Step down transformer temperature rating

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Hv&Lv

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Rating depends on ambient.
all transformer rise temp are based on 40c
so 55c above 40c (95C) the transformer is rated at 25 MVA in open air. That is the loading that will produce a 95C hot spot rating.
55C was done away with years ago in ANSI, and insulation has gotten better over the years.

65C shows a higher MVA rating.
when the transformer is loaded to 28MVA, it will show an average hot spot of 65C above 40C ambient. (105C)
 
mbrooke

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I'm sure it obvious on your reply, but why are two temps listed even for brand new transformers? 65*C is based on 30*C air temps?
 
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Hv&Lv

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Use is dependent on location.
104F ambient in the shade isn’t usual around here, although it seems like it sometimes.
in the sun in middle of summer though, ambient can be higher. When it is, the ambient difference from 40C(104F) needs to be reduced from the rise temp for loading.

example, in the middle of winter when it’s 10F, I wouldn’t worry about your transformer above running at 35-37 for a while with no fans.
the problem is, if it’s that overloaded in the winter, I bet it’s close to that in the summer also. Then the ambient is much higher, resulting in lower MVA ratings.
overloaded XFs don’t last years like they should.

I noticed above I wrote open air, that not what OA means before someone chimes in to correct it...

OA. Oil/air

you also have two FA ratings. That’s for two stages of fans that the transformer probably came with.

FA. Fan assist
 
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Hv&Lv

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Manufacturers still list it and make it available. It makes people happy.
if I were to order one today, it would be a 65C rating.
It will probably come in with a 55/65 rating, with my specified MVA listed on the 65 row.
 
mbrooke

mbrooke

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Hv&Lv said:
Use is dependent on location.
104F ambient in the shade isn’t usual around here, although it seems like it sometimes.
in the sun in middle of summer though, ambient can be higher. When it is, the ambient difference from 40C(104F) needs to be reduced from the rise temp for loading.

example, in the middle of winter when it’s 10F, I wouldn’t worry about your transformer above running at 35-37 for a while with no fans.
the problem is, if it’s that overloaded in the winter, I bet it’s close to that in the summer also. Then the ambient is much higher, resulting in lower MVA ratings.
overloaded XFs don’t last years like they should.

I noticed above I wrote open air, that not what OA means before someone chimes in to correct it...

OA. Oil/air

you also have two FA ratings. That’s for two stages of fans that the transformer probably came with.

FA. Fan assist
Click to expand...


Is it not Forced Air? (nit picking, I know) :p
 
mbrooke

mbrooke

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Hv&Lv said:
Manufacturers still list it and make it available. It makes people happy.
if I were to order one today, it would be a 65C rating.
It will probably come in with a 55/65 rating, with my specified MVA listed on the 65 row.
Click to expand...


Doesn't the IEEE have something like a 115-125% safety factor? That a 100MVA unit is something like 115-125MVA? I know I read that somewhere when looking at pirated copies of the IEEE standards lol.


FWIW- I know of some POCOs that will actually overload a two unit substation for the loss of one unit on the premise of a 1% loss of life based on the fact the overload will only take place for a few hours on the hottest days of the year- mobile unit or cooler weather to follow in 48 hours.
 
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