1. State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Beijing 102206, China; 2. State Grid Tianjin Maintenance Company, Tianjin 300000, China;3. State Grid Economic and Technological Research Institute Co.Ltd., Beijing 102209, China

In order to effectively evaluate the voltage stability of the wind-thermal-bundled high voltage direct current(HVDC)sending system and determine the upper limit of penetration ratio of wind power that satisfies voltage stability, the short-circuit ratio index of wind-thermal-bundled HVDC sending system is deduced, and the influence of ratios of wind/thermal power and ratios of local load are taken into account. Firstly, the electromagnetic transient simulated model of wind-thermal-bundled HVDC sending system is established by PSCAD/EMTDC. The correctness of the short-circuit ratio formula is verified by setting short-circuit fault of the commutating bus. Then, the simulations are carried out by setting different local load ratios, wind power permeabilities, fault types and fault locations, and the critical short-circuit ratio of wind-thermal-bundled HVDC sending system that meets voltage stability is determined. In the end, a simple and practical method for calculating the maximum penetration ratio of wind power for wind-thermal-bundled island DC sending system which is applicable to long-term planning is summarized.