| Microgrid is a product of the transition from the traditional power grid to the future smart grid in order to provide a variety of energy consumption,energy saving and emission reduction,and adapt to the diverse needs of different users.In order to improve the micro network reliability and large-scale new energy network acceptance capacity,the domestic and foreign scholars put forward the virtual synchronous generator(Virtual Synchronous Generator SVG)technology,which can make the fast and sensitive power electronic converter to simulate the external characteristics of synchronous generator because of adding damping and inertia,In this paper,we mainly study on the control strategy of a single VSG island / grid connected and two VSG parallel network multi-mode,summarized in the following aspects.Firstly,this paper introduces the background of the research,several typical control strategies of the parallel operation of the micro grid converter and the research status of the virtual synchronous generator at home and abroad.Secondly,this paper introduces the basic principle of virtual synchronous generator and put forward the improvement,through the small signal modeling and analysis of the operating characteristics of VSG on single islanding and grid connected mode,the mechanism of influence of each parameter on the system stability and power frequency response is obtained.Adaptive control and constant voltage constant frequency control method have been put forward to ensure the reliable operation of single machine;the pre-synchronization method based on the virtual impulse current is proposed,which is used to track the phase of the grid and reduces the current shock before the grid is connected;the inertial voltage droop control strategy is proposed to retard the reactive power-voltage drop process,which is based on the differential negative feedback.The correctness of the proposed control strategy is verified by simulation and experiment.Then,the operating characteristics of the VSG multi-machine parallel mode are analyzed,and the relationship between the AC bus voltage,the VSG output current and the load,the VSG terminal voltage is obtained by the parallel mathematical model,based on the analysis of the generation mechanism of the micro source,the output power of each VSG is deduced according to the power control diagram,which provides a theoretical basis for improving the power distribution accuracy and suppressing the circulation.Finally,according to the effect of line impedance on the reactive power allocation,an improved excitation magnetic loop control strategy is proposed.the influence of various parameters in the improved algorithm on the stable operation of microgrid is analyzed by the small signal model.Analyze the operation characteristics of VSG parallel networking with improved control strategy,contrary to circulation,power oscillation problems caused by machine combined and the load increase,it provides parameter design method based on impedance matching principle,the optimization method can improve the dynamic performance of VSG,inhibition of circulation,stabilize the power oscillation.The correctness of the proposed control strategy and parameter selection scheme is verified by simulation and experiment. |
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