1.China Electric Power Research Institute，Beijing100192，China;2.Sichuan Electric Power Corporation，Chengdu610041，China

Following the Fulong-Fengxian ultra-high voltage direct current（UHVDC）transmission project put into operation at the end of the11th five-year plan，Xiluodu-Zhejiang±800kV/8000MW UVHDC will be put into operation in2014.Because the supporting generators are commissioned ahead of the UHVDC project，Xiluodu hydropower station will have to be connected to Sichuan power grid by long-chain alternating current（AC）channel in2013in order to make full use of hydropower.Studies show that AC fault occurred at the outlet of Fulong converter station will cause the second swing transient instability and oscillations of increasing power angle amplitude due to AC and direct current（DC）coupling interactions.Based on the disturbed dynamic trajectory，the mechanisms that the second swing transient instability caused by DC power fast recovery and weakening of oscillation damping caused by DC power fluctuation are revealed.Based on the stability influence factors analyzed for the transition of the UHVDC construction，the corresponding control measures are proposed.The conclusion provides important technical support to enhance hydropower output capacity.