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Transformer porcelain bushings spare parts
Transformer porcelain bushings spare parts (Pure porcelain structure, copper conductor, with low temperature rise and good heat dissipation.)

Transformer bushings //

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.

Transformer with 420kV RIP bushings
Transformer with 420kV RIP bushings (photo credit: power-technology.com)

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

About Author //

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Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

5 Comments

  1. […] parts include transformer bushings, which connect LPTs to transmission lines, as well as tap changers, power cable connectors, […]


  2. Adnan Munir
    May 21, 2016

    My E-mail address:
    adnanmunir_93@yahoo.com

  3. […] function of these bushings is to let the high-voltage transformer connection pass through the grounded metal tank without […]

  4. […] the business case for creating a substation or making major modifications. Planners, operating and maintenance personnel, asset managers, and design engineers are among the various employees typically involved […]

  5. […] is usually some readily accessible point inside the tank, but it can also be brought through a bushing on the tank wall or top for external access.This grounding point should be removable for testing […]

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