Research On Resonance Detection And Suppression For Multi-inverter Grid-connected System

At present,China’s demand for electricity is increasing day by day,and the development and utilization of renewable energy are increasing.In the process of renewable energy generation,it is necessary to use inverters to convert the electric energy from renewable energy into electric energy that meets the requirements of the power grid,which leads to more and more inverters being incorporated into the power grid.At present,there are two main types of filters for grid-connected inverters: L-type or LCL-type.In the same filtering effect,the volume,cost and loss of LCL filter are smaller than that of L filter,and the application is more extensive.However,LCL filter has resonance peak,which will cause the resonant of the grid-connected inverter.At present,the research on the suppression of grid-connected resonance of single inverter is more mature.However,in the multi-inverter grid-connected system,the resonant frequency of the system will change with the change of the number of parallel inverter,so the single inverter grid-connected resonant suppression measures can not eliminate the resonant problem of the multi-inverter grid-connected system.In this paper,the resonant detection and suppression methods of the multi-inverter grid-connected system are studied.For the multi-inverter grid-connected system with LCL filtering,the resonant frequency will change with the control mode of grid-connected inverter,the number of parallel units and the equivalent impedance of the grid,etc.,resulting in the existing impedance analysis method to detect the resonant frequency is too complicated and not applicable.At the same time,the variation of resonant frequency makes it difficult to apply the resonant suppression measure of single inverter to the grid-connected system of multiple inverters.Aiming at the above problems,this paper mainly completes the following work:Firstly,by deducing the current source equivalent model of the grid-connected inverter,the equivalent model of the multi-inverter grid-connected system is established,and the mathematical model of the multi-inverter grid-connected system is established by using the equivalent model.Through the mathematical model of the multi-inverter grid-connected system,the resonant characteristics of the multi-inverter grid-connected system are analyzed,and the resonant characteristics of the multi-inverter grid-connected system are further verified through the decoupling analysis of the single grid-connected inverter in the multi-inverter grid-connected system.Then,according to the advantages and disadvantages of the existing algorithms in extracting the resonant frequency of the multi-inverter grid-connected system,a resonant frequency detection algorithm based on wavelet transform and SOGI-FLL is proposed,which can detect the resonant frequency of the multi-inverter grid-connected system in real time and accurately.In addition,the discretization method of SOGI is improved,which not only improves the accuracy of the resonant detection algorithm proposed above,but also has a good application in MSOGI.Finally,according to the resonant frequency detected by the resonant detection algorithm proposed in this paper,a resonant suppression strategy for multi-inverter grid-connected system based on resonant frequency feedback is proposed.Simulation and experimental results show that the resonant suppression strategy has a good effect on the resonant suppression of multi-inverter grid-connected system.