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Can a transformer operate until maximum load?

  1. Aug 10, 2012 #1
    Hi, I have one 2500KVA transformer with 11kV/433V. The maximum current at low voltage side for this transformer provided by producer is up to 3333.49A. Currently, this transformer is running with 2700A which is about 80% of transformer usage and the temperature of the transformer has reached to 65 degreeC. The current is mainly supply to air compressor and big motors. If I improve the air circulation in the transformer room to reduce its temperature, can I still connect some air compressor to it?

    Please enlighten me! Thanks!
  2. jcsd
  3. Aug 10, 2012 #2

    Simon Bridge

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    You can usually run a new transformer at max spec ... that's just the manufacturers rating and it will still run for a time at higher loads. It's not like a fuse which will blow should the peak current exceed the max rated value. In fact, there should be a fuse or a circuit breaker on the low V side.

    At some load you'll have too much stray capacitance, the insulation breaks down and so on. If you can keep it cool you do reduce the risk of melting and other Bad Things. The transformer will still be dissipating the same energy.

    Naturally if you routinely exceed the manufacturers max current you can void your warranty.

    It is not normally advised to run at max spec for long periods of time.
    Best practice usually has you turning something off when you turn something else on or getting another transformer.
  4. Aug 10, 2012 #3
    Like Simon Bridge said, there isn't something intrinsically wrong with high currents. The thing that kills is over temperature. That being said, manufacturers have provide the 'rated current' figure based on how much temperature rise it might create and whats the temp withstand capacity of your transformer.
    But since everything depends upon temperature, the manufacturer most have specified the 3333.49A for a given ambient temperature. If your room temperature is above that, you can't flow that much current. That being said, please note that, all transformers have short term overloading capacity. The short term may be as large as 3-4 hrs for your transformer.
  5. Aug 11, 2012 #4
    Neither can i install a new transformer or change to a higher kVA one, it's restricted by the space available. I need to reduce the temperature 1st while thinking a way to reduce the load's current consumption.
    Thanks a lot guys!
  6. Aug 11, 2012 #5

    Simon Bridge

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    .. OR, a way to manage the loads so you can drive each of them when and as they are needed.
    It may be that you don't absolutely have to run everything at the same time. Explain to the client that it is either accept a little inconvenience or do without something.
  7. Aug 11, 2012 #6
    You said, it is 80% loaded currently, so you may not need to do anything extra and you could just hook extra loads, provided you don't exceed 100% for too long.
    But the important aspect is temperature rise. What temperature do you mean by 65 degreeC, the external case temperature or internal winding temperature? How did you measure it.
    I learned something from this site
    http://www.copper.org/applications/electrical/energy/trans_efficiency.html [Broken]
    From the site:
    "Dry-type transformers are available in three standard temperature rises: 80C, 115C, or 150C. Liquid-filled transformers come in standard rises of 55C and 65C. These values are based on a maximum ambient temperature of 40C."
    So, whatever be the 65 degreeC, it doesn't seem too hot, and hence you can increase your load.
    Last edited by a moderator: May 6, 2017
  8. Aug 11, 2012 #7

    jim hardy

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    you have not told us whether the room is well ventilated or closed up, and what is temperature in the room.

    Circulating air inside a closed room achieves very little.
    You need to move the heat OUT of the room so the transformer can cool itself.

    IF i assume your transformer is 99% efficient, it is rejecting something like 20 kilowatts to the room - it would take nearly six tons of air-conditioning to pump that much heat out.

    Improve air circulation THROUGH the room, to a place outside where the heat can go up.
    Logic is - Hot air wants to rise, so let Mother Nature help you out.
    Get that heat outdoors where she can carry it away.
    Put an exhaust fan near ceiling and an intake fan near floor.

    Look up heat capacity of air and calculate how many cfm it takes to carry away 20kw with perhaps 25 deg temperature rise.

    old jim.
  9. Aug 14, 2012 #8
    Yes,is a oil-filled transformer,there is a temperature indicator for the oil provided by manufacturer. The standard temperature given by manufacturer is around 50-60degreeC,and currently the temperature rose to 65degreeC.
    For the temporary solution, i have to cool down the temperature of the transformer before i can think of other way to reduce the load.

    It is installed in a closed room, some wall fans are installed to assist the air circulation in the room, but due to too much heat dissipated from the transformer, I plan to install two more wall fans to blow directly on the lamina plates.I'm not sure whether the way is correct,just try and see.
  10. Aug 14, 2012 #9

    jim hardy

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    is it hot inside the room?
    how does the heat get out of the room?


    looks to me like you need to move a LOT of air THROUGH the room, not just stir it around inside . Maybe a couple thousand cfm...
  11. Aug 14, 2012 #10
    very hot inside the room (those heat dissipated from the transformer itself), even wall fans have installed the room still in hot condition. Two blower fans installed(top right side of the room) at the wall to blow out the hot air and one air blower installed near the door (bottom left side of the room) to suck outside air into the room. The wall fans help to circulate the air inside the room.
    My colleague has proposed to install air conditioner inside the room, but has banded by management.
  12. Aug 14, 2012 #11

    jim hardy

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    my advice: open the door and set some fans in place to move fresh cool air into the room.

    Then, two additional things:

    1. Contact whoever sold you the transformer and get from them two things:
    ...a: the design heat load that the transformer dissipates when fullly loaded; that'll be either an efficiency or a direct number for heat in BTU or KW
    ...b: their recommendations for ventilation when installing the transformer

    2. Advise your management they have an architectural design problem with the room where your transformer is located. It's too darn hot.

    Rule of thumb - a really good transformer can be 98% or 99% efficient.
    With your size transformer, 1% loss (99% efficient) is about 20KW of heat that must be removed.
    When you have the heat load information from transformer supplier, you have real facts to present to management. You'll know how much heat is going into room.
    That link i gave you gives a simple formula for how much air must be moved to carry away that much heat. How do your blower and exhaust fan's CFM rating compare?

    A ton of airconditioning moves 12,000 BTU/hour or 3.5 kw of heat and you may well need to move six or twelve times that.

    good luck - old jim

    ps EDIT we had a big transformer that ran hot because the area it was in drew in very hot air from around some steam pipes. We replaced the metal roof over the pipes with a grate so the hot air could escape straight up, problem solved. Sometimes these things are not obvious to the architect when he is laying out a building.
    Don't be hard on the poor guy, he has only the drawings to look at....
    Last edited: Aug 14, 2012
  13. Aug 16, 2012 #12
    2 incoming blowers (bring air from outside into the room) total up are 3200CFM, and the exhaust fan is 3500CFM.
    Base on the calculation you provided to me, I'm not sure how much KW heat dissipated from the transformer, and I can't contact back the manufacturer.So, I will plan to install another exhaust fan with 3500CFM for trying.
    Thanks for your idea.
    Last edited: Aug 16, 2012
  14. Aug 17, 2012 #13

    jim hardy

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    AHA !!!

    Now you have a rough idea of the airflow, assuming niothing is blocking either fan

    using two inexpensive thermometers you could measure the temperature rise and, using that handy formula , calculate the heat being carried away by that much air moving through the room. Just measure inlet and outlet temperature of the room, right at the fan intakes.

    I'd get average of hourly readings over maybe a day

    and see if the number looks reasonable compared to transformer throoughput.

    Also check your incoming voltage and the transformer's tap settings. Operating at higher voltage increases your iron losses substantially, i think to the 1.4th power.
    So if you have flexibility to tweak it so it's running slightly undervoltage instead of slightly overvoltage you'll reduce heat input to the room.

    always remember - one experiment is worth a thousand expert opinions !!
  15. Aug 21, 2012 #14
    Hmm, whatever you do please consider possible reasons one would stick a massive heat source such as a transformer (oil filled!) in a tightly closed room.

    The main reason is usually, in a word, FIRE!!!

    Please do not prop open any normally closed transformer/electrical rooms in an occupied building. And if one has to go the route of exhaust venting you may want to take measures to ensure that a potential fire won't cause your heat exhaust path to fill another area with lethal smoke in the case of a transformer failure. I will say again, please do not prop open fire doors in your buildings. The architects definitely knew what they were doing when they placed those!
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