Alexandrov - Saint-Petersburg State Technical University Russia
Gerasimov - Saint-Petersburg State Technical University Russia

Insulation conductors suspension of he overhead transmission lines is characterized by high electrical strength when there exist switching overvoltages impulses. 50% flashover voltages of air gaps between conductor and tower elements are 20-30% higher than for a gap between conductor and earth. For this reason the strings of plate insulators selected according to the condition of safe operating under the operating voltage in the long-term operating regime have significant resource of electrical strength when switching overvolt-ages. The lightning flashover voltages of insulation suspensions are higher than switching overvoltages. Moreover when increasing of a string length the S0% lightning flashover volt-age increases practically proportionally. However while investigation of long glass plate insulators of ultra high volt-age overhead lines it was discovered the significant decreasing of the average flashover gradient for strings with more than bat length under the influence of lightning impulses of negative polarity [1,2,3]. The cause of small increasing of f lashover voltages of the long insulators strings is the circumstance that because of high electrical field strength at the metallic elements of insulators, arranged near conductors, the spark initiates id-ten relatively low voltage. As it is shown in [3], it is possible to increase the flashover voltages by screening the bottom part of a string or by using long-rod composite insulators. In this case it is not expedient to use the plate insulators on the ultra high voltage overhead lines. Up to present, days investigations of electrical strength of long strings of different- types with different switching impulses were not. carried out.