Research On Frequent Deviation-free Control For Islanded Microgrid Multi-inverters Based On Virtual Synchronous Generator

With the increasingly serious energy and ecological problems,the clean energy,such as solar energy and wind energy,has received more and more attention due to its renewable and environmentally friendly characteristics.In order to solve the problem that clean energy cannot be directly connected to the grid,scholars at home and abroad have proposed a microgrid structure.When there are large power fluctuations in the island microgrid,the system frequency will easily deviate from the allowable range and overshoot will occur,which is not conducive to the stable operation of the system.Therefore,it is of great significance to study the frequent deviation-free control in the island microgrid.However,the microgrid inverter,as the interface connected to the distributed power supply,has problems such as small output impedance,small capacity,fast response speed,and lack of inertia,which makes the traditional frequent deviation-free control strategy unable to guarantee good frequency dynamic response performance and achieve no static differential control,the frequent deviation-free control strategy of virtual synchronous machine came into being.This paper studies the problems existing in the frequent deviation-free control strategy of multiple virtual synchronous machines in the traditional island microgrid.The specific research contents are as follows:First of all,research the basic principles of the virtual synchronous machine,establish a single virtual synchronous machine small signal model and a dual virtual synchronous unit network equivalent model,and analyze the power response characteristics and stability of them respectively,and obtain the key parameters of the control strategy for the system Stability and control performance are affected,and the control parameters for subsequent research are selected.The simulation results verify the accuracy of the theoretical analysis.Secondly,in order to solve the problems of uneven power distribution and coupling of primary and secondary frequency modulation in the frequency recovery control of traditional multiple virtual synchronous machines,a virtual synchronous machine frequent deviation-free control strategy with automatic switching delay is proposed.The automatic switch delay control link is added to the control strategy to realize the decoupling between the primary and secondary frequency modulation of the virtual synchronous machine.The stability analysis of the system is carried out,and the value range of the proportional integral coefficient K is given.The simulation results show that the proposed control strategy can automatically realize the primary and secondary decoupling of the frequency adjustment and the proportional distribution of the output power under the premise of satisfying the non-difference adjustment of the frequency,which is beneficial to improve the stability of the system.However,the dynamic response characteristic of the output frequency of this control strategy becomes worse when the load is switched on and off.Finally,when the load fluctuates greatly,the proposed control strategy has the problem of the deterioration of the frequency transient response performance,and a virtual synchronous machine frequency non-difference control strategy based on adaptive parameters is adopted.On the basis of the above control strategy,an adaptive parameter control link of moment of inertia and damping coefficient is added,and the virtual synchronous machine system model under the adaptive parameters is analyzed to obtain the selection range of the key parameters of the model.The simulation results show that the improved control strategy effectively improves the dynamic response performance of the system frequency and suppresses the frequency overshoot,which is consistent with the theoretical analysis while retaining the advantages of the original control frequency modulation.