| With implementation of "West to east power transmission, North-south power exchange and Nationwide interconnection", the grid is gradually developing into AC/DC hybrid power system. AC-DC hybrid transmission brings great benefits, it would also bring subsynchronous oscillation at the same time. Therefore, the study of subsynchronous oscillation caused by HVDC system is of great significance.From the viewpoint of electrical damping, the paper derives electrical damping coefficient of system that a single generator connected to an infinite grid by AC-DC hybrid transmission by complex torque coefficient method in detail. Then according to the Gaoling back-to-back HVDC power transmission project, it analyzes the electrical damping characteristics of AC/DC parallel operation and DC islanded operation, and summarizes the influence of ac bus strength of rectifier side, generator output, DC power, rectifier controller parameters and reactance of DC line to electrical damping characteristics.Based on the actual DC project, it simplifies the units not to be studied in the he multi-machine system with different approaches for different operating conditions. Then the electrical damping curves obtained by test signal method are compared with the results of Prony modal identification in order to obtain application conditions in multi-machine systems.Finally, based on electrical damping, a power grid equivalent method for subsynchronous oscillation caused by DC system is proposed for the first time. It mainly includes the screening of the units, equivalence of external network at sending end and equivalence of external network at receiving end. It is applied to reduce an actual grid which involves an 800kV UVDC Project. |
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