Transformer Inrush multipliers for a 55°C/65°C ONAN/ONAN unit 1

[kartracer087]

Hello,

If you have a transformer that is designed as a 55°C/65°C rise, what should the 12 x FLC at 0.1 seconds and 25 x FLC at 0.01 seconds multipliers be based upon? The 55°C rating, or the 65°C rating?

For example, if you have a 3000kVA transformer with 55°C rise, it is capable of being loaded up to 3360kVA at 65°C rise (a 12% increase over base 55°C rating). Would you use the 3000kVA base at 55°C to determine inrush, or 3360kVA at 65°C? With 3000kVA, at 34.5kV primary, the inrush is 602A at 0.1 seconds and at 3360kVA the inrush is 674A at 0.1 seconds for example. Depending on the fuse selection you might be very close between these two values so I'm not sure if you should use the 55°C base or the 65°C base in the calculation.

Any thoughts?

Thanks.

 

[RRaghunath]

Are you sure 65degC is not winding temperature rise and 55degC is oil temperature rise!
If both are winding temperature rises, then assuming rated winding temperature rise is 55degC, 65degC will be only permissible overload for a short time.
If yes, the transformer design would have been based on 55degC rise only.
So, the inrush calculation should be based on 55degC temperature rise.

 

[waross]

what should the 12 x FLC at 0.1 seconds and 25 x FLC at 0.01 seconds multipliers be based upon?

12x and 25x are rules of thumb to avoid nuisance fuse blowing.
The actual maximum inrush is dependant on the impedance of the transformer.
The impedance of the transformer is relatively unchanged from 55c to 65C.
The impedance will be slightly greater at 65C due to the increased resistance of the hotter windings, but this is slight enough to be mostly ignored.
Impedances are measured and calculated at working temperature.
However the impedance difference of a cold transformer compared to the impedance at working temperature is ignored and the hot value is used. The impedance difference between 55C and 65C is much less.
I anticipate that the %impedance ratios between 3000KVA and 3360 KVA will be in the ratio of 3000/3360.
A calculation of the available fault current and the maximum asymmetric current will be the same at either rating.
Using x12 and x25 at the 3000 KVA rating should also be adequate for the 3360 KVA rating.
The 0.1 sec and 0.01 sec currents should be almost identical at either rating.
If you are nervous, use the higher rating.
At x12 and x25, we are looking at short circuit protection, not overload protection.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[jghrist]

Back when transformer insulation was improved to withstand higher temperatures, and the standard temperature rise was changed from 55°C to 65°C, many utilities, to be conservative, continued to make the base kVA rating on 55°C rise. This provided higher loading if the temperature rise was allowed to go to 65°C.

Percent impedance is based on the 55°C ONAN rating. I suggest doing the same for the 12 X FLC inrush estimate.