Research On Grid-connected Stability Of Microgrid Inverter Based On Virtual Synchronous Generator

As the interface between the distributed generators and microgrid and the large power grid,the microgrid inverter has a great influence on the stable operation of the microgrid.Traditional PQ control,Vf control and droop control are difficult to compensate for the shortcomings of low inertia and underdamping of the microgrid.The Virtual Synchronous Generator(VSG)technology introduces the characteristic equation of the synchronous generator into the control strategy of the microgrid inverter,simulating the external characteristics of the synchronous machine and the voltage regulation and frequency modulation function.However,virtual synchronous generators are essentially power electronic devices,and their operating characteristics are different from those of synchronous generator systems.In this paper,the stability of the microgrid connected with the virtual synchronous generators is studied.The main contents of this paper are as follows:1.The background and typical control strategies of microgrid are introduced.The research status and latest development of virtual synchronous machine technology are analyzed.The research status of virtual synchronous machine grid connection stability is also analyzed and summarized.2.Based on the research of the mathematical model and control method of synchronous generator,combined with the topology of the microgrid inverter,the synchronous machine characteristic equation is introduced into the control strategy of the microgrid inverter.By simulating the rotor motion equation,stator voltage equation,and the control strategy of governor and exciter,the complete control strategy of the microgrid inverter based on virtual synchronous generator is established.Then,the characteristics of the micro-grid inverter simulating the synchronous generator characteristics and participating in the frequency regulation and voltage regulation are researched by establishing the dynamic small-signal model of the virtual synchronous generator.Then the simulation examples are used to verify the correctness and feasibility of the established control strategy.Finally,a hardware-in-loop experimental platform based on RT-Lab was built.The hardware-in-loop experiments are used to further verify the correctness and feasibility of the established virtual synchronous generator control strategy.3.The basic steps of eigenvalue analysis method and parameter optimization method for small signal stability of microgrid system are analyzed.Then,the basic step of grid-connected stability of microgrid inverter based on virtual synchronous machine,namely state space modeling method,is analyzed in detail.The state space modeling method is used to model the micro-grid system connected with the virtual synchronous generators in detail,and the detailed mathematical expressions for modeling each part in the virtual synchronous generator control strategy are given,and the universal modeling method for different operating modes of microgrid systems a connected with the multi virtual synchronous generators is obtained.Finally,the correctness and feasibility of the state space modeling method are verified by simulation examples and hardware-in-loop experiments.4.Based on the established state space model,the stability research methods for microgrid systems are researched in detail,including small signal stability determination method,eigenvalue analysis methods and system parameter optimization methods.The system parameter optimization methods include a preliminary optimization method based on root locus anlysis and an advanced optimization method based on particle swarm optimization.In order to verify the correctness and feasibility of the above methods,three simulation examples are established,including microgrid connected with single virtual synchronous generator,virtual microgrid connected with double synchronous generator in parallel running in on-grid mode and off-grid mode.The state space modeling method are used for these three simulation examples.By calculating and analyzing the characteristics of the state space model of the examples,the laws of the influence of system parameters on the stability of the system are obtained.The system parameters are optimized by root locus method and particle swarm optimization algorithm,and the optimal results of system parameters are obtained,the correctness and feasibility of the stability analysis method and parameter optimization method proposed in this paper are verified.