Analysis And Improvement Of Control Performance Of LCL Three-phase Grid-connected Converter

With the gradual increase of the scale of new energy grid connection,the power of the grid-connected converter is gradually increased.In order to reduce the switching loss,it is usually necessary to reduce the switching frequency of the converter.The delay of the digitally controlled grid-connected converter will become non-negligible as the switching frequency decreases,so it is necessary to further analyze the control performance of the LCL grid-connected converter.The dynamic characteristics of the system are mainly studied on the dq axis coupling problem,and the stability is mainly studied on the problem of low frequency oscillation.The specific issues include:(1)Giving the converter topology and control,using complex vectors to model converters of different power levels,and analyze the dq-axis coupling problems under different sampling methods and control variables through double side frequency response and pole-zero maps;Through the established complex vector model,the decoupling effect of the existing decoupling strategy under different sampling methods and control variables is studied,and its shortcomings are pointed out.Comprehensively compare the characteristics of coupling under different power levels,sampling methods,control variables and the effectiveness of decoupling strategies.Finally,the above theoretical analysis is verified by simulations and experiments.(2)According to the impedance theory of the cascade system,take the LCL grid-connected converter with inductor current control of the converter side and asymmetric regular sampling as an example to establish a small signal impedance model of the system;simply analyze different control strategies and sampling methods in impedance characteristics.Taking the established model as an example,analyze the influence of control parameters on low-frequency oscillation under unit power factor through the dominant admittance element;then study the influence of power factor on low-frequency oscillation of the system by drawing a curve of power factor and admittance amplitude.Further,according to the generalized Nyquist stability criterion and ratio matrix analyze the influence of the number of parallel converters,the impedance of the grid,and the interaction between the converters on low-frequency oscillations.As for the interaction between the converters on the low-frequency oscillations,the impact is mainly to analyze the influence of a certain converter's impedance change on other converters after changing the control parameters of a certain converter.Finally,simulations and experiments are used to verify the correctness of the conclusions.(3)According to the above analysis,a control strategy based on virtual resistance is proposed to suppress low-frequency oscillations.Another control strategy based on virtual reactance reduces the dq-axis coupling.The virtual resistance and virtual reactance are generalized to virtual impedance.An improved grid-connected control strategy,which simultaneously solves the system low-frequency oscillation and dq-axis coupling with virtual impedence and phase compasention is given.The parameters setting of virtual resistance value,virtual reactance value and phase compensation value is given.Finally,based on the existing phase compensation,an improved phase compensation strategy based on infinite impulse response is proposed to further improve the dynamic performance of the system,and the effectiveness of the strategy to suppress low-frequency oscillations and improve dq-axis coupling is verified through simulations and experiments.