Analysis And Suppression Of Low Frequency Oscillation Of Direct-drive PMSG Based On Damping Torque Method

Large-scale wind power is connected to the grid via long transmission lines,the transmission line impedance cannot be ignored,the weak grid characteristics of the system into which wind power is integrated are significant,and oscillation accidents occur frequently,which seriously threaten the safe and stable operation of the power system and restrict the rapid development of wind power.At present,there is still no unified conclusion in academic circles about the induced mechanism of direct-drive PMSG oscillation.How to reveal the oscillation mechanism and suppress the oscillation of direct-drive PMSG an important issue to be studied urgently..In this paper,the low-frequency oscillation characteristics of direct-drive PMSG under weak grid are studied based on the damping torque method,and the main research contents are as follows.（1）A linearized model of grid-connected direct-drive PMSG is constructed to analyze the low-frequency oscillation characteristics of a direct-drive PMSG connected to a weak grid;the factors that play a dominant role in the two low-frequency modes of the system are determined to be the DC voltage loop and the phase-locked loop through participation factor calculations,and then the effects of grid strength,operating conditions and control parameters on the low-frequency oscillation characteristics are analyzed.（2）A damping torque model is established for the stability analysis of DC voltage loop and phase locked loop,and the damping torque method is used to reveal the destabilization mechanism of direct-drive PMSG and analyze the influence of various influencing factors on the damping characteristics of the system;for the dynamic coupling of parallel direct-drive PMSGs under weak grid,a damping torque model of the two-machine system is established,and the interaction between the units is portrayed by the damping transfer function,and the interaction is analyzed with the grid strength,operating conditions and control parameters.The damped torque model is developed for the two-machine system,and the damping transfer function is used to characterize the interaction between the units,and analyze the trend of the interaction with grid strength,operating conditions and control parameters.（3）A new direct-drive PMSG stabilizer is designed based on the phase compensation method to suppress the instability of direct-drive direct-drive PMSG in low-frequency modes（dc-voltage loop mode and phase-locked loop mode）.Based on the established damped torque model,the phase compensation loop is introduced by analyzing the damping phases of dc-voltage loop and phase-locked loop,and the phase and amplitude of the damping transfer loop are adjusted,which can effectively enhance the damping of dc-voltage loop and phase-locked loop and suppress the damping of direct-drive wind turbines.The effectiveness of the stabilizer is verified based on the eigenvalue analysis and time domain simulation results.