Pollution performance of double insulator assemblies

Some outdoor insulator assemblies have to be designed as two parallel insulators due to the structural or mechanical requirements. One example is a double suspension or tension insulator set for overhead lines. Another example is a double post insulator of some disconnector designs, where one post insulator serves as a support insulator for disconnector contacts while the second post insulator is used as an operating insulator for moving contacts. The parallel insulators design itself influences pollution performance (i.e. flashover probability) of the insulators due to the increased (double) number of insulation points. But the results of the measurements have shown that AC pollution flashover voltage of such an assembly is also affected by the distance between the two parallel insulators. If this distance is small enough, the discharges could bridge the air gap between the two parallel insulators due to a different voltage distribution along each insulator. This can lead to a flashover propagation different to that on a single insulator, resulting in lower flashover voltage. This paper discusses the results of artificial pollution tests performed on double tension insulator sets and double post insulator assembly