Research On Control Strategy Of Micro-grid Inverter Based On Virtual Synchronous Generator

With the wide application of distributed power generation technology,the development and utilization of renewable energy in power systems is continuously improved,but the inverter power supply with the power electronic inverter as the interface does not have the working characteristics of synchronous generators,which easily causes system instability.The virtual synchronous generator realizes the synchronization mechanism through the control strategy,provides certain inertia and damping support to the grid,enhances the stability of the system,and provides a new technical direction for the development of distributed generation.Micro-grid is a system that integrates the advantages of distributed generation.It has two operating states: grid-connected and off-grid.Seamless switching is an important guarantee for stable and uninterrupted operation of micro-grids.When the microgrid multiple inverters are operated in parallel,the difference between the equivalent output impedance and the line impedance affects the power sharing and the circulation suppression effect.Therefore,the research on micro-grid inverter based on virtual synchronous generator and off-grid seamless switching and its parallel control technology is of great significance to enhance the stability and adaptability of micro-grid system.The paper mainly does the following work:Firstly,the research status and development trend of control strategies and key operation control technologies of microgrid and inverters are summarized.This paper introduces the principle of virtual synchronous generator,designs its main circuit and power loop,establishes its small signal analysis model,presents the method of setting inertia and damping coefficients,and analyses the influence of inertia and damping coefficients on the dynamic performance and frequency variation of the system.Secondly,aiming at the problem that traditional control methods can not meet the requirements of micro-grid and off-grid switching,a seamless off-grid switching method based on virtual synchronous generator is proposed.The seamless switching process of traditional microgrid inverters is analyzed in detail,and the pre-synchronization control method is improved.The output voltage of grid-connected inverters can quickly track the amplitude and phase of grid voltage.The instantaneous switching current can smoothly transition and restrain the impact of power.The seamless switching control target of off-grid inverters is realized.Then,the power transmission and circulation characteristics of multi-inverters in parallel are analyzed,and an improved parallel control strategy based on virtual synchronous generator is proposed.Voltage feedback and integrator are added to the reactive-voltage loop to make the voltage stability and reactive power independent of the output impedance.Dynamic virtual complex impedance is introduced to reduce voltage sag and circulating current.A parallel pre-synchronization link is designed to ensure that no current shock occurs at the instant of parallel connection.The improved control strategy achieves power sharing and circulating current suppression when multi-inverters are connected in parallel.Finally,the simulation and experiment platforms of virtual synchronous generator are built in MATLAB/Simulink simulation software and laboratory respectively,which verifies the correctness of the above theoretical analysis and the effectiveness of the proposed control strategy.