Does continuous ONAF operation reduce the transformer life time?

[cirtcele]

I believe not since (I am thinking) as long as we keep the oil temperature same (55 deg C or 65 deg C rise whatever it is) then the life expectancy should be the same. Am I right here?

Maybe the overall losses would be increased exponentially..
Anyone can comment? Thank you again….

 

[dpc]

The key to transformer life is the average temperature of the windings. As long as the temperature is the same, the lifetime will be roughly the same.

But operating with fans running all the time implies that the minimum temperature never gets below the point where the fans can shut off. This would generally mean a higher average temp than most self-cooled applications.

Transformer losses are comprised of two components:

Core losses - these are constant regardless of the transformer loading (while energized)

Load (copper) losses - these vary with the square of the current through the transformer.

 

[cirtcele]

Thank you. This is a good explanation.

 

[msteen]

In our applications transformer heating is a huge issue. After researching the problem I have come to understand a couple of things in regard to the handling of heat and its affect on the life span of transformers.

The temperature rise is a figure the transformer is designed to operate below. If you have a 65 degree rise on a 30 degree ambient you should have a transformer that will operate at or around 95 degrees at full load. In our business we apply oil immersed step up transformers to variable speed drives (output side) to run submersible pumps. In this application the harmonics of the drive cause huge losses in the transformer due to eddy currents and skin effect.

A transformer is designed to operate at its maximum temperature rating (here 95 degrees) for its entire life, the span of which should be 30 years. For every ten degrees above the maximum rating you can expect the life span to be halved. So for 105 degrees the transformer should survive for 15 years.

The application of fans will dramatically reduce the operating temperature of the transformer thereby bringing it back into its operating range. I have seen single fans reduce the operating temperature of my transformers by 25 degree at ambient temperatures of 55 degrees Celsius. Utilities commonly use fans to increase the MVA rating of their transformers to handle momentary overloads at peak load periods.

For our applications we have succesfully used fans to manage transformer heating and have had years of service with no failures.

 

[bacon4life]

When you say the same temperature, is that top oil, bottom oil or winding hot spot? The temperature gradient would be larger with ONAF, so if you kept the actual hot spot temperature the same in both, everywhere else in the ONAF would be cooler and thus last longer. Maybe?

Perhaps we could also compare running at for example 80 degree hot spot ONAF or 70 degree hot spot ONAN. From the tables in C57.91, it appear that if it runs below about 85 degrees hot spot, it will run pretty much forever. Or at least until the fans fail, or something besides transformer heating causes a failure.

msteen
Are you referring to IEC designs? IEEE C57.91 lists transformer design life at maximum temperature as 7.42 years to 20.55 years depending on the end of life criteria.

Do you run the transformers cooler to take into account that harmonics would heat up parts of the transformer that are different that the cooling designer planned?