Stability Analysis And Control For PMSG-based Wind Power Generation System During Fault In A Weak AC Grid

Since wind farms are often located far away from load centers,high-impedance and long-distance transmission has become a critical factor for wind farms to connect to the grid.In addition,with the continuous growth of the penetration of wind power in the power system,the supporting effect of AC power system on wind farm is gradually weakened,i.e.,the grid strength becomes weaker,namely the weak grid.The interaction between the grid impedance and different controllers of wind power generation system will become more serious with a weak grid connection.Besides,the coupling will be aggravated during low voltage ride-through,thus destroying the system stability,which will seriously threaten the reliability and stable operation of the power system.Therefore,to study the small-signal stability of the system during LVRT with a high impedance grid connection,the eigenvalue analysis method is employed to investigate the oscillation characteristics and influencing factors of the system stability.Furthermore,based on the complex torque coefficient method,an additional damping control strategy which is on the basis of generation system's regulation potential is proposed in this paper.The main research contents are as follows:Firstly,the detail mathematical model of the PMSG-based wind power generation system for the analysis of the small-signal oscillation is established,which includes the mathematical model of the PMSG,back-to-back pulse width modulation(PWM)converter and its control system,phase lock loop(PLL),wind turbine,shafting and grid transmission line etc.Based on the method of Lyapunov,the linearization model of the system is deduced,and then the interaction among grid impedance,PLL and current controller is illustrated.Secondly,based on the eigenvalue analysis method and the linearization model of the system during LVRT,the corresponding oscillation characteristics of the different system modes can be obtained,and the oscillation mode of which the damping is weakest can further be found.Moreover,the influencing factors of the system stability are investigated by using eigenvalues locus.And then the time-domain simulations and experiments are conducted to verify the theoretical analysis.Finally,referencing to the traditional power system stabilizer,based on the system's regulation potential and the interaction between grid impedance and system's controllers,a small-signal oscillation suppression strategy is proposed by add an additional damping control loop in the inner current control.Furthermore,based on the complex torque coefficient method,the corresponding parameters of the proposed oscillation suppression strategy is designed.In addition,the time-domain simulation and experimental results prove the effectiveness of the proposed control strategy.The research of this paper may lay the foundation for improving not only the oscillation characteristics,but also the stability and LVRT capability of the PMSG-based wind power generation system.In addition,the study in the paper can also provide the theoretical analysis for parameters tuning and control strategies designing of the system under a weak grid condition.