Insulator pollution performance at high altitude: major trends

A survey of the data available from the literature was conducted in order to assess major trends in insulator performance with altitude. The literature comprised tests on individual insulator strings under natural and artificial conditions along with actual live-line performance. The data covered pollution performance as well as the air-gap breakdown performance of insulators under reduced pressure conditions for AC and DC applied voltages. The major trends reported here suggest a pressure dependence (p/p₀) ^0.35 for DC and (p/p₀)^0.5 for AC, of V₅₀ for standard polluted insulators at altitudes lower than about 4 km (p~0.6 atm). These pressure dependences impose a derating of the relative performance of leakage path length or flashover voltage with altitude, namely 6%/km for AC, 5%/km for positive DC, and 4%/km for negative DC applied voltages. Dielectric breakdown, in contrast, imposes a 10.5%/km degradation of insulating air gaps with altitude