Premium Membership ♕

Save 50% on all Video Courses with Enterprise Membership Plan and study specialized LV/MV/HV technical articles and guides.

Home / Technical Articles / Maintenance and inspection of the transformer bushings
Maintenance and inspection of the transformer bushings
Maintenance and inspection of the transformer bushings (photo credit: Bradley Woods via Flickr)

Six different inspections

There are six different inspections to be performed on the bushings of the power transformer:

  1. Routine inspection
  2. Regular inspection (Once every two years)
  3. Inspection due to excessive partial heatings
  4. Local damages inspection (fissures) on the bushings
  5. Inspection for oil leaks
  6. Storage

1. Routine inspection

Excessive local heating

Pay attention to the clamping section of the terminals. It is convenient to paint this section with heat indicating paint.


When there is much dust and salt, a clean up must be performed and to do so, the transformer must be place out of service and use water, ammonia or carbon tetrachloride. If they are very dirty, use concentrated hydrochloric acid diluted 40 or more times in water.

The solution should not be in contact with any metallic part; after the cleaning the porcelain parts, these must be neutralized with water that contains sodium bicarbonate in a proportion of 30 grams by liter.

As long as it uses a chemical solution, make sure of washing it after with fresh water, so that no strange elements are left.

In systems in which will be difficult to stop the operation of the transformer for cleaning, or in zones where there are many damages by the dust or the salt, it is being using recently a washing method designated “of hot line”. It is a method to wash the equipment without stopping its operation, and there are 2 or 3 forms of doing it.

In any case, it must be verified the degree of dust and salt, the quality of the water to wash and the method of waterproofing when the cleaning is done.

Mechanical damages

Check if there are any damages or oil leaks in the bushings.

Go back to Index ↑

2. Regular inspection (Once every two years)

Evaluation of the deterioration of the insulation

The methods to detect the deterioration of the insulation are the measurement of the insulation resistance and tan ∆.

The measurement of the insulation resistance in the bushings is not simple, since the bushing and the winding of the transformer should be independent; nevertheless, the measurement must be made the best way possible.

The measurement of the tan ∆ is also difficult, since the bushings should be separated from the transformer in most cases. The evaluation of the result of the measurement should not depend solely on the absolute values obtained, but on the values obtained each year and from the variation among them. If there are large discrepancies in the values, special attention is necessary.

When the insulation resistance is superior to 1000 MΩ at normal temperatures, it can be considered as good condition, but the value of the tan also must be taken into consideration for the evaluation.

3. Inspection due to excessive partial heatings

The excessive heating of the terminals in most cases is due to loosening. If this condition is observed, eliminate the dust or dirt from the parts from contact and tighten firmly.

Go back to Index ↑

4. Local damages inspection (fissures) on the bushings

The cleaning of the bushings must be done according to what was mentioned. If the damages are very serious it must be replaced with new ones.

Go back to Index ↑

5. Inspection for oil leaks

Check the various pieces of the bushings to see if there is any oil leaks. If oil is leaking through the gasket, tighten it or replace it. If there are bushings of immersed in oil type and the oil leak is through other part of the bushing, report it to the manufacturer.

Keep the bushings in a vertical position and in a dry place. It is recommended to keep them in their original packaging.

Go back to Index ↑

6. Storage

Keep the bushings in a vertical position and in a dry place. It is recommended to keep them in their original packaging.

Go back to Index ↑

Reference: Operation and Maintenance for Power Transformers – ABB

Premium Membership

Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! It helps you to shape up your technical skills in your everyday life as an electrical engineer.
More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Luke Smith
    Feb 25, 2022

    It was very informative to know that loosening is one of the causes of excessive heating in a transformer equipment. My father recently opened a home depot establishment in town, and he’s looking for relevant equipment for the business. That being said, I’ll make sure to help him find the best transformer suppliers near our area.

  2. Shammy Peterson
    Sep 01, 2021

    It made sense to me when you said that transformers should be inspected once every two years, and the measurement of the insulation resistance must be made the best possible way. This reminded me of manufacturing companies where transformers must always work efficiently and safely. I could imagine how necessary it is for maintenance checks to be done regularly to remedy issues that could possibly cause downtime on operations.

  3. Nezar
    Dec 09, 2020

    How to prevent the bushings from wetness

  4. nmr
    Aug 06, 2015

    how to calculate no. of earth electrodes in a substation 11kv/440v three phase 500 kva transformer two nos.

  5. nmr
    Aug 06, 2015

    PL GIVE THE ALLOWABLE GAP OF ROD TYPE LIGHTINING arrestors provided across various bushings under various voltages 250v ac, 440v ac, 11kv ac, 33 kv ac, 25 kv ac and how to calculate the gap with respect to voltage and rating of transformer etc.,

  6. Girdhar Gopal Chhipa
    Feb 20, 2015

    Hello edvard
    I join as a chief engineer in a textile company . I found there installed two transformers of 3000 kva each 11kv to 440 volt. . While total sanction load is only 3000 kva. And only aprox 1200 kva running load on each transformer , which is 40 % of transformer capacity. So I wants help to know that is ok or not ok ? And what is losses and how much energy losses ? Because utilisation is less then 85 % .

  7. JOSE
    Oct 20, 2014


  8. oded itzhak
    Aug 02, 2014

    hello edvard

    Could you explain how to do the routine inspection
    Did you mean to use with the Thermal device?
    can you tell me more about all inspections?
    Especially about tan ∆

  9. hanane
    Aug 02, 2014

    that’s good but plz add pics!!!

Leave a Comment

Tell us what you're thinking. We care about your opinion! Please keep in mind that comments are moderated and rel="nofollow" is in use. So, please do not use a spammy keyword or a domain as your name, or it will be deleted. Let's have a professional and meaningful conversation instead. Thanks for dropping by!

six  ×    =  twelve

Learn How to Design Power Systems

Learn to design LV/MV/HV power systems through professional video courses. Lifetime access. Enjoy learning!

Subscribe to Weekly Newsletter

Subscribe to our Weekly Digest newsletter and receive free updates on new technical articles, video courses and guides (PDF).
EEP Academy Courses - A hand crafted cutting-edge electrical engineering knowledge