Transformer Ratings

[kingpb]

I guess I should also mention that I have another similar transformer that has only one rating which is 15/20/25 MVA @65deg rise.

This only has a 65 deg rise rating. I'm assuming that this transformer actually has less capacity because it has less of a rating a 65deg. If that is its rating at 65deg what would its ambient rating be? 15MVA?

When reviewing a power study I see a transformer has two rating options. Nominal KVA which is listed at 15000 and Full load KVA which is listed at 25000. Are the 65deg ratings always used for these nominal and full load KVA's?

If it is an older transformer it may only have a 55 deg C rating. In which case that is what you would use. If it has a 65 deg C rating then typically that is what you would also use. I say typically, because it really depends on what the study is trying to accomplish.

 

[kingpb]

For everyones, benefit this is directly from IEEE C57.12.00

When air-cooled, the temperature of the cooling air (ambient temperature shall not exceed 40 ˚C, and the average temperature of the cooling air for any 24 h period shall not exceed 30 ˚C.

 

[bob]

Type: ONAN/ONAF/ONAF
MVA: @55deg C Rise - 15/20/25 MVA
@65deg C Rise - 16.8/22.4/28 MVA

I have just looked at a power study from a couple of years ago and found that the Max MVA plant load at that time was aprox 17MVA. (I'm sure its grown since then)

With this 17MVA I'm assuming it was at a 55deg rise? Is there any way of telling?

Mull
You still don't have this temp thing down yet. The transformer has 2 ratings. One at 55C and one at 65C. Since the 65C rating is higher, forget the 55C rating. You will not need to use it. The 17 MVA load has nothing to due with the 55C or 65C rating. The rating is what it is. The rating. The max rating is what you need to be concerned with and that's 28 MVA.

So lets say its currently loaded up to 17MVA. This is above the ambient rating and probably into the single fan cooling stage. How much more load can safely be put on this transformer. From what I've heard, it sounds like 11 more MVA to bring us up to the max rating of 28MVA at 65deg?

17MVA is above the 65C rating of 16.8 MVA(no fans). IF you put one stage of fans on, the rating is now 22.4 MVA. IF you install another stage the rating is 28 MVA.
Suggest you install all of the fans now. Don't just put a single stage now and have to come back and do it again later.

 

[rcwilson]

Transformer Capability

Transformer Capability

Your transformer should last 30 years operating at 65C rise (28 MVA) assuming everything else is perfect. Similar to a car going 250,000 miles running on a smooth road at 60 mph continuously with proper maintenance.

At one plant we operated several banks of transformers about 25% over their nameplate rating. This was possible because they were in a cooler climate and were designed for frequent short circuits that occur in mercury arc rectifier service. When the rectifiers changed out to solid-state that eliminated the shorts and freed up some reserve capacity. We monitored oil temperatures and put soaker hoses on the fins in hot weather. It worked for over 20 years with no failures. A thorough study was done on all our equipment including bushings and transformer internals before we boosted load.

One thing to check is the rating of the cables, bus and breakers on both sides of the transformer. Verify that the equipment connected to the transformer can handle the full 28 MVA.

As others mentioned, high energy prices may make the increased losses in the transformer too expensive. Look at the capital cost savings versus the ongoing energy costs. It may justify putting a more efficient (bigger) transformer on order.

 

[jghrist]

This only has a 65 deg rise rating. I'm assuming that this transformer actually has less capacity because it has less of a rating a 65deg. If that is its rating at 65deg what would its ambient rating be? 15MVA?

You are correct about the transformer having less capacity. I suppose by "ambient rating" you mean what load will produce an average winding equal to the ambient temperature. This load is zero kVA because any load will heat the windings above ambient.