Research On Impedance Modeling And Passivity Method Of Virtual Synchronous Generator

Energy is an important material foundation for the stable development of national economy and the long-term stability of people’s livelihood society.Energy security is related to the overall situation of national security.Facing the global energy crisis and climate and environment change,the green development direction of new energy as the main energy is recognized by the international community as the future energy development strategy.With the continuous growth of the demand for new energy generation to connect to the grid,the permeability of grid connected converter,as the interface between new energy generation equipment and grid,is increasing in the grid.The frequency stability of high grid converter permeability power grid system is becoming more and more serious,and it does not have the ability of short-term overload,which leads to the sharp decline of the stability of the grid system.In order to solve the above problems,the concept of virtual synchronous generator is put forward.The concept of virtual synchronous generator is based on the simulation of the dynamic characteristics of traditional synchronous generator.By adding the dynamic equation of rotor rotation to droop control,the grid connected inverter has the same inertia and damping as the traditional generator,which makes the simulation of synchronous generator more real and helps to improve the stability of power grid.This paper takes the virtual synchronous generator as the research object,and carries out the research work.Firstly,based on the topology of the virtual synchronous generator,namely,the three-phase voltage inverter topology,the static coordinates of three-phase ABC and two phases of the three-phase voltage inverter are derived according to the knowledge of circuit analysis and automatic control principle αβ The mathematical model of static coordinate system and DQ rotating coordinate system.According to the control structure and working principle of the virtual synchronous generator,the small signal model of the virtual synchronous generator is deduced.Finally,according to Mason Gain rule,the equivalent circuit and port equivalent impedance of virtual synchronous generator considering the main circuit and control algorithm are derived.Secondly,based on the mathematical model of virtual synchronous generator,according to the typical system design method,the control parameters of the virtual synchronous generator are designed,including: 1)decoupling analysis of voltage and current control double loop;The PI controller parameters of voltage loop and current loop are designed by using the conditions of zero pole cancellation,phase angle stability margin and damping ratio;2)The power control loop of virtual synchronous generator is modeled by small signal and its decoupling analysis is carried out;3)The parameters of active power control loop and reactive power control loop are designed by using the constraints of bandwidth,phase angle stability margin and attenuation of twice power frequency wave momentum.Finally,the stability of the system under different short circuit ratio of power grid is analyzed by using the stability criterion based on impedance ratio,and the simulation results are carried out.The simulation results show that when the virtual synchronous generator is incorporated into the grid with small short circuit ratio,the grid connection is stable;On the contrary,it is unstable.Finally,according to the equivalent circuit and the port equivalent impedance model of the virtual synchronous generator derived in Chapter 2,the influence of the control parameters of each functional unit on the port impedance of the virtual synchronous generator is analyzed.Then,under the constraints of expanding the passive area of port impedance,the control parameters are optimized to expand the equivalent impedance passive area of the virtual synchronous generator port,so as to improve the grid stability of the virtual synchronous generator.Finally,the model built by MATLAB further verifies the grid stability of the virtual synchronous generator after passive optimization.Similarly,the stability of the virtual synchronous generator under different short circuit ratio of power grid is analyzed after port impedance passive optimization,and the simulation results are carried out.The simulation results show that the stability of the virtual synchronous generator is improved after passive optimization.