Overloaded Transformer Question

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KevinWhitten

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I was posed this question in the office today:

Suppose you are using a 75kVA transformer in the field to power equipment. You then notice that the transformer is severely overheated due to the fact that current is higher than allowed for said transformer, and the breakers were oversized and have not blown. You cannot get another transformer on site for at least another day; what is a simple way to temporarily reduce the current on the transformer using on site appliances?

I was told I'd be kicking myself when I found out the answer, but up until now I haven't come up with a solid solution. Any suggestions?
 
What percentage is the overload?
Assuming you can't reduce loads by shutting sthings off?
You could force air cool (fan) the transformer.
 
does the transformer have taps if so, change tap to higher voltage; amount of effect will depend on ratio of no load to full load losses in its design. i have seen melting down xfmrs run well below max temp rating simply by re-tapping from just 460 to 480v tap.

fan on it?

in an enclosure leave the door/top/side open.

kick?
 
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outdoor transformer can be cooled with a sprinkler.

I saw one once that had steam coming off it on a hot day. I was told it was too expensive to replace it with a bigger one for the few days a year that they needed more juice so they just turned a sprinkler on it when it was hot.

Not a lot different then places that turn the sprinklers on their A/C condenser units when it gets hot.
 
There may arise a shock hazard to water tap users, when attempt is made to cool a transformer with water streams.
 
T.M.Haja Sahib said:
There may arise a shock hazard to water tap users, when attempt is made to cool a transformer with water streams.
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He said outdoor transformer - it should at least have a 3R enclosure, but that means the water must be falling downward onto the transformer just like when it rains or you may get water where you don't want it.

That said assistance from water may not be as effective with a typical dry type transformer as all you are cooling is the case, increased air flow will be more effective. An oil filled transformer will cool better with water flowing over the case as the oil simply moves the heat to the case to be dissipated, the water will take the heat away faster than air alone will (maybe not if the air is really super cold, but that is not going to happen at this time of year, if it does, turn off the air cooler and you will remove a significant load from the transformer:lol:
 
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kwired said:
He said outdoor transformer - it should at least have a 3R enclosure,
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I thought a transformer like this. :)

images
 
mike_kilroy said:
does the transformer have taps if so, change tap to higher voltage; amount of effect will depend on ratio of no load to full load losses in its design. i have seen melting down xfmrs run well below max temp rating simply by re-tapping from just 460 to 480v tap.
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Is not the reverse that is true? Lowering the transformer voltage may reduce core losses as well as in partly loaded motors in addition to lower wattages in resistive loads.
 
T.M.Haja Sahib said:
Is not the reverse that is true? Lowering the transformer voltage may reduce core losses as well as in partly loaded motors in addition to lower wattages in resistive loads.
Click to expand...

No, although it would seem logical if you don't think it thru.....

example: supply is 460v. You have it in 460v tap. output is right on.
now re-tap instead to HIGHER 480v tap; now output is Lower, core losses are less (460 into 480v tap), I2r losses in xfmr are lower (since output voltage is lower and most external loads will pull less current), & kva load on xfrmr is less for same reason......
 
mike_kilroy said:
No, although it would seem logical if you don't think it thru.....

example: supply is 460v. You have it in 460v tap. output is right on.
now re-tap instead to HIGHER 480v tap; now output is Lower, core losses are less (460 into 480v tap), I2r losses in xfmr are lower (since output voltage is lower and most external loads will pull less current), & kva load on xfrmr is less for same reason......
Click to expand...
Anyway, it depends on whether the incoming high voltage remains constant or not.
 
The person who came up with this question did not think it through. :happyno: First of all, if the existing 75KVA transformer has too much current flowing through it, then replacing it tomorrow with another 75KVA transformer will not solve the problem. If the transformer is undersized for its application, then someone made a design error, and the result was a violation of 110.3(B). But if the transformer you bring in tomorrow has a higher rating, say 112.5 KVA, then you need to look at the primary and secondary conductors, the primary and secondary overcurrent protection, and the ratings of the downstream panel. For the immediate problem (i.e., what to do today), you can makes things better by using fans to help carry away the heat. But you would still be violating 110.3(B). The correct appliance to use to deal with this situation will be one or more of the appliances being served by this transformer: turn them off! :happyyes:
 
charlie b said:
The correct appliance to use to deal with this situation will be one or more of the appliances being served by this transformer: turn them off! :happyyes:
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While the fans and water are a good way to increase to current handling capacity of the transformer, they won't bring down the current. Cutting off the transformer to change the taps might not be an option even though it would help. Capacitors might help if the power factor is poor but they are not really much of an on-site appliance (I supposed you could build some out of some materials laying around).

So I'll cast my vote with charlie.
 
Usually high quality pole top transformers are built with excess cooling capacity so that overload up to 200% may be sustained by them for many hours . I do not know whether this is applicable in the OP's case.
 
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KevinWhitten said:
I was posed this question in the office today:

Suppose you are using a 75kVA transformer in the field to power equipment. You then notice that the transformer is severely overheated due to the fact that current is higher than allowed for said transformer, and the breakers were oversized and have not blown. You cannot get another transformer on site for at least another day; what is a simple way to temporarily reduce the current on the transformer using on site appliances?

I was told I'd be kicking myself when I found out the answer, but up until now I haven't come up with a solid solution. Any suggestions?
Click to expand...

the transformers I have worked with often have this 110% overcapacity for short periods of time. Even came across one that is constantly 105% loaded but not overheating due to the airconditioned environment it was in.

for "at least another day", I would suggest that blowers can be used to bring down the heat. water sprinkling if the transformer can take it.

tap changing the transformer (IMHO) is too much trouble just to last for "at least another day"
 
T.M.Haja Sahib said:
iwire:
I quoted this

Usually high quality pole top transformers are built with excess cooling capacity so that overload up to 200% may be sustained by them for many hours .
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from a US reference book.
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When they do allow a transformer to be loaded that heavily they take into consideration how long it is expected to be loaded, at what level, how long of a cool down period expected before loaded again, and ambient temperature.
 
A few years ago a local municipal utility actually did run hoses up a few of their poles to cool overloaded transformers.
 
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