Electrical Strength Reduction of Porcelain Suspension Insulators on AC Transmission Lines

Porcelain suspension insulators that were in service for up to 38 years on 115-500 kV AC transmission lines were evaluated for residual electrical strength. It was seen that the reduction in electrical strength or aging, was significant for some insulators causing them to puncture. The reasons for the reduction in electrical breakdown strength were investigated using scanning electron microscopy and x-ray diffraction. The marked reduction in electrical breakdown strength with time has been correlated with large number and size of defects in the porcelain microstructure. It has been shown that local field enhancement created by these defects is sufficient to initiate progressive dielectric breakdown that ultimately leads to puncture.