Research On Parallel Operation Of Virtual Synchronous Generators In Microgrid

In recent years,the problems of energy crisis and environmental pollution have become increasingly prominent,so new energy fields such as renewable energy and distributed energy have received more and more attention.Distributed power can effectively improve reliability and quality of power supply,also can reduce system cost.It is significant to solve distributed power supply problems in remote areas.However,lacking the inertia and damping of conventional generators for distributed generation systems,independent microgrid systems are severely affected by intermittent renewable energy and unpredictable load.Therefore,the parallel system of Sychronverter virtual synchronous generator with inertia and damping characteristics is studied in this paper.Firstly,the control methods of microgrid inverter is introduced in this paper,and a comparative analysis on its application background and control performance is made,at the same time several different control technologies of virtual synchronous generator are introduced in detail.The results show that the virtual synchronous generator based on Synchronverter technology has excellent grid-connected performance of inertia and damping,and can adjust the frequency and voltage of the micro-grid system,which has a great development prospect.Then,the main circuit topology and control structure of Synchronverter virtual synchronous generator are introduced.The open-loop transfer function of the active-frequency loop and the reactive-voltage loop are obtained by establishing the small-signal model of the virtual synchronous machine.The principle of selecting the parameters of the active loop inertia coefficient and the reactive loop gain coefficient is determined according to the method of cross-over frequency and stability margin.The influence of key parameters on the stability and dynamic performance of the virtual synchronous generator is analyzed,and the parameter tuning and stability verification are carried out.At the same time,the virtual synchronous generator grid-connected pre-synchronization control strategy is studied.The single-machine simulation of the virtual synchronous generator is established.The results verify the feasibility of the parameter selection and the great dynamic performance of the virtual synchronous generator.Finally,the startup scheme,the circulating current restraining strategy and the power distribution method of parallel operation of virtual synchronous generators under microgrid conditions are studied.The purpose of restraining the circulating current and distributing the power is achieved by introducing the virtual impedance strategy and tuning of control parameters.At the same time,the method of restraining the zero-sequence circulating current is studied for the two-machine parallel system of the common DC bus.The parallel system simulation and experimental system is established,and the results verify the feasibility and superiority of the research scheme.