Research On MMC Grid Connected System Based On Virtual Synchronous Generator Technology

Modular multilevel converter(MMC)is a kind of voltage source converter(VSC).It is more suitable for high voltage and high power occasions.Most of the existing MMC inverters adopt vector control technology.However,with the access of flexible DC transmission system,the MMC system under vector control shows zero inertia and zero damping characteristics.Deteriorating the operating environment of the system.However,the virtual synchronous generator(VSG)can simulate the inverter as a synchronous generator,so that it has the corresponding inertia and damping characteristics,and the system has the ability of independent frequency and voltage regulation,which reduces the adverse impact of renewable energy access to a certain extent.For this reason,this paper proposes to use VSG technology to control MMC system grid connection.This makes MMC have the characteristics of participating in system frequency modulation and voltage regulation at the same time of energy conversion,which improves the stability of the system.In this paper,the basic structure of MMC system is given at first.Through the analysis of the main circuit structure,the working principle of MMC system is introduced,and then the circulation of MMC system is analyzed.In addition,two modulation technologies,carrier phase-shifting method and nearest level approach method are analyzed,and their advantages and disadvantages are compared.Finally,the sub module capacitor voltage equalization control technology is analyzed to ensure that MMC has good output characteristics during power conversion.Next,the grid connection control method of MMC system is analyzed.We decide to design a strategy of using VSG technology to control the grid connection of MMC system based on vector control and its advantages.The mathematical models of MMC system and synchronous generator system are established.By comparing the structural parameters and motion equations of MMC system and synchronous generator system.Derive the parameter correspondence between the MMC system and the synchronous generator system.The relationship between virtual inertia parameters and damping parameters and MMC specific structural parameters is obtained.Then the optimization method of the key parameters in the main controller of MMC grid connected system is determined.Add optimized inertial parameters and damping parameters to the active-frequency controller.Add magnetic chain feedback loop and reactive differential compensation link in reactive-voltage controller.The complete active frequency controller and reactive voltage controller of virtual synchronous generator are given.Implemented MMC grid-connected control strategy based on virtual synchronous generator technology.Based on MATLAB/Simulink,the simulation models of MMC system under vector control and VSG control are established respectively.The output voltage,power and frequency characteristics of the system under the two control modes are compared by changing the system grid connection control parameters.The proposed VSG control method is verified and the stability of the system is analyzed.The simulation results show that the control strategy designed in this paper is correct.