Optimal Control Of Dynamic Response Of Transmission System Of DFIG Under FRT

Vigorously developing clean,green,and renewable new energy sources is currently a common choice for countries around the world in the energy industry.Among various types of new energy,wind power has been widely used worldwide due to its own advantages.With the increasing penetration of wind power in the power grid,the requirements for the fault-ride-through(FRT)capability of wind turbines are also increasing.Doubly-fed induction generator wind turbine(DFIG)is the main model in the wind power field at present.Due to its structural limitations,the disturbance of the active power and electromagnetic torque output by DFIG during FRT is severe,resulting in dynamic response of the transmission system,including significant and long-lasting fluctuations in generator speed,as well as low-frequency torsional oscillation of the shaft.These issues limit the FRT capability of DFIG and may cause damage to itself.Therefore,this paper conducts research on the dynamic response of the transmission system during DFIG fault traversal,reveals its generation mechanism and influencing factors,and proposes corresponding suppression strategies.This article first establishes a mathematical model of DFIG and its control system,analyzes the power output characteristics under FRT,and builds a simulation testing platform on DIgSILENT PowerFactory.The formation process of speed fluctuation and shaft torsional vibration under FRT was analyzed in principle,and the factors affecting the dynamic response strength of the transmission system were preliminarily analyzed from two aspects:input characteristics and output characteristics through simulation.Then,a speed fluctuation suppression strategy for improving pitch control is studied.Based on the analysis of small-signal modeling and the current pitch control,a forecast feathering+integral locking pitch control strategy is proposed to improve the speed response under FRT.The forecast feathering module is used to improve the performance of the wind turbine in absorbing wind power of following electromagnetic power,accelerate pitch response speed,reduce speed overshoot,and prevent generator overspeed.The integral locking module achieves system order reduction,which can avoid excessive drop of speed and shorten transient duration.Furthermore,the mechanism of shaft torsional vibration is studied from an energy perspective.Base on the two-mass block model of the shaft system,the transient energy directly related to the torsional vibration of the shaft system is defined,and the impact of electromagnetic torque disturbance on the transient energy during FRT is analyzed.The analysis results indicate that the recovery of electromagnetic torque can play a role in dissipating transient energy and suppressing torsional vibration under specific circumstances.Based on this,a fast torsional vibration suppression strategy for FRT is proposed,which recovers active power within a specific time period after fault clearance,and utilizes this process to suppress torsional vibration and accelerate active power recovery.Finally,by combining the above two control strategies,a comprehensive optimization strategy for the dynamic response of the transmission system of DFIG under FRT is obtained.Each strategy has been simulated and validated on the DIgSILENT PowerFactory platform,and the results show that the suppression strategies for speed fluctuations and shaft torsional vibration have good control effects.The comprehensive optimization strategy achieves the superposition of control effects,reduces the speed fluctuation amplitude and duration of DFIG during FRT,reduces the degree of shaft torsional vibration and fatigue torque,and accelerates the recovery speed of active power after fault clearance.This comprehensive optimization strategy has good feasibility and adaptability,which can improve the FRT capability of DFIG.