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07302008, 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 Advertisement 
07312008, 12:42 PM  #2 
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Optimal Spare Factor for Transformer
Please, more details. A lot more. . .
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08032008, 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 residencesaccording to IEC standard). Do you think I need a spare factor for my transformer?? 
08032008, 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 drytype, I would add 30%.
Rob 
08042008, 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 
08042008, 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?

08052008, 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. 
08062008, 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? 
08062008, 12:56 PM  #9  
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Optimal Spare Factor for TransformerQuote:
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. Advertisement 

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