Modeling And Stability Analysis Of Three-Phase Grid-Connected Inverters System Based On Harmonic Linearization

In recent years,new energy power generation industry has developed rapidly in China,and new energy accounts for an increasing proportion in the primary energy structure.With the development of large-scale distributed power generation technology and distributed energy grid power level increasing,three-phase grid inverter as an important and indispensable in new energy power generation device has been widely used so as to the characteristics of weak power grid are increasingly presented in power grid,and the stability of the interconnected system between inverters and power grid is becoming increasingly prominent.The resonance phenomenon between the inverter and the power grid due to the impedance interaction will lead to the instability of the system.The study of small signal stability based on impedance model has become a hot research topic nowadays.When analyzing the stability of three-phase grid-connected inverter system,harmonic linearization modeling is a common impedance modeling method,and it has irreplaceable advantages when applied to the distributed power generation system with complex structure and unknown parameters.With the in-depth study of the frequency coupling phenomenon in the three-phase grid-connected inverter,some studies have given the complete output admittance model of L-type grid-connected inverter with single closed-loop current control.The output admittance model of LCL-type grid-connected inverter with the double-closed-loop current control structure has not been established.In addition,when analyzing the stability of the impedance model of three-phase grid-connected inverters,the three-phase system is usually simplified to a single-phase system,and the equivalent circuit of the parallel system is obtained by using the simplified single-phase structure in the study of multi-inverters parallel system.This can effectively simplify the analysis process,but the conclusions obtained also lose some accuracy.Therefore,this paper takes the parallel system of L-type and LCL-type three-phase grid-connected inverters as the research object.Firstly,a complete output admittance model of L-type and LCL-type three-phase grid-connected inverters is established,which is verified by simulation experiments.Then,the influence of system parameters on the output admittance characteristics of inverters is further analyzed.In order to ensure the accuracy of stability analysis conclusion and simplify the analysis process of impedance stability of interconnected system between inverters and power grids,a Bode diagram criterion based on determinant of return matrix of MIMO system is proposed,which helps to obtain the stability of interconnected system with complex model intuitively.The validity of the criterion is verified by simulation experiments.Based on the criterion,the inherent relationship between system stability and system parameters including grid impedance,current controller and phase-locked loop is studied for L-type and LCL-type three-phase grid-connected inverters and their parallel systems.Finally,in order to meet the stability requirement of grid-connected inverters in very weak power grid,a simple and practical voltage q-axis feed-forward control method is proposed.The remodeling effect of the control on admittance model is studied.The stability analysis and simulation experiments verify that the control improves the stability of the system on the premise of ensuring the dynamic performance of the phase-locked loop.