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Old 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

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Old 07-31-2008, 12:42 PM   #2
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Optimal Spare Factor for Transformer


Please, more details. A lot more. . .

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Old 08-03-2008, 11:00 PM   #3
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Optimal Spare Factor for Transformer


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??
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Old 08-03-2008, 11:22 PM   #4
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Optimal Spare Factor for Transformer


If the transformer is oil filled it is probably OK, if it is a dry-type, I would add 30%.

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Old 08-04-2008, 07:49 AM   #5
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Optimal Spare Factor for Transformer


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
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Old 08-04-2008, 10:25 PM   #6
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Optimal Spare Factor for Transformer


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?
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Old 08-05-2008, 09:21 AM   #7
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Optimal Spare Factor for Transformer


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.
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Old 08-06-2008, 06:24 AM   #8
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Optimal Spare Factor for Transformer


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?
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Old 08-06-2008, 12:56 PM   #9
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Optimal Spare Factor for Transformer


Quote:
Originally Posted by Minh Tam Nguyen View Post
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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