Stability Analysis Of Three-phase Voltage Source Converters And Switching Control Of Mode

Most of China’s wind energy resources are located in the northeast,northwest and north China,so the wind power generation adopts the development mode of centralized development and long-distance transmission.The centralized connection of long-distance lines leads to the weak grid characteristics of the point of common coupling,and the interaction between the direct drive fan and the grid leads to various stability problems of the system.Moreover,the traditional vector control method of wind power grid-connected converter is lack of inertia and damping,so it cannot support the grid voltage.The grid-connected converter using voltage source virtual synchronous control simulates the operation characteristics of synchronous generator,and can provide voltage and frequency support for the power grid,which is especially suitable for grid-connected power generation under weak grid conditions.However,due to the operation characteristics of the simulated synchronous generator,the stability problem of the virtual synchronous generator is similar to that of the synchronous motor.In this paper,the voltage source converter as the research object,using the eigenvalue analysis method based on the state space model,the stability of the voltage source converter grid-connected system is studied,and the stability control method to solve the system oscillation is proposed.Then,the mode of the direct-drive fan grid-connected system considering the generator side converter control is studied,and a smooth switching control strategy is proposed to reduce the output power fluctuation during the mode switching process.Firstly,the control strategy of voltage source-type virtual synchronous machine is introduced.The voltage source-type control is realized by the grid-side converter in the grid-connected power generation system.The state space modeling of the voltage source-type grid-side converter is carried out,and the line impedance,virtual synchronous control and inner loop control are fully considered while the machine side dynamics are ignored.Then the system simulation model is built and the input admittance frequency characteristics of the system are measured by frequency scanning method.The frequency characteristics were drawn by transforming the system state space model into admittance model,and the accuracy of the model was verified by comparing with the measured admittance frequency characteristics.Based on the state space model,the eigenvalue analysis method is used to analyze the main parameters affecting the stability of the system by using the participating factors,and the change trend of the dominant eigenvalue when these parameters change is further analyzed by using the root locus,so as to determine the influence of the parameter change on the stability of the system.On this basis,a stability control method to suppress the low frequency oscillation of the system is proposed.Due to the grid-connected generation system of direct-drive fan needs to switch the grid-connected mode,this paper proposes a mode switching method considering both the generator side converter and the grid-side converter,which can minimize the output power fluctuation when mode switching between the voltage source control and the traditional vector control.Finally,the stability analysis results are verified by using the converter platform in the laboratory,and the effectiveness of the proposed control method for smooth switching of grid-connected mode is proved.