Research On Analysis And Suppression Methods Of Sub-synchronous Oscillation In DFIG Power System

In our country,load demand and energy storage present a reverse distribution pattern,making large-capacity and long-distance power transmission an inevitable trend.The series capacitor compensation transmission technology is one of the important ways of wind power transmission.While increasing the power transmission capacity,it will induce sub-synchronous oscillation in the DFIG,which will seriously affect the safety of the power system.The DFIG can realize the decoupling control of active and reactive power,and realize high-efficiency power generation with variable speed and constant frequency,so it becomes one of the most widely used wind turbines today.Therefore,this thesis focuses on the sub-synchronous oscillations that occurred in the DFIG power system in Hexi,Gansu.The main contents in the thesis are as follows:First,establish mathematical models of DFIG power systems suitable for sub-synchronous oscillation research,including wind turbine models,doubly-fed induction generator models,dual PWM converter control system models,and series capacitor compensation transmission line models.On this basis,the generation mechanism of sub-synchronous oscillations of DFIG is analyzed.The established mathematical model provided a theoretical foundation for the later use of eigenvalue analysis method,complex torque coefficient method and time domain simulation method to study the sub-synchronous oscillation of the DFIG power system.Secondly,based on the eigenvalue analysis method,the influencing factors of the sub-synchronous oscillation of the DFIG power system are studied.Derive the linearized model of the system at the steady-state operating point,determine the state matrix of the system,and solve the eigenvalues,eigenvectors,and participation factors of the state matrix,through actual cases,analyze the influencing factors of the sub-synchronous oscillation under low wind speed operating condition,then identify the oscillation mode according to the eigenvalues,and determine the influencing factors of the oscillation mode by the participation factor,after that the influence of series compensation,wind speed,rotor-side converter inner loop gain and integral time constant on the sub-synchronous oscillation were simulated and verified in PSCAD/EMTDC.Finally,based on the complex torque coefficient method,the transfer function between the electromagnetic torque increment and the rotational speed increment of the wind turbine is deduced,so make the study of the sub-synchronous oscillation of the DFIG power system from the electrical damping level clearer in physical significance.On this basis,focus on the shortcomings of band stop filters in suppressing sub-synchronous oscillation,install an supplementary damping controller at the d-axis output end of the grid-side converter current inner loop of doubly-fed wind turbines is proposed to suppress sub-synchronous oscillation.Then the improved particle swarm optimization is used to optimize the gain coefficient of the supplementary damping controller.The simulation results show that the supplementary damping controller optimized by the improved particle swarm optimization has better suppression performance on the sub-synchronous oscillation of the DFIG power system.The simulation results show that the supplementary damping controller designed in this thesis can suppress the sub-synchronous oscillations that occur in the DFIG power system,and its suppression performance is better after the improved particle swarm optimization is optimized.