Influence of low air pressure on flashover voltages of polluted insulators: test data, generalization attempts and some recommendations

110 to 500 kV mountain power transmission lines in operation in the USSR are discussed. Since 1967 a 100 kV line has been in service at an altitude up to 3.58 km above sea level. A recent 100 kV overhead line is at =4 km, its entire 224 km length running above 1.8 km and at least 65% of its right-of-way lying in mountains over 3 km high. A number of 2220 to 500 kV lines cross passes whose altitude exceeds 3.2 km. Construction of overhead lines in the mountains of the Caucasus, central Asia and in the eastern USSR made it necessary to conduct unique studies of outdoor insulation behavior at air pressure p below the normal pressure values p₀. Some of the studies were made in altitude chambers and some at special high-altitude outdoor test facilities. An attempt is made to generalize the basic results of these studies