Sub-synchronous Torsional Interaction Mechanism Analysis Of Doubly-fed Wind Generator

With wind power capacity increasing fast, the issues about sub-synchronous oscillation in wind turbines is highlighted in China. Doubly fed induction generator(DFIG) becomes the mainstream type of wind turbine for its decoupling control of active and reactive power, but its rotor excitation converter system may send negative damping to exciting shaft oscillations.This paper discusses the mechanism of the doubly fed wind Generator connected to the grid which may inspire the shaft torsional vibration problems due to the unreasonable setting excitation control system parameters. The main work is as follows.First, a doubly fed wind power generation system model including shaft model,induction generator model, excitation transformer models and transmission line models is established for the analysis of sub-synchronous oscillations. Describe the stator and rotor converter working principle, vector control strategies in detail. A doubly fed wind turbine via the transmission line connected the power system simulation model is built on Matlab/ simulink simulation platform.Then, linear each dynamic element including the shaft model, induction generator model, excitation transformer model, transmission line model, and form a whole systems of linear equations based on doubly fed wind power generation system of infinite single system topology. Obtain the characteristic roots and participation factors in the steady-state operating point, and identify the dominant influence of the state variable to shaft torsional vibration. The results show that: the shaft torsional vibration is only associated with the shaft state variable, and there is no correlation between excitation converter controller parameters, and excitation converter controller parameters can not excite SSTI phenomenon.Finally, in order to further validate the correctness of the conclusions through the eigenvalue analysis, derive the expression of transfer function from the rotor speed deviation amount to the increment of stator output active power, select a representative RSC dual closed loop PI controller parameters, draw amplitude frequency characteristics and phase frequency characteristic curve of the transfer function through the MATLAB program, analyzes SSTI phenomen through amplitude-frequency and phase-frequency curve in the shaft torsional vibration frequency range. The results show that: the change of proportional coefficient inner and outer ring of the rotor side converter does not affect the nature of the damping; the change of integral coefficient can change the nature of the damping. But the magnitude of the torque is small; the damping effect can be ignored.The rotor side converter parameter does not excite SSTI phenomenon. Finally, the conclusions of the two methods are verified by time domain simulation.