There are six different inspections to be performed on the bushings of the power transformer:
- Routine inspection
- Regular inspection (Once every two years)
- Inspection due to excessive partial heatings
- Local damages inspection (fissures) on the bushings
- Inspection for oil leaks
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 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.
Check if there are any damages or oil leaks in the bushings.
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 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.
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.
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.
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.
Reference: Operation and Maintenance for Power Transformers – ABB