Research On Subsynchronous Oscillation Of The Wind-Thermal-Bundled System Connected To LCC-HVDC Lines

With the rapid development of new energy and the reverse distribution of wind energy and load centers in China,wind-thermal-bundling system via HVDC transmission has become a major way.However,unlike thermal power generator,new energy mostly use power electronic converters to connect to the power grid,which may cause new subsynchronous oscillation problems.For wind-thermal bundling system connected to LCC-HVDC Lines,the subsynchronous interaction among wind power,thermal power and LCC-HVDC becomes more complex.At present,there are few studies on the subsynchronous oscillation of wind-thermal bundling system connected to LCC-HVDC Lines,and its mechanism has not been fully explained.This paper mainly explores the subsynchronous oscillation characteristics of the wind-thermal-bundled system connected to LCC-HVDC lines.The small signal models based on FSIG and DFIG connected to LCC-HVDC Lines and the time domain simulation model of wind-thermal-bundling system connected to LCC-HVDC Lines are established.The subsynchronous oscillation characteristics based on FSIG and DFIG connected to LCC-HVDC Lines are analyzed by eigenvalue analysis method.The wind-thermal bundling system connected to LCC-HVDC Lines is carried out by time domain simulation analysis.Specific contents include the following aspects:The small signal models based on FSIG and DFIG connected to LCC-HVDC Lines are established and its subsynchronous oscillation characteristics are analyzed.According to the characteristics of the two kinds of wind generator,the SSTI of the FSIG wind turbines connected to LCC-HVDC lines system is analyzed emphatically.Three main oscillation modes associated with SSTI were identified in the entire system by small signal analysis,including two torsional modes and an electromechanical mode.The dominant factors analysis demonstrated that a system with a higher wind farm capacity or a longer distance between the wind farm and the rectifier station is more susceptible to drive the system less stable.The results indicate that wind farms with FSIG-based connected to LCC-HVDC lines may not cause the unstable SSTI phenomenon.For the wind farms with DFIG wind turbines connected to LCC-HVDC lines system,the subsynchronous interaction between the DFIG and the LCC-HVDC system is analyzed.In order to improve the accuracy of the model,the step response curves of the detailed electromagnetic transient simulation model and the small signal model are compared to verify the correctness of the model.Small signal analysis and participation factor analysis are then applied to investigate the interaction characteristics between the Wind Farms and the LCC-HVDC system.It is shown that the interaction between the twosystems is not obvious.Impact of dominant factors analysis show that the system stability margin is large under working conditions,but there is a possibility of subsynchronous oscillation when changing system parameters.The time domain simulation analysis of wind-thermal-bundling system connected to LCC-HVDC Lines is carried out.The influence of wind power integration on SSTI of thermal power unit connected to LCC-HVDC transmission system is mainly analyzed.According to the position of the wind power and the actual installed capacity,it is simplified as a subsynchronous current source.When the oscillation frequency matches the natural frequency of the shafting of the nearby thermal power unit,the torsional vibration of the shafting will occur.The subsynchronous oscillation characteristics of the system to be studied are observed by comparing the subsynchronous oscillation currents injected with different amplitudes and different modes of shafting oscillation.When the values of the current frequency of the subsynchronous oscillation associated with the shafting are injected,all the corresponding oscillation modes will be excited to oscillate.With the increase of injection current amplitude,the corresponding oscillation amplitude of each mode also increases.