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Southeast Tibet in China is rich in clean energy such as water energy, light energy, and wind energy. As the key means of large-scale clean energy transmission in southeast Tibet, the ultra-high voltage (UHV) power transmission projects will be constructed at an altitude of nearly 4000 m. To accurately acquire the external insulation strength of electrical equipment operating at different altitudes, the standard positive switching impulse voltages were applied to a rod electrode and a 1.1 m diameter shielding sphere at the altitude of 3400, 2100, and 80 m. The switching impulse discharge characterization and altitude correction factors of the 1.1 m diameter shielding sphere-plate gap under different clearances at different altitudes were obtained. The discharge voltage of the rod-plate gap at the altitude of 80 m was corrected to the altitude of 3400 m using altitude correction methods recommended by GB 311.1, GB/T 16927.1, and DL/T 2305. Further comparison was made between the calculation results using the three altitude correction methods mentioned above and the test results at an altitude of 3400 m. The discharge characterization and the minimum clearance of the end fittings of the electrical equipment in the valve hall at the altitude of 3800 and 4500 m were obtained by fitting and extrapolating the test data. The research results provide important reference significance for the external insulation configuration of UHV converter stations constructed in areas with an altitude of 3000 m and above. [ABSTRACT FROM AUTHOR]