Research On Control Strategy Of Virtual Synchronous Generator Based On MMC

With the advent of the global energy crisis and the environmental problems caused by traditional fossil energy consumption,renewable energy power generation technology has become one of the important development directions in the energy field.However,the increase in the permeability of renewable energy units has led to a reduction in the inertia of power systems,and the problem of frequency and voltage stability has become increasingly prominent,which undoubtedly restricts the large-scale development of renewable energy power generation.Virtual Synchronous Generator(VSG)controls the grid-connected converter to simulate the inertia,damping and frequency modulation characteristics of synchronous generators,which can achieve good compatibility with the grid and reduce the instability caused by renewable energy access,influences.In view of the large-scale renewable energy power generation demand,Modular Multilevel Converter(MMC)has been widely used in high-voltage direct current transmission systems in recent years.To this end,the MMC-based Virtual Synchronous Generator based Modular Multilevel Converter(MMC-VSG)control technology is proposed to enable the MMC to support the frequency and voltage of the medium and high voltage system while performing energy conversion.Stability.Firstly,based on the analysis of MMC topology and operation characteristics,the MMC circulation model is established.The MMC circulation suppression strategy and the capacitor voltage equalization control strategy are studied to ensure that MMC has good output characteristics when performing power conversion.Then the mathematical model of MMC-VSG is established and the MMC-VSG control technology is proposed.According to the operation characteristics of the traditional synchronous generator,the MMC-VSG power frequency controller and the excitation controller are designed to realize the support of the MMC-VSG to the voltage and frequency of the high voltage power grid.Then,the pre-synchronous grid-connected technology used in the modal switching of the MMC-VSG from the off-grid operation to the grid-connected operation is studied.It is pointed out that the direct grid connection of the MMC-VSG under high-voltage and high-power conditions will have a huge impact.Current,which jeopardizes the safe operation of the system.Combined with the characteristics of MMC-VSG control technology,a pre-synchronous grid-connected scheme using phase-locked loop and PI controller is designed,which effectively reduces the MMC-VSG grid-connected current impact and ensures that the MMC-VSG can be safely and reliably connected to the grid.Finally,a detailed experimental study was carried out on the experimental platform.The experimental results proved the validity of the proposed MMC-VSG theory and control technology.