage outer loop cut-off frequency have no obvious influence on the impedance of the AC-DC-AC system.2.The phase-locked loop and current-loop of the inverter cut-off frequency have a significant influence on the impedance of the AC-DC-AC system.In the low frequency band,increase the bandwidth of the phase-locked loop of the inverter and the current inner loop,the amplitude and phase of the system impedance will change;in the high frequency band,increase the bandwidth of the phase-locked loop and current inner loop of the inverter,and the system impedance amplitude There is no change in the value phase.Finally,this paper analyzes the stability of the system based on the established impedance model.According to the Nyquist stability criterion,the Nyquist curve of the ratio of the grid impedance to the system impedance is plotted in the complex plane,and whether the system is stable is determined by observing whether the curve includes the point of(-1,j0).Finally,the analysis results are obtained: 1.The cut-off frequency of the rectifier current loop and voltage loop have no significant effect on the stability of the system;2.The cut-off frequency of the inverter phase-locked loop and the current inner loop controller are closely related to the system stability.Increasing the bandwidth of the inverter phase-locked loop and the current inner loop makes the system more and more unstable.