Research On Resonance Mechanism Analysis And Suppression Methods Of Multi-inverter Grid-connected Systems

With the development of society and advances in science and technology,the solar photovoltaic power generation industry has continuously expanded the scale of development,resulting in a sharp increase in the number of inverters,making the resonance generated between inverters more complex,resulting in egregious effect on power quality of the State Grid.Therefore,resonance is a problem that needs to be solved in large-scale promotion of photovoltaic grid-connected technology.In this paper,the mathematical models of L-type,LC-type and LCL-type typical filters are constructed firstly,and they are expressed in the dq rotating coordinate system and the a? stationary coordinate system,respectively,to facilitate the construction of a simulation model in Simulink.Then,L-type,LC-type and LCL-type filters are compared respectively,and their filtering performance is analyzed.Matlab simulation software is used to obtain their respective Bode diagrams.Through comparative analysis of the graphs,it is found that the Harmonic attenuation of LCL-type filters at high-frequencies area works best.Then,a passivity analysis method is proposed,and it is pointed out that the negative real part of the equivalent admittance in the impedance model is the cause of resonance.For the single-inverter system,the output impedance model of the LCL-type single inverter is constructed by means of grid side current feedback,and the resonant characteristics of the single-inverter grid-connected system are analyzed using the passive analysis method.In this paper,the three position relationships between the equivalent admittance and the grid impedance are carefully studied and compared,and the correctness of the obtained results is verified by the Nyquist diagram.The multi-inverter system is based on the single inverter system,and the output impedance models of two grid-connected systems with multi-LCL inverters are deduced.The whole system is a linear system.The superposition theorem can be used,which is equivalent to the parallel connection of a total current source and total admittance.The relationship between the number of multi-inverters and grid impedance is studied by the passive analysis method.In the impedance model constructed by n inverters equivalent to a single inverter,the interaction effects of multi-inverter grid connection are analyzed,and grid impedance and the number of grid-connected inverters are used to analyze the system resonance peaks.Impact analyses frequency characteristics of multiple inverter systems.What's more,for the single-resonator resonance problem,passive damping method and active damping method based on capacitive current negative feedback are used in this paper.Write the transfer functions of the six kinds of grid-side currents to the output voltage of the inverter,and use the Bode plot to represent the filter performance changes of the LCL filter under the three grid resistance values.Based on this,this paper finally builds a three kinds of passive damping simulation models of series connection of resistor R and inductor L1,series connection of resistor R and inductor L2,and series connection of resistor R and inductor C.A single LCL inverter output impedance model based on capacitive current negative feedback is constructed.Passive analysis method is used to analyze the effect of active damping on resonance suppression.This method uses current feedback to change the equivalent admittance and remove/reduce the non-passive region,so as to achieve the purpose of suppressing resonance.In the simulation model,when the grid impedance takes three kinds of values,the current changes at the PCC and the inverter side are observed to verify the correctness of the passive analysis method.For the resonance induced by the grid impedance,the resonance is removed by the suppression method based on capacitive current negative feedback.Finally,on the basis of the output impedance model of the single LCL inverter based on the negative feedback of capacitance current,the output impedance model of the inverter system with active damping is built on the basis of the resonant problem of the multi LCL inverter parallel system.Passivity analysis includes the relationship between the equivalent admittance of the active damping and the impedance of the grid,and the stability of the system is judged by Nyquist diagram.In the Matlab simulation model,two identical inverters are used in parallel to observe the changes of the current in the PCC and the inverter net side,and then a single inverter can be added to verify the correctness of the passivity analysis method.Finally,the suppression method based on capacitive current negative feedback is used to remove resonance.