Stability Analysis And Control Of Doubly-Fed Wind Farm Transmitted Via Flexible DC System

With the large-scale grid-connected operation of wind power,double-fed wind farm through flexible DC transmission has a complex and huge influence on the dynamic characteristics of traditional power system,which makes the safe and stable operation of the system under large disturbance face new challenges.When the doubly-fed induction generator is subjected to large disturbance through the flexible direct feed,the interaction between the rotor side converter,the grid-side converter and the control link of the flexible direct converter station may induce the risk of system oscillation instability,which is easy to cause the large-scale off-grid of doubly-fed wind turbines and the outage of the flexible direct transmission system.Therefore,by studying the energy flow path and distribution law of the system with flexible direct feed of doubly-fed induction generator under large disturbance,this paper analyzes the oscillation energy response characteristics of each control link,locates the key energy path that causes the system oscillation,and designs the oscillation suppression control strategy accordingly.The main contents and achievements of this paper are as follows:In order to solve the problem of system oscillation caused by large disturbance of doubly-fed induction generator system,a dynamic energy modeling method based on subsystem division is proposed in this paper.Firstly.the dynamic mathematical model of the outgoing system in the unified synchronous coordinate system is established,and the analytical expressions of transient current in the fault traversing process and fault recovery process are solved.On this basis,the outgoing system is divided into subsystems,and the high order complex system which is difficult to be analyzed by traditional energy function is decomposed into low order subsystems.The dynamic energy models of different subsystems are established by first integration method,and the energy flow paths between different subsystems are analyzed.In view of the influence of large disturbance on the stability of the flexible direct feed system of doubly-fed induction generator,this paper determines the main energy redistribution path of the system by analyzing the energy flow direction and value of the energy flow path between subsystems in the process of fault crossing and fault recovery.On this basis,based on Lyapunov’s second stability principle,the storage energy of the system is used to reflect the stability level of the system.The mutual conversion law of the storage energy and the interaction energy of each subsystem in the process of fault crossing and fault recovery is analyzed.From the perspective of energy,the mechanism of large disturbance affecting the stability level of the system sent by the doubly-fed induction generator through the soft direct is explained.In order to solve the problem of large disturbance oscillation tracing and oscillation suppression control of doubly-fed induction generator system through flexible direct transmission,this paper proposes an oscillation tracing method based on the oscillation energy response characteristics of the subsystem and an oscillation suppression control method based on the oscillation transmission path suppression.First,according to their contributions to the oscillation energy of the system,the interaction links among subsystems in the oscillation process are divided into"oscillation gain link","oscillation maintenance link" and "oscillation damping link".By analyzing the oscillation gain link corresponding to the leading interaction energy item of each subsystem,the transfer path of the leading oscillation energy with continuous gain in the flow process is determined.Find the source of the energy that causes the oscillation of the system.On this basis,the key control links that play a role in amplifying and transmitting the oscillation energy are screened out,and additional control links are designed to inhibit the propagation of the oscillation energy in the system,so as to suppress the oscillation caused by large disturbance.