| With the depletion of fossil energy,the proportion of new energy in distributed generation is increasing.The new energy grid needs inverter as a bridge,so grid connected inverter is the key link of the whole grid connected power generation system.At present,the main type of grid connected inverter is LCL type.The main advantages of LCL type inverter are: it can better suppress switching harmonics,and from the cost point of view,it has the advantages of small size and small inductance,so it is widely used.However,LCL inverter has inherent resonance peak,which can easily lead to resonance in grid connected system and endanger the safe and stable operation of the system.At the same time,the new energy grid connected power generation system often adopts the structure of multi inverter grid connected,which will make the whole system more complex,and the risk of losing stability is higher and higher.Therefore,based on the model of multi inverter connected to the grid,this paper analyzes the resonance characteristics of the system and puts forward the resonance suppression strategy.In view of the problem of multi inverter grid resonance,the traditional method of inverter resonance analysis based on system transfer function can get basic information such as resonance frequency,but there are some deficiencies in the analysis of system resonance distribution and resonance sensitivity.In this paper,the modal analysis method is used to study and analyze the multi inverter grid system.Based on the transfer function,the Norton equivalent model of multi inverters connected to the grid is established.Through modal analysis and sensitivity analysis of the admittance matrix of the system,the information of resonance frequency,Resonance Center and the participation degree of various components in the resonance of the system are obtained.At the same time,the influence of the number of parallel inverters and the control parameters of the system on the resonance characteristics of the system is analyzed,which is mutually related to the Bode diagram of the transfer function The validity of modal analysis method is verified by comparison.For the suppression of harmonic resonance in the system,a strategy based on PI regulator is proposed.Firstly,the control block diagram of the system is obtained,based on which the influence of grid impedance and control parameters on the system is analyzed.The results show that the larger the grid impedance is,the more serious the instability trend of the system will be which will eventually lead to the resonance of the system and make the system lose stability.Even if the control parameters are adjusted,the stability of the system cannot be improved.Then the resonance suppression strategy based on PI regulator is proposed.Finally,the simulation model is built on MATLAB / Simulink platform.The results of modal analysis and the law of system resonance are verified by simulation analysis.At the same time,the effectiveness of resonance suppression strategy is verified by comparing the simulation results before and after adding PI regulator. |
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