Research On Virtual Impedance Control Technology Of MMC Converter Based On Adaptive Rotational Inertia

With the rapid development of social modernization,fossil energy can no longer meet the requirements of human society's forward development.Building a stable,reliable,safe and environment-friendly new generation power network has become an inevitable trend of the times.The flexible interconnected distribution network based on flexible power electronic switches has many advantages,such as flexible adjustment methods,low operation cost,fast response speed and small fault influence range,etc.,and has gradually become a hot spot of power system research and application.However,the converter station of the flexible distribution network using the traditional control strategy has too fast response speed and almost no inertia and damping,which makes it difficult to provide the necessary voltage and frequency support for the power network.Therefore,this paper takes the flexible interconnected distribution network based on Modular Multilevel Converter(MMC)as the research object,and proposes a grid-connected Virtual Synchronous Generator(VSG)inertia damping adaptive control strategy.In the meantime,in order to make the system more stable,a VSG control method to improve the damping performance of the weak grid is also proposed.The specific research content is as follows:Firstly,the basic mathematical model of Synchronous Generator(SG)is described,and the VSG technology is studied by referring to its working principle.In order to enable the VSG to simulate the external characteristics of the traditional SG,the virtual governor and virtual excitation controller of the inverter are designed.Secondly,the basic topological structure,working principle and traditional control strategy of hybrid submodule MMC are introduced.On this basis,taking MMC type flexible interconnected distribution network at both ends as the research object,VSG control technology is introduced into the control of MMC converter.Then,aiming at the problems of the traditional VSG control strategy,such as the lack of obvious improvement of grid-connected power and frequency and poor system robustness,the influence of important parameters such as rotary inertia and damping coefficient on the output characteristics of VSG was analyzed from the physical characteristics,and an adaptive control method of inertia damping was proposed,and the design principles of inertia and damping were given.At the same time,the simulation is verified by PSCAD/EMTDC.Finally,in order to further improve the stability of the system,a MMC control strategy based on virtual resistance is proposed and verified by PSCAD/ EMTDC.