Research On Control Method Of Microgrid Inverter Based On Impedance Passive Theory

In order to effectively deal with the global fossil energy crisis and pollution problem,vigorously developing and utilizing new energy has become an international consensus and trend.Under China’s "double carbon" development strategy,the vigorous development of new energy power generation system has been raised to a new height.Microgrid is the interface between a variety of new energy power generation systems and power grid,and it can interact with a large number of new energy power generation units and operate on an island.It is an important way to organize new energy power generation system.Taking microgrid inverter as the research object and impedance passive stability judgment as the theoretical tool,this thesis studies the stability and optimal control method of microgrid inverter under different working modes,namely grid structured（GFM）island operation mode and grid following（GFL）grid connected operation mode.Firstly,starting from the topology of microgrid inverter system,this thesis models the main circuit of LCL three-phase inverter in abc,αβ,and dq coordinate systems respectively,gives the main circuit structure block diagram in three coordinate systems,analyzes it,and selects dq coordinate system to carry out the research work of the full text.The coupling electric quantity in dq coordinate system is given,which lays the foundation for the later control and modeling in dq coordinate system.It also lays a foundation for the later analysis of port impedance characteristics and system stability under grid connected and off grid conditions.For the grid connected operation state of microgrid inverter,firstly,a current control loop is added based on the LCL main circuit model to establish the inverter port impedance model.Through the macro analysis of the system,repetitive control and PI control in parallel are selected as the control strategy of the current loop,and its parameters are designed to ensure the internal stability of the grid connected inverter.On this basis,using the impedance frequency domain passive analysis method,the frequency domain passive analysis of the port impedance on the side of the grid connected inverter is carried out to find out the non-passive area of the port impedance in the full frequency band.The impedance modeling of different types of grid impedance is carried out,and the stability of the system when the grid connected inverter is incorporated into different types of grid impedance is analyzed.Then add the dynamic influence link of PLL in the grid connected state to the above impedance model,update the impedance model,and analyze and compare the impact of the dynamic influence link of PLL on the impedance model and the stability of grid connected system.The correctness of the above theoretical analysis is verified by simulation.For the islanding operation state of microgrid inverter,firstly,based on GFM control structure,the islanding inverter system is established by using power synchronization control（PSC）,and the port impedance model of AC voltage loop and frequency synchronization loop is added.On the premise of designing controller parameters to ensure internal stability,the non-passive section of islanding inverter is analyzed by impedance frequency domain passive analysis method and connected to specific pure inductive load,the stability of islanding inverter is analyzed.Then,the isolated island microgrid GFM inverter is regarded as a simulated power grid,and the grid connected inverter analyzed and studied above is connected to the simulated power grid as a load inverter to analyze the stability of the whole system.Considering that multiple inverters are often connected in parallel in microgrid system,this chapter will also analyze the system stability after grid connected inverters are connected in parallel to the simulated power grid.The correctness of the theoretical analysis is verified by simulation.