Grid-connected Three-phase Inverter Control Based On Virtual Synchronous Generator Techniquo

With the shortage of fossil fuels and the strict regulation of carbon emission,the proportion of renewable energy generation in the power grid is continuously increasing.Renewable-energy power is fed to the grid via power electronic converters without the inertia of the traditional synchronous generators,which does harm to grid stability.In response to this problem,Virtual Synchronous Generator(VSG)technology enables renewable energy generation nodes to demonstrate the same external characteristics as the traditional synchronous generators.providing an effective way to improve the stability of a large number of new energy grids.In this paper,the three-phase grid-connected inverter is taken as the research object.Firstly,the mathematical models of LCL three-phase grid-connected inverter and second-order hiddenpole synchronous generator are established and compared respectively.The operating mechanism of the synchronous generator,namely the active power-adjustment frequency control loop and the reactive power-regulation voltage control loop,and the working characteristics of the damping inertia are studied,which is the theoretical basis for the realization of the VSG technology.Secondly,the influence of VSG parameters on system stability is analyzed.Subsequently,the VSG system was fully designed from different angles.1.There are two problems in the traditional VSG parameter adaptive adjustment:First,the adjustment range of the damping droop coefficient and the virtual moment of onertia is large,which will require the system to have a high energy storage margin;Another point is that frequency fluctuations and power overshoots are large during transient adjustment,which has a negative impact on power equipment.This paper introduces the output speed feedback controlto adjust the system damping to improve the system transient stability.At the same time,according to the transient power angle characteristics of the system,an adaptive control strategyof virtual moment of inertia and autput speed feedback coefficient is proposed.The transient performance of the grid frequency adjustment is improved,the adjustment time is effectively shortened,and the system frequency deviation during the transient adjustment process does not exceed the rated threshold.Morever,the power overshoot of the system is suppressed.2.For the problem that the VSG system has poor open-loop power quality and poor antiinterference ability,a closed-loop control structure of the capacitor current inner loop and the output voltage outer loop is adopted.Firstly,the internal model principle is used to obtain the structural characteristics that the controller should have under the two-phase stationary coordinate system(αβ),and the PR controller is selected.Secondly,the root locus method is used to analyze the selection of inner loop feedback,and from the perspective of system antiinterference performance,the optimal control performance is obtained when the capacitor current is selected as the inner loop feedback.Finally,considering the system’s immunity and response speed and the grid-connected standard,the frequency analysis method is used to adjust the controller parameters.The results show that the power quality of the VSG system has been significantly improved and improved.3.For the physical counterpart of the virtual moment of inertia,an energy storage system with a super-capacitor as the energy storage unit is designed.On this basis,the linear model of bidirectional DC-DC converter is derived.According to the control requirements of the VSG energy storage system for different working modes of the bidirectional DC-DC converter,different control structures are adopted,and the tuning of the controller parameters is completed.At the same time,the corresponding energy storage adjustment algorithm is designed to realize the primary frequency modulation characteristics of the synchronous generator and improve the stability of the grid-connected system.Finally,the corresponding simulation system of each chapter is built on the Simulink platform,which proves the correctness and rationality of the control algorithm.The LCL-type three-phase grid-connected inverter platform of the laboratory was used to verify the gridconnected experiment and VSG parameter adaptive regulation,which further proved the feasibility of the proposed control strategy.