Stability And Resonance Analysis Of Microgrid With Multi Distributed Generation

In recent years,with the development and utilization of photovoltaic,wind power and other new energy,microgrid technology based on distributed generation has been developing continuously.As a key interface for new energy access to microgrid,DC change and AC,the control system of inverter has natural multi-time scale characteristics and extremely wide control frequency band.The impact of its access to microgrid on Microgrid is mainly reflected in the following aspects: on the one hand,under grid connection mode,multi inverters connecting to weak grid may inject negative damping of different frequency bands into the grid and interact with the LC resonant frequency of filter and line in the power grid.These factors may induce broadband oscillation and even instability.On the other hand,under island mode,the inverter with different control strategies produces dynamic control interaction,which leads to harmonic circulation.Therefore,it is necessary to deeply study the stability and vibration of microgrid system.The main work of this paper is divided into the following parts:A simplified modeling method of harmonic linearized impedance for inverters with various control strategies is proposed,and the impedance models of current controlled and voltage controlled inverters are established.PQ control strategy controller is selected as current controlled inverter.The frequency coupling effect brought by PLL is deduced.The impedance matrix model characterizing the frequency coupling effect is defined,and the impedance matrix is deduced by the proposed simplified modeling method.VSG control strategy inverter is selected as voltage controlled inverter.Considering the small signal disturbance of phase angle caused by power loop,the positive and negative sequence impedance model is established based on the simplified modeling method.Impedance frequency sweeping is carried out based on PSCAD simulation platform to verify the correctness of harmonic linearization simplification method and impedance model.The stability criteria of typical single bus microgrid in grid connection mode and island mode are proposed.In grid connected mode,considering the frequency coupling effect,the grid connected stability criterion of multi current controlled inverter is obtained by separating the positive and negative sequence system of inverter impedance matrix.The effects of grid impedance,control parameters and the number of inverters on system stability are analyzed through Nyquist curve and Bode diagram.In islanding mode,considering the parallel connection of multi voltage controlled and current controlled inverters,the stability criterion of microgrid islanding mode is deduced based on the judgment method of grid connection stability.The influence of the number combination of the two inverters and line impedance on the system stability is analyzed.PSCAD simulation proves the accuracy of the theory and analysis.The resonance modal analysis(RMA)method is introduced into the multi distributed generation microgrid,and the resonance characteristics of typical single bus microgrid are obtained.Based on the impedance model of microgrid with voltage controlled and current controlled inverters,the principles of traditional frequency domain analysis method and modal analysis method are described.The example analysis shows that resonance modal analysis method is more intuitive and can get more information such as node participation.The resonance characteristics are analyzed in microgrid grid connection mode and island mode.The effects of the number of inverters and filter parameters on the resonant characteristics of the system are studied in grid connection mode.The effects of the number combination of two inverters,line impedance,grid or load impedance on the resonance characteristics of the system are studied in island mode.Finally,the simulation in PSCAD proves the correctness of the theory and analysis.