The Study Of SSO Caused By PMSG-based Wind Turbines Connected To Weak AC System

As the fastest growing new energy generation technology,wind power generation has been developed and applied on a large scale in many parts of the world.Wind farms are mostly located in remote areas,where the power grid is weak AC system due to long transmission distance,small local load and relatively weak grid structure.In recent years,the proportion of direct-driven permanent magnet synchronous generator(D-PMSG)in China has increased steadily.However,existing research and practical projects show that there exists sub-synchronous oscillation problem when D-PMSG is connected to the grid via weak AC system.At present,there is less in-depth analysis on this issue,and further research is urgently needed.This paper describes the research background and significance of subsynchronous oscillation about D-PMSGs at home and abroad,including the generation mechanism,main research methods and suppression methods.Based on the aggregation equivalent model of wind turbine,the electromagnetic transient model of D-PMSG grid-connected system is established on PSCAD platform,and the subsynchronous oscillation of the D-PMSG connected to the weak AC system is simulated.A small-signal model of D-PMSG connected to the weak AC system with phase-locked loop dynamics in synchronous rotating coordinates is established.The accuracy of the small-signal model is verified by comparing the simulation results of the small-signal model in MATLAB and the electromagnetic transient model in PSCAD.By means of eigenvalue analysis and time domain simulation,the effects of AC power grid strength,generator output,grid-side converter control parameters and phase-locked loop control parameters on sub-synchronous oscillation are studied,and improvement measures are put forward.In order to solve the sub-synchronous oscillation problem of D-PMSG connected to weak AC system,a broadband additional damping control method is designed based on the calculation results obtained by eigenvalue analysis method.The additional damping control is introduced into the control section of the grid-side converter with a DC voltage as a feedback input signal,thereby enhancing the sub-synchronous mode damping of the D-PMSG grid-connected system.In addition,the control parameters and feedback action positions are optimized and analyzed,and the optimal parameters suggestion for improving the suppression effect are given.The simulation model is built in the PSCAD platform to verify the correctness of the above research conclusions and the effectiveness of the proposed control strategy.