Transformer temperature ratings (Insulation ratings) and fault current

[mshields]

If you have a 2500kVA transformer and say it’s has a 55/65 degree such that you get 12% more at the 65 deg rating. I believe the maximum fault current is nevertheless based on the 2500kVA value, divided by the decimal value of the impedance and then converted to amps (or switch to amps first and divide it by the decimal value of the impedance).

But a) am I correct that you would never use the 65 deg KVA rating and b) why is that so?

Thanks,

Mike

 

[xptpcrewx]

Good question. Transformer %Z is with reference to the Base kVA rating.


Sent from my iPhone using Tapatalk

 

[Hv&Lv]

But if you specify a 55/65 transformer isn’t 55 just a holdover? I was thinking 55/65 was removed from ANSI, but some manufacturers kept it because people were just used to it.
like the :5 designation for CTs. Its hard to change...
I believe if you order a 55/65 you would get a 65 transformer and the Z is based on worst case.
It would probably cost more to order and have an XF tested for 55.

if you truly have an old 55/65 XF then the Z, losses, regulation, etc would have to be adjusted to the 65 values.

 

[mshields]

According to Eaton, 65 does appear to be the standard, It is at least the fist configuration they list. Then they list (55, 55/65) in parenthesis implying they are the same thing I guess, next they have 65/75 and then 75

So what are you saying Hv&Lvl. Regardless, of the voltage options, is the impedance always based on the first temp rating?

 

[Hv&Lv]

According to Eaton, 65 does appear to be the standard, It is at least the fist configuration they list. Then they list (55, 55/65) in parenthesis implying they are the same thing I guess, next they have 65/75 and then 75

So what are you saying Hv&Lvl. Regardless, of the voltage options, is the impedance always based on the first temp rating?

While I’m not exactly sure which number, it shouldn’t make that big a difference. The transformer should be designed so that the %Z is at operating temperature.

%Z is pu. The temperature will make a difference in actual ohms, it won’t really change the nameplate value %Z