Transformer Underloading and Life

[Max1996]

In my workplace, we had a good discussion on how to extend the life of a transformer. A question was raised on the effect on the transformer lifespan between 100% loading and 90% or 60%. Many argued that the 60% and 90% loading would extend the life of the transformer, with the 60% adding more life-years to the transformer. A small number of us argued that 60% or 90% loading will have the same effect compared to 100% loading. This is because the transformer is designed at a 100% load so any loading below that will not add more years to the transformer lifespan, assuming of course other things not exceeded, like the temperature. Are we correct in our reasoning?

 

[gar]

150114-1154 EST

I suggest your reasoning is incorrect. Fundamentally there is a thermal problem.

Most chemical reactions occur more rapidly as temperature increases. Most materials deteroriate more rapidly as temperature rises. Most materials soften or lose strength as temperature increases, etc.

The 100% rating of a transformer is designed based on some general empirically derived expected life (experimental accelerated life testing).

The temperature rise of the materials in a transformer increases as loading goes from 0 to 100%. The rate of deteroriation is not linearly related to temperature rise, and will accelerate rapidly somewhere above 100%.

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[Jraef]

But at 60% loading, the relative efficiency is going to drop, meaning you will likely spend way more on wasted energy over the life of that transformer than the value of the extra life you attain. At 90%, the difference in efficiency will not be that great, plus you will be able to expect longer life.

All that said, are you experiencing transformer life issues? In your lifetime? I know of many many transformers that have been in service for well over 50 years, trouble free. Relatively easy routine maintenance and testing to foresee impending trouble is the key to service life. If you are having transformers fail, it might be worth a good study of your systems.

 

[jim dungar]

By far and away, temperature has the most impact on the life of a dry-type air cooled transformer. A rough number is: a 6-10C winding temp change could result in a doubling (or halving) of the life of the insulation.
Other than reduced air flow (e.g. dirt build up), the predominate source of transformer heating is its loading. Reduce the transformer's continuous load and you will extend its life fairly significantly.

 

[big john]

Heat is the enemy of electrical insulation. The hotter it gets, and the longer it stays that way, the more stress is placed on the insulation, and the faster it degrades.

ANSI publishes loss-of-life curves that estimate how routine overloads above 100% will reduce the overall life-expectancy of a transformer winding insulation, so it stands to reason that by reducing the loading below 100% you would absolutely extend the expected life of the transformer.

... A rough number is: a 6-10C winding temp change could result in a doubling (or halving) of the life of the insulation....

I run into that 10? rule often, in literature for everything from winding-insulation to bearing grease to battery electrolyte. It comes from the Arrhenius equation for chemical reaction rates, and seems to be pretty universal.

 

[Electric-Light]

You didn't tell us the type of load. K factor?
Many transformers are also designed to accept an overload conditions especially poco ones. If the overload is only a few hours a day, the wasted energy of higher standby loss of a next size up can actually be more.

A higher efficiency transformer certainly helps limit heating. 92 vs 96 efficiency doesn't seem like a big deal, but the heat production is halved. (8% vs 4% of load kW loss)

 

[Max1996]

Thanks guys for your answers.

This discussion in our workplace was purely for educational purposes although this may lead to asset strategies later on.

To extend the discussion answer by Gar, if the transformer paper insulation, let's say 110 deg C class paper, is subjected to heat by transformer loading, at what temperature will the paper starts to deteriorate? What equation/graph can be used to model the additional life gained by loading the transformer below 100%?

Additionally, was there ever a study made regarding loading the transformer below 100% versus life-years added? I am searching the internet for this for a while now.

 

[jim dungar]

You might try; IEEE C57.96-2013 IEEE guide for loading dry type transformers.

 

[gar]

150116-0914

Do a Google search with --- component life vs temperature --- then modify the search string to more directly address your specific concern. Try this string --- transformer life vs loading --- .

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[Electric-Light]

All you ever wanted to know about transformers
http://www.copper.org/environment/sustainable-energy/transformers/education/trans_efficiency.html


Less efficient units may run the insulation much closer to the limit. There's a balance point. When you increase the heat generation, the temperature goes up until you reach the balance point.

 

[petersonra]

Want to extend the life of a transformer?

Add a fan kit with thermostat.

Most cases the fan won't run all that much but if the xfmr does warm up, the fan will limit the heat rise and in the case of overload limit the damage.