transformer loading

[jcraft]

We have a 500 kva internal substation in our plant that are about 6 years old. 4160v to 480/277. The fla of this transformer is 602 amps. The transformer is located in an air conditioned manufacturing area that is very clean. The sub presently has about 6 pieces of machinery on it and about 5 hvac units on them that run ac about 4 onths out of the year. The question is most of the time this transformer runs about 350 amps and in the summer we have noted on a 90 deg. hot day that it runs about 525-550 amps. Now my question is we want to add another piece of equipment that is about 100 amps so at some point throughout the year the transformer may be overloaded by about 10%. Does this look like it could be a problem. I have heard that transformers can take overloads. We could always put exhaust fans on them if neccesary. Please advise. Thanks

 

[DanDel]

Transformers can take overloads for limited times.

That being said, you should realize that any overload that raises the operating temperature will in effect reduce the expected life of the transformer.

I do not recommend that a private company runs a transformer continuously above its rating. Utilities do this all the time, but they are less concerned with outages than I expect you would be, and they have a warehouse/parking lot full of spare equipment.
You have mentioned that it sometimes runs at 525-550A (over 90% load), but you didn't mention the operating temperature at that time, or if you even have a temp gauge to tell you. This is the key issue. If you can keep the operating temperature always significantly below the nameplate rated temp rise, you should be OK, though the best solution for you is to install the correctly sized equipment for the load served.

If you really want to run the existing system with the new loads, first, look at your overcurrent protection, and verify that you can allow for your expected overcurrent. Then, make sure that you have or install fans, and a winding temp gauge (I'm assuming it's a dry-type transformer) with a temperature alarm/fan control relay which will control the fans. I'm also assuming that the 500kVA rating is 'AA'(air-cooled), and that it doesn't have an 'FA'(fan-cooled) rating.

 

[Marmite]

As a guide, for a self cooled oil immersed transformer running at 90% full load in a 20C ambient temperature you should be able to get away with daily overloads of 16% for 24 hours, 33% for 8 hrs,50% for 4 hrs, 74% for 2hrs, and 102% for 1hr. These figures are based purely on thermal and insulation aging considerations to give a minimum 20 year life expectancy. They are taken from GE Power Distribution Transformer Manual. Unfortunately I don't have an electronic link to the document.
Regards
Marmite

 

[dpc]

I would be cautious about extrapolating overload data for an oil-filled xfmr to a 500 kVA xfmr that is probably dry-type.

There is an ANSI standard for dry-type transformers that should address overload capacity, but this transformer will be strongly impacted by the ambient air temperature. On a hot day, the room temperature may get pretty hot, especially with the transformer running in overload and generating plenty of its own heat.

Transformer ratings are based on a 24 hr average ambient temperature of 30 deg C. Since the indoor space will likely average more than that in hot weather, some caution is advisable. I would not count on a long transformer life. I agree that some type of embedded thermal detectors would be advisable.

 

[Marmite]

I wasn't suggesting extrapolating the figures for an oil filled transformer to a dry transformer.I agree that the figures I have given do not apply to a dry type transformer. The original post doesn't state whether it's dry or oil filled. It could be either. The post does however indicate that the transformer is in an air conditioned manufacturing area which is why I gave the figures for 20C ambient.
Regards
Marmite

 

[advidana]

just install a new transformer for the new load.

 

[itsmoked]

Geez I'd be trying to get that transformer out of a "clean" area that's air conditioned.

As the transformer gets hotter it has to work even more to supply power to the A/C that it's demanding more of. A transformer can be happy in an ambient that people would hate so why pay to keep it comfortable? A waste of energy that would probably pay back the cost of moving the humming monster outside.

The "clean" area likely won't be, after that transformer burns out someday.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com