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#66533 - 06/07/06 09:49 AM Three Phase Transformer Questions  
JamesMI  Offline
Junior Member
Joined: Mar 2003
Posts: 5

I am an electrician working in a small industrial plant. I was asked to look into a possible transformer overheating problem and had a few questions about the transformer ratings in general. I apologize for the length of the post, but I just wanted to be as complete as possible.

What we have is a bank of three single phase transformers hooked up as a 4 wire delta system (with a 208 high-leg). The transfers are each rated at 150kVa at 240 Volts with a 65 degree C temperature rise.

The first question I have is just on how you would determine permissible line current for the system. This is what I figured:

Total bank capacity = 150+150+150 = 450 kVa
Current = 450 *1000/(1.732*240) = 1083 amps.

So the line current would be 1083 amps at full load. And would be fused at 125% of this or 1356 amps

The current in the winding would be 1083/1.732 = 625 amps.
This could also be found by using the individual kVa ratings by:
150*1000/240 = 625 amps

Are the above calculations correct?

The second question I have is on the temperature rating of the transformer. If it is rated at 150 kVa with a 65 degree temperature rise wouldn't that mean that when under full load the transformer would be running at 65 degrees above ambient temperature? Or if I convert to farenheight and figured ambient to be 70 degrees F, the rise would be 150 degrees F, and the transformer would be running at about 220 degrees F.

If this is correct how is the temperature normally measured, we currently just shoot the transformer case with a infrared temperature gun. The temperature of the center transformer (the highest) is about 120 degrees F. Our high voltage maintenance company has my supervisor concerned, but I don't think this is a problem based on what I posted above.

We also measured the current in each of the transformer windings and the center transformer was running at 380 amps. The high voltage guy told my supervisor it is only rated at 360 amps,which is the rating for a 3 phase 150 kVa transformer, but wouldn't are system be rated at 450 kVa? Am I doing the above calculations wrong, or is the high voltage maintenance guy giving us incorrect information?

Thanks for any help you can give,


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#66534 - 06/08/06 06:14 AM Re: Three Phase Transformer Questions  
winnie  Offline
Joined: Sep 2003
Posts: 649
boston, ma
Your calculations appear correct to me. I'll comment on the temperature rise issue:

The temperature rise of a transformer is related to the insulation system of the coils, and is the temperature rise of the hottest point (or presumed hottest point) of the coil at full load operation. The temperature of the outside of the can or enclosure gives you very little clue of what is going on _inside_, and it is the temperature _inside_ that you care about.

I am not familiar with any easy way to test such temperatures in the field; I do electric motor research, and we have our motors built up with embedded RTD temperature sensors.


#66535 - 06/08/06 06:54 AM Re: Three Phase Transformer Questions  
JamesMI  Offline
Junior Member
Joined: Mar 2003
Posts: 5
Thanks for the reply,

Unfortunately these transformers don't have any imbedded temperature sensors in them. I'm not sure if it is something that can be added or not.

The load on these transformers changes very little from day to day. I understand that as the load changes the temperature of the windings changes as well. There is also a delay between the temperature of the windings the oil and the case.

If the load stays consistent wouldn't the case temperature be very close to the oil temperature?


#66536 - 06/08/06 08:27 AM Re: Three Phase Transformer Questions  
JBD  Offline
Joined: Jul 2001
Posts: 599
Your load calculations look correct.

The transformer temperature rise of 65C is based on the interior of the center coil winding, theoretically the hottest spot in the transformer.

If the "surface" of the cooling oil was at the same temperature as the windings then very little heat transfer would be occuring occuring.

For a typical liquid filled transformer the temperatures are:

ambient = 40C
average conductor rise = 55C
hot-spot gradient = 10C
Total permissible ultimate temperature = 105C

#66537 - 06/08/06 08:49 AM Re: Three Phase Transformer Questions  
winnie  Offline
Joined: Sep 2003
Posts: 649
boston, ma
I am even less familiar with the details of oil filled transformers, but the same basic physical principals should hold. Heat is being generated in the coils and core, and heat always flows from hot to cold. Thus in equilibrium the coils must be hotter than the oil, and the oil hotter than the can, and the can hotter than the surrounding air. The actual temperature differences will depend upon the thermal conductivity of the materials, any convection taking place in the oil, and the rate of heat production.

I would expect temperatures to be more uniform in an oil filled transformer than in a dry transformer, but I have no experience to say what sort of temperature variation from the coil to the can is to be expected.


#66538 - 06/08/06 08:51 AM Re: Three Phase Transformer Questions  
JamesMI  Offline
Junior Member
Joined: Mar 2003
Posts: 5
OK, that makes sense.

I guess my question now is given the situation where we have a transformer with now way of telling the internal temperature, can we use the case temperature at all to determine if they are running to hot?

From what I have read online, for some transformers it is completely normal for them to run so hot that you can not touch them.

If my calculations are correct, we are definitely not overloading the transformers.

My supervisors are concerned about the heat because it showed up in an infrared temperature scan. The center transformer is about 20 degrees hotter than the others. We I checked the current for each transformer was 240, 380, 140. With 380 amps being the center transformer. The temperatures in farenheight are respectively 101, 120, 97.

There concern is mainly because of the infrared scans and the age of the transformers (about 20 years). But I told him because of the imbalance, the temperatures will not be the same. We may be able to balance some of the loads, but most of the loads are from single phase lighting panels.

Does anyone think this is really a problem? I'm going to try to get ahold of the transformer manufacturer to see if I can get any specific information, or to possibly see if thermometers can be added to these transformers, but can certainly use any advice anyone can give.

Thanks again for the help


#66539 - 06/08/06 10:25 AM Re: Three Phase Transformer Questions  
Zog  Offline
Joined: Sep 2005
Posts: 120
Charlotte, NC
I also agree your calcs look right, and that these can run hot to the touch without being a problem.

Your best bet is to install oil temp gauges, this is pretty easy if you have the ports on the tank, if you do not have the ports, forget about it.

My company does this type of work, and we are in Detroit. I could have one of my transformer guys stop by and look at the tanks for you if you like.

MV/HV Testing Specialist, "BKRMAN"

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