Custom ONAF2 Rating

[cyriousn]

I've tried reaching out to a couple transformer vendors without any luck so I figured I'd give it a shot here.

IEEE C57.12.10-2017 Table 2 says for typical transformer ratings, the 1st stage and 2nd state cooling ratings are 133% and 166% of base rating however there is a line that reads “actual ratings shall be mutually agreed between the user and the manufacture”.

With that being said, is it possible to get a ONAF2 rating higher then 166% if there is a larger radiator / more robust fan system without increasing the base rating?

 

[JJ_Roy]

There's a lot of stuff that goes into transformer rating (winding materials, winding design, oil flow within the windings, etc.) so I'd be careful assuming you can increase the top rating without talking to a manufacturer.

Just curious, is there a reason you don't want to increase the base rating?

 

[cyriousn]

There's a lot of stuff that goes into transformer rating (winding materials, winding design, oil flow within the windings, etc.) so I'd be careful assuming you can increase the top rating without talking to a manufacturer.

Just curious, is there a reason you don't want to increase the base rating?

The response I'm getting from vendors via sales reps has not been helpful. One said it was "outside of their scope".... which makes no sense to me.

The ISO's system impact study says there was a thermal violation for a condition that is not very likely to happen so they are making the project owner show that the secondary cooling rating is going to be higher than 166%. If we increase the base rating of the transformer we could decrease the base impedance to keep the same per unit impedance but the transformer already has a low impedance to start with causing high fault current at the 34.5kv system so we may be changing more data points with the transformer anyways. Its a messy one.