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.

 

[bacon4life]

On possible alternative is to ask them for calculated ratings under alternate operating conditions. For example, I have asked vendors to calculate nonstandard conditions such as:
an ambient adjusted rating for a zero degrees C ambient
a rating assuming the long time overloading parameters in IEEE C57.91.

I suspect this kind of special rating calculation is much easier for the vendor to document as compared to deviating from the normal ONAN/ONAF1/ONAF2 ratings.

Based on the heat run tests I have reviewed, I have not see a transformer reach rated temperature at both the ONAN and the ONAF2 rating. Typically the hotspot temperature has been quite a bit different. By analyzing the heat run test info, the manufacturer can calculate which condition is more limiting, and might be able to give you an as-built rating a few percent higher than the standardized value.

My vendor requests have all been based on purchasing a normal rating, and then asking for post testing calculations. The bidding process would probably be more complicated if you include specific requirements into the bid documents. That being said, I have seen utility specifications that require transformers to withstand various overload levels.

On thing to note is that there are some components inside transformers that are not strictly thermal related, so it is important for the vendor to analysis all components. A couple examples are the interrupting ratings of LTC contacts and the oil expansion volume. Other items like tank heating due to stray flux can be nonlinear.

 

[JJ_Roy]

I'm assuming the system impact study thermal violation is for a "N" condition and not a "N-1" or "N-1-1".

If it's for a "N-" condition, I've seen typical emergency loadings for 115/34.5kV xfmrs at 115% [Rate 2] and 140% [Rate 3] above the normal (highest) MVA rating [Rate 1].

How close are you to the 166% for ONAF2? If you're only a few percent off then I'd maybe roll the dice and bump it up, but if it's more... I'd probably wouldn't do it....
Your engineering judgement prevails here!!!!

Word of caution: Don't design a xfmr with some strange impedance that nobody can build or cost big $$$ on a small project, as I've seen that trap before....