| In recent years,the penetration rate of distributed power sources in the grid has been continuously improved.Due to the lack of inertia and damping of power electronic device connected to the grid interface,it poses a potential threat to the safe and stable operation of the power systems.The virtual synchronous generator(VSG)introduces the virtual inertia and damping for the system by simulating the rotor motion equation of the traditional synchronous generator,and can participate in voltage and frequency regulation of the grid,which is of great significance to improve the stability of distributed power sources grid-connected systems.Combined with the characteristics of small inertia and fast response speed of distributed energy generation system,this paper mainly studies the control strategy of virtual synchronous generator under low inertia,and puts forward the microgrid inverter equalization control strategy and parameter design method.Then,small signal modeling and stability comparison analysis of small signal of low inertia virtual synchronous generator control system are given.The main contents of this paper are as follows:1.The basic principle of low inertia virtual synchronous generator control strategy is expounded.Then a microgrid inverter equalization control method based on low inertia VSG control is proposed,which can realize the independent adjustment of droop and damping coefficient through combined feedback control of inverter frequency and grid frequency;suppress the overcurrent under frequency disturbance in large scale through switching the power outer loop according to the grid frequency logic judgment link;solve the contradiction between the dynamic response of the output voltage of the microgrid inverter and the parallel current sharing characteristic in island operation through the weighted control based on the inductor current and the capacitor current.In terms of the equalization control strategy,a parameter design method for low inertia virtual synchronous generator is presented.The feasibility and effectiveness of the proposed control strategy and parameters design method are verified by simulation results with the model under Matlab/simulink.2.High-order small-signal models of the low inertia virtual synchronous generator in island operation are established by the modular modeling method,in which the modules are independent and easy to modify and expand.At the same time,the stability of the system under small disturbance is analyzed by eigenvalues,eigenvectors,sensitivity and participation factor.And the influence of power loop controller parameters on system stability and dynamic performance is explored by trajectory,which provides a theoretical basis for the optimal design of low virtual inertia.3.In order to analyze and compare the performance of two inner loop controllers,single voltage loop and voltage-current double loop,small-signal models of VSG single-machine and dual-machine system based on the above two inner loop controllers are established respectively.Based on the models,the evolutionary phenomenon of the oscillation modes of the microgrid system from single-machine to dual-machine and the stable adjustment range of the controller parameters are given.The adjustable range of controller parameters of the dual-machine system significantly reduced compared with that of the single-machine system.Especially,for the voltage proportional coefficient of the single voltage loop,as well as the current integral coefficient of the voltage-current double loop in dual-machine system,their adjustable ranges are limited.In addition,the inductor current feedback loop in the double loop control can increase the system damping and significantly suppress the system oscillation.Finally,the simulation results prove the correctness of the small-signal model and stability analysis.4.The simple experimental verification of low inertia virtual synchronous generator control is completed based on the low power experimental platform. |
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