Investigation of electrical failures in porcelain cap and pin line insulators

Suspension and dead-end porcelain insulator strings were removed after 25-30 years in service for various reasons, and the insulator bells (units) were evaluated in the laboratory. Some units failed the puncture test and these were cut open and analyzed. The electric field due to the presence of microscopic defects in porcelain was modeled using a 3D software package. It has been shown that the local electric field enhancement can be high enough to cause dielectric breakdown of porcelain. The electrical deterioration is a slow process. The breakdown process creates phase changes in the porcelain microstructure and this was confirmed by scanning electron microscopy.