The Simulation Analysis And Mitigation Method Research Of Subsynchronous Oscillation In 800kV UHVDC System

Generally,increasing the voltage level of transmission line could significantly increase the transmission power,improve the distance of transmission,decrease the current of transmission line,greatly reduce the line loss,and simultaneously reduce the amount of metals used for conductors and overhead tower constructions.Therefore,it is suitable to apply High Voltage Direct Current(HVDC)or Ultra High Voltage Direct Current(UHVDC)system when long-distance and large-capacity power transmission is required.However,Subsynchronous Oscillation(SSO)phenomenon would occur due to the interaction between mechanical subsystem and electrical subsystem during the operation of UHVDC transmission system,resulting in fatigue,crack,and even fracture of generator shaft,which seriously threatens the safety of generator and power system.Based on the ?800kV UHVDC system,the SSO phenomenon is analyzed and studied in this thesis.The main research contents of this thesis are as follows:1.The causes of SSO induced by the series compensation device s and the UHVDC fast control system are analyzed in detail.Then for the subsynchronous oscillation induced by UHVDC system,according to the switching function of commutation process of the converter in UHVDC system,the transfer process and relationship of the electrical disturbance between AC and DC system is derived.2.Several model analyses of key subsystems in the UHVDC system are done including equivalent circuit model,generator shaft model and control regulation system model.For the structure of the UHVDC system,the equivalent circuit model is analyzed and the relationship among basic parameters is explained by the model.For the generator shaft system at power plant side,shaft segmental concentrated mass spring model is selected,and the 6-mass block shaft model is analyzed and studied in detail.For the UHVDC control and regulation system,the hierarchical structure and the basic control methods are researched separately.3.Based on the data of IEEE SSR first benchmark and Yunguang UHVDC project,a ?800kV bipolar ground return UHVDC system is established according to the key models and control methods of previous analyses and researches.The correctness and effectiveness of the established system are verified by time domain simulation software.The phenomena of subsynchronous oscillation occurred in the system are analyzed and studied.4.The torsional modes of the generator shaft in this thesis are analyzed in detail.And the simulation results are used for the spectrum analysis to determine the subsynchronous torsional modes that need to be suppressed.Then,according to the results of generator shaft analysis and parameters set in UHVDC system,2 types of Supplementary Subsynchronous Damping Controller(SSDC)are designed based on the phase compensation method.Finally,2 SSDCs are added to the UHVDC system separately and verified by time domain simulation.