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The meteorological causes of wind deflection and tower collapse of a 500 kV transmission lines are analyzed. The wind speed distribution characteristics in the fault area of the line are obtained by using the observed wind speed of an automatic weather station and the three-dimensional wind field based on radar inversion. The results show that the maximum wind speed and the extreme wind speed observed by automatic weather stations around the two accidents are small. The three-dimensional wind field of radar inversion better indicates the fault's location and time. Compared with strong convective weather which can cause the wind deflection， the strong convective weather which can cause the tower collapse has a higher echo height and stronger updraft. The core height of its reflectivity factor drops more than 3.5 km within 6 minutes， causing greater ground damage， but its locality is strong and the influence range is small. The results show that the current design wind speed of the transmission line is safe under the strong convective weather that causes wind deflection， but adverse terrains that can accelerate the wind speed must be avoided. It is suggested to take “cascaded collapse prevention” as the main idea for the microburst which can cause the tower collapse. In addition， under the influence of strong convective weather， the gust coefficient calculated by the observed wind speed is significantly larger than the value in the relevant specifications.

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The icing flashover characteristics and discharge processes of 500 kV ac transmission line long insulator strings energized in the ice accretion were studied. The influence of pollution and grading rings (including simulated conductor) on heavy ice-covered insulators' flashover voltage were considered in this paper. Based on the artificial icing test results, it was found that 1) under the severe icing conditions and three salt deposit density (SDD) levels of ρSDD=0.1, 0.05, 0.025 mg/cm², compared with 500 kV transmission line rated phase to ground voltage, the flashover voltages of 28 units XWP2-160 porcelain double-shed insulator are 20.5% lower, 13.1% lower and 2.3% higher respectively; and the values of the FXBW4-500/160 composite insulator are 18.7% lower, 12.0% lower and 4.3% higher respectively. 2) equipped with the given grading ring and simulated conductor the icing flashover stress of 28 units XWP2-160 is lower than those without grading ring and simulated conductor under the three pollution degrees. The discharge process of the icing flashover and relevant factors were investigated also, which shows that 1) the locations of the air gaps on icing insulators are not completely random. 2) arcs on long insulator strings require more time to reach a critical length, hence, the arc propagation might be easily influenced by the decreasing of melted water conductivity and ice shedding than short insulator strings. [ABSTRACT FROM AUTHOR]