| The development of China’s electric utility industry needs Extra High Voltage, large capacity and long-distance transmission modes for the reversal distribution of energy resources and electrical load. With the increase of DC transmission capacity in the proportion of the AC/DC hybrid power system, the risk of Subsynchronous Oscillation caused by DC system is increasingly high. Therefore, the study of Subsynchronous Oscillation caused by HVDC system is of great significance.In this thesis, the time-domain simulation modeling problem of HVDC control system has been studied. Following the discussion of the basic principle of HVDC control systems, the time-domain model of Suizhong back-to-back converter station is built. This thesis focuses on the control system modules related to the fault transient process. Based on the traditional CIGRE benchmark model, VDCOL modules are improved and CFPREV module is added. Finally, the effectiveness of the built model of HVDC control system is verified by comparing the time-domain simulation results and the actual recorded data.Electrical damping coefficient of system that a single generator connected to an infinite grid by HVDC system is detailed derived. Then, risk of system Subsynchronous Oscillation occurrence is analyzed on the basis of the calculated electrical damping characteristic curve. Test signal method is the time-domain form of the complex torque coefficient method, its implementation steps and application conditions in multi-machine systems are summarized. By test signal method, the influence of DC power, DC voltage, and rectifier controller parameters to electrical damping characteristics in the Suizhong system is studied based on time-domain simulation model.The physical mechanism of the transient torque amplification is theoretically analyzed. On the basis of the decoupled shafting multi-degree of freedom vibration equation, shafting modal vibration process is divided into two stages of the forced oscillation and the free oscillation, then relationship between electromagnetic torque and each Subsynchronous Oscillation modal components is analyzed. The initial maximum values of shafting subsynchronous oscillation modal components are closely related to the electromagnetic impact torque generated during a large disturbance process. |
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