Analysis Of Subsynchronous Oscillation Mechanism Of Grid-Connected Inverters Based On Impedance Modeling

With the implementation of "clean fuel alternative,electric power replacement"strategy,large-capacity of new energy connected to the grid and transmitted to elsewhere have become an inevitable trend.While the new energy brings clean energy,it also produces subsynchronous oscillation.However,both photovoltaic and wind power generation systems use grid connected inverter as their main power interface unit.Therefore,the study of subsynchronous oscillation caused by grid connected inverter is of great significance.From the grid connected inverter system,this paper firstly derives the positive and negative sequence impedance analytical formula of inverter in three-phase static coordinate system,and verifies the impedance formula by using the impedance characteristic curve measured by harmonic impedance.The relationship between the impedance formula of different frequency components and the phase sequence is proved by theoretical deduction and simulation,and the transformation rules of harmonic signals in the grid connected system is clarified.Next,in view of the fact that the traditional Nyquist criterion can only judge the oscillating characteristic of the intersection point,a new criterion based on the RLC series resonance principle is proposed and verified for the grid connected system,and the stability of the system is analyzed by using the criterion.According to the independent variables in impedance formula,the influence of the number of units,output,LC filter parameters and control system parameters on the impedance characteristics of inverter is summarized,and the stability trend of grid connected system is studied.Finally,for the same grid connected inverter system,its output impedance matrix is derived in the d/q coordinate system.The harmonic impedance measurement of the d/q axis is realized and the correctness of the impedance matrix is verified by the measurement results.Under the d/q coordinate system,influence of output voltage,voltage controller parameters and current controller parameters on the inverter impedance is analyzed,and the stability of the grid connected system under is analyzed.The relation and difference between the impedance matrix in d/q coordinate system and the impedance formula in three phase stationary coordinate system are explained by theoretical derivation.