| The microgrid has developed rapidly in recent years,the micro power supply connected to the external power grid is mostly through the power electronic converter and LCL filter.Then multiple inverters are often used in parallel to increase the capacity of the microgrid system.In view of the circulation phenomenon occurring in parallel with multi-inverter and the resonance problem caused by LCL filter,this thesis makes an in-depth study of the parallel control strategy for the microgrid multi-inverter,and puts forward the virtual impedance droop control and grid voltage feed-forward and grid current feedback control method in series of active virtual damping resistors respectively.The circulation suppression and resonance suppression are realized,the dynamic response performance of the system is improved,and for the complex system of the multi-inverter parallel connection of the microgrid with LCL filter,combining the above two control methods for modeling and simulation,the validity of the proposed control method was verified.First of all,the principle analysis and modeling simulation of three control methods(PQ,V/f and Droop control)of the inverter have been studied in this thesis,focusing on the comparative analysis of the dynamic performance of the two methods: the P-f/Q-U Droop and f-P/U-Q Droop control.The P-f/Q-U Droop control with simple control block diagram and better dynamic response performance has been selected as the basic control of the inverter.Secondly,in view of the coupling problem that the traditional droop control is directly applied in the low voltage microgrid,the virtual impedance droop control strategy is adopted.Comparative analysis of the equivalent output impedance of these two droop controls,through the corresponding Bode diagram,the appropriate control parameters and virtual impedance values are selected to make the equivalent output impedance of the virtual impedance droop control be inductive,modeling and simulation on Digsilent realized the power sharing and circulation suppression.However,it cannot suppress the resonance of the LCL filter.Third,aiming at the resonance problem of LCL filter,the thesis analyzes the principle and topology structure of LCL filter.According to the transmission characteristics of the current,through the corresponding Bode diagram,the influence of the value of LCL filter parameter on the filtering effect is studied and the design flow of a set of parameters is given.In order to save the number of sensors and reduce the power loss of the system,a control of grid voltage feed-forward and grid current feedback control method in series of active virtual damping resistors is proposed.According to the equivalent transformation of the block diagram and equivalent principle of the transfer function,the corresponding control coefficient is obtained,and modeling and simulating on Digsilent.The simulation results showed that this method had better resonance suppression ability and dynamic response performance.However,it cannot suppress However,it cannot suppress the circulation between multiple inverters.Finally,for the phenomenon that the circulation and the resonance may simultaneously exist in the actual microgrid,combining the above two kinds of improved methods,the comprehensive control of the circulation and the resonance has been applied to a microgrid multi-inverter parallel system with LCL filter.The simulation results verified the feasibility and validity of the proposed control strategy. |
