Insulator failures under transient voltages

The operation of a high-voltage transmission line involves changes in energy distribution that are very conducive to high-frequency disturbances and transients of very steep front. These are often superposed on the normal frequency voltage of the line in such a way as to impose great stresses on the insulators. The mechanism of failure of an insulator is of great importance to those designing and operating transmission lines. This paper presents the results of recent investigations on the failure of insulators under impact and combined impact and normal-frequency voltages. Microphotographs of the resulting failures are included. The breakdown of a dielectric involves energy which is a time function and the importance of the duration of the stress in determining the magnitude of the voltage necessary to puncture an insulator is discussed. Due to the short duration of transients, insulators are often punctured repeatedly by them, the procelain in the puncture solidifying again on account of the small energy involved. These sealed punctures however weaken the insulator, lowering its dielectric strength materially. The importance of the elimination of air holes and defects in the porcelain is shown. Some essential features of a successful line insulator are stated.