The two most common types of bushings used on transformers as main lead entrances are solid porcelain bushings on smaller transformers and oil-filled condenser bushings on larger transformers.
Solid porcelain bushings consist of high-grade porcelain cylinders that conductors pass through. Outside surfaces have a series of skirts to increase the leakage path distance to the grounded metal case. High-voltage bushings are generally oil-filled condenser type.
Condenser types have a central conductor wound with alternating layers of paper insulation and tin foil and filled with insulating oil.
This results in a path from the conductor to the grounded tank, consisting of a series of condensers. The layers are designed to provide approximately equal voltage drops between each condenser layer.
Acceptance and routine maintenance tests most often used for checking the condition of bushings are Doble power factor tests. The power factor of a bushing in good condition will remain relatively stable throughout the service life.
A good indication of insulation deterioration is a slowly rising power factor. The most common cause of failure is moisture entrance through the top bushing seal.
This condition will be revealed before failure by routine Doble testing. If Doble testing is not performed regularly, explosive failure is the eventual result of a leaking bushing.
This, many times, results in a catastrophic and expensive failure of the transformer as well.
SOURCE: Transformers: Basics, Maintenance and Diagnostics – U.S. Department of the Interior Bureau of Reclamation