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07-30-2008, 11:00 PM   #1
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## Optimal Spare Factor for Transformer

Dear all,
I have a new topic is:
What is the optimal spare factor for one designed transformer?
Best Regards,
Minh Tam

07-31-2008, 12:42 PM   #2
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Please, more details. A lot more. . .

 08-03-2008, 11:00 PM #3 Newbie   Join Date: Jul 2008 Location: Viet Nam Posts: 6 Rewards Points: 10 Sorry about late reply. Detail of my question is: I have a building (has 65 residences). After I calculated the required power for my transformer (ofcourse having the simultaneous factor 0.4 for building has over 50 residences-according to IEC standard). Do you think I need a spare factor for my transformer??

 08-03-2008, 11:22 PM #4 Idiot Emeritus   Join Date: Mar 2008 Location: Fernley, Nevada (near Reno) Posts: 1,849 Rewards Points: 1,492 If the transformer is oil filled it is probably OK, if it is a dry-type, I would add 30%. Rob
 08-04-2008, 07:49 AM #5 Member   Join Date: Aug 2008 Location: Israel Posts: 63 Rewards Points: 75 I'd not load anything above 80 - 90 % of its rated load But consider what happens if all pepole turn on air conditioning etc at the same time (which may happen). Maybe the 0.4 factor is too small
 08-04-2008, 10:25 PM #6 Newbie   Join Date: Jul 2008 Location: Viet Nam Posts: 6 Rewards Points: 10 Thanks for your proposes. Ks = 0.4 is the min factor when the building has over 50 resistances. But I often choose ks = 0.5 for safety and the trans has spare factor is 10 %. Do you think is it ok?
 08-05-2008, 09:21 AM #7 Member   Join Date: Jul 2008 Location: NW of D.C. Posts: 5,990 Rewards Points: 2,000 From another post, I know now to ask "What is the (I^2) T curve for the transformer?" You mix this in with the probability of all the loads being on at once.
 08-06-2008, 06:24 AM #8 Newbie   Join Date: Jul 2008 Location: Viet Nam Posts: 6 Rewards Points: 10 Thank you so much Yoyilit. I already have the load diagram (I*t) for one residence. I can understand your idea is choose capacity of Transformer according to the peak time in the load diagram. But with this method the capacity of transformer is so big. Because I think maybe the peak time of this residence diffence with another residences, because of that IEC had given a ks min for us. And the trouble of mine is how to choose a correct factor to make sure the Transformer not too big or too small with it's loads. Could you give me an advice?
08-06-2008, 12:56 PM   #9
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Quote:
 Originally Posted by Minh Tam Nguyen Thank you so much Yoyilit. I already have the load diagram (I*t) for one residence. I can understand your idea is choose capacity of Transformer according to the peak time in the load diagram. But with this method the capacity of transformer is so big. Because I think maybe the peak time of this residence diffence with another residences, because of that IEC had given a ks min for us. And the trouble of mine is how to choose a correct factor to make sure the Transformer not too big or too small with it's loads. Could you give me an advice?
I'm a little out of my depth here, but the transformer doesn't respond to a brief overload by failing catastrophically. It forgives brief overloads according an I^2 T curve, just like a fuse.
The probability calculations for this circuit should take that into account.

If the transformer instantly failed on a 1 microsecond overload the probability calculations would designed to prevent even the shortest overload [to some level of certainty].
This kind of safety factor would be way too conservative for a real world, massive, transformer with considerable Thermal Inertia and oil cooling.

Ultimately the problem comes down to how many dollars are you willing to pay for how much certainty against a catastrophic failure?
If there is such a failure, will it be just property damage or human life?
You'll be getting into Confidence Levels and whatnot, unless these are already factored into the tables you have.

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