transfomer loading

[tish53]

First I have to commend everyone on this site. By monitoring the threads over the last month, I have learned and relearned so much.

My question is about a 1500 kva, pad mount oil filled transformer 12,500 wye primary-- 480/277 wye secondary. Normally at most of our plants this incoming service transfromer is owned by the utility. In this case, we own the transformer and I am concerned about the load that has been added over the years.

I calculate 1804 full load amps capacity for the 1500 kva transformer at 480 vac. All our loads are 3 phase 480 vac. Mostly motor loads. We installed a data logger and the average load is around 2200 amps ( balanced across the 3 phases) for 7 hours per day 5 days a week. The load for the other 17 hours is around 500 amps.

Is there a thermal overload capacity, that this pad mount oil filled transformer can handle since it cools for 17 hours and then stresses for 7 hours. If so how do I calculate it and determine if 2200 amps is oK or is only 1900 amps ok.

I really apprciate any guidance anyone might be able to give.

Thanks again

 

[templdl]

What is the temp rise and what is the average ambient temperature?

 

[jim dungar]

Is there a thermal overload capacity, that this pad mount oil filled transformer can handle since it cools for 17 hours and then stresses for 7 hours. If so how do I calculate it and determine if 2200 amps is oK or is only 1900 amps ok.

Yes there are guides to loading power transformers, like C57.96-1999.
You could also try a manufacturer like Cooper Power.

 

[mivey]

Yes there are guides to loading power transformers, like C57.96-1999.
You could also try a manufacturer like Cooper Power.

C57.96 is for dry-type. C57.91 is for the oil-filled.

 

[broadgage]

Transformers will stand appreciable overloads without harm, provided that they dont get too hot.
It is overheating that kills transformers, not current as such.

The ambient temperature and amount of airflow are very important, as well as the load in amps, and the duration of that load.

The manufacturers of the transformer should be able to state the maximum permitted oil temperature, and this should be measured to ensure that it is not exceeded.
Continous monitoring would be ideal, but a maximum recording thermometer is usefull, some transformers are factory fitted with such.

If you find that the transformer is only slightly over the permitted temperaure, then you may be able to relieve matters by improved cooling.

You may be able to reduce the current by improving the power factor of the load, that may be cheaper than a larger transformer.

I have seen transformers loaded to 150% of the proper loading, and with the oil well below the maximum permitted temp, but that was outdoors at freezing point, in a high wind.
I have seen other transformers alarmingly hot, due to limited ventilation and high ambients, whilst only loaded to 80%.

 

[iwire]

Just figured I would point out that if you own and control the transformer that the NEC likely applies and you would have to size the unit per the NEC. 1804 amps max load.

I do not mean to take away from the other posters comments.

 

[tish53]

This transformer is a typical utility style pad mount transformer with no external guages. I will the check the temp gauge at the next opportunity. I believe this transformer has a peak temp indicator.

The plant is located in Northern Virginia. The outside temperature has been 100 deg F for the last 4 days.

I will check C57-91 guide.

Thanks

 

[jim dungar]

Just figured I would point out that if you own and control the transformer that the NEC likely applies and you would have to size the unit per the NEC. 1804 amps max load.

I do not mean to take away from the other posters comments.

Bob, where do you find transformer loading restrictions in the NEC? Everything I find is about transformer protection sizing.:roll:
In fact, there are several situations in table 450.3(A) where transformers can be protected past 250%.

 

[kingpb]

For anyone who care, this is the generic thermal damage curve for a 1500KVA Xfmr with 480V LV winding.

http://i815.photobucket.com/albums/zz74/kingpbk/1500KVA%20Xfmr/1500KVAXFMR.jpg

As you can see that 2200A is well underneath the thermal damage curve, assuming the transformer heating is limited to a maximum of 65 deg C above the design ambient of 30 deg c average, with a max of 40 deg C over 24 hrs.

Basically, I think what you will find is that instead of lasting 30yrs or more, it will last some time less.

 

[tish53]

Thanks for the transformer damage curve. That helps alot.

I also have seen a table in the c57.91 guide that would limit the overload for 8 hours to 128 %. That would indicate my situation would be ok. ( given the other assumptions are all true) The bottom line seems to be the temperature rise of the transformer. I will need to monitor the temperature going forward to ensure we are not reducing the life of the transformer signigicantly.

Thanks again for the information

 

[skeshesh]

Fine discussion as usual gentlemen.

I think the guys summed everything off nicely, but if you want a fun bedtime read on it I found this paper - it has some interesting survey data from utility companies about their design standards:

http://weps.weidmannb2b.biz/WACTI/TechLibrary/LoadingAboveNameplate.pdf

 

[mivey]

I will need to monitor the temperature going forward to ensure we are not reducing the life of the transformer signigicantly.

In addition to the references listed in C57.91 you might also find the following to be of interest:

IEEE Std C57.119-2001: IEEE Recommended Practice for Performing Temperature Rise Tests on Oil-Immersed Power Transformers at Loads Beyond Nameplate Ratings

 

[rcwilson]

You might want to look at the cables and terminations for signs of overheating at the next opportunity to do it safely. The cables have less thermal mass than a transformer and may heat up faster.

We ran a transformer overloaded (150%) for a few hours during a repair outage. We monitored transformer temperatures and had large fans to help with cooling. We had to replace the secondary cables later due to insualtion melting at the terminations.

 

[zog]

. I will need to monitor the temperature going forward to ensure we are not reducing the life of the transformer signigicantly.

A common thumb rule for life expectancy of transformers is as follows:

-Class ?A? insulation - 8C above max temp rating= ? life expectancy
-Class ?B? insulation - 12C above max temp rating= ? life expectancy

 

[wasasparky]

Bob, where do you find transformer loading restrictions in the NEC? Everything I find is about transformer protection sizing.:roll:
In fact, there are several situations in table 450.3(A) where transformers can be protected past 250%.

Proof that the NEC is not a design manual.