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External insulation design of UHV power project is important. The lightning impulse (LI) voltage is one of the main factors that determine the air gap distance. Based on existing LI voltage results of power project typical gap, impact of gap structure and distance on LI discharge correction is analyzed. And from a theoretical aspect, existing LI discharge correction methods are summarized. Furthermore, by the use of common correction methods, LI voltage correction factors of rod - plane were calculated and analyzed. Results show no significant impact of gap type or gap distance on LI voltage correction is found out. Average atmospheric conditions are effective when atmospheric parameters method is used for LI voltage correction. DL/T 620, IEC 60060-1 and IEC 60071-2 can be used for LI voltage correction at areas with altitudes of 1000-5000m. While GB311.3-improvement method can be used for LI voltage correction at areas with altitudes blow 4000m.