Do you derate oil filled transformer capacity for high ambient temps?

[bdn2004]

We have a 2500kVA/2800kVA 55/65 deg C oil filled tranformer that sits outside of the switchgear room where it feeds a 480V switchgear that feeds two large power users that consume about 2000kVA.

This is in the Midwest part of the USA, where routinely the temperatures in the summer approach 100deg. Per the NEC cables have to be derated based on that temp is it the same with transformers?

They are talking about increasing the load so that it will consume 2400 kVA, over 90% of the capacity on the 55deg temp capacity. I'm questioning if this is a good move on an old transformer as this is.

I thought you weren't to load a transformer but to 80% and not to use mechanical means of cooling except for peaks, these loads however are continuous.

Any equation I can use here to prove that we shouldn't do this without bumping the size of the transformer?

 

[dpc]

Transformer ratings (ANSI) are generally based on an AVERAGE ambient temperature of 30 deg C.

If the average temperature exceeds 30 deg C, ANSI C57 provides guidance on derating. For a 24 hr average of 40 deg C, the transformer kVA rating is reduced to 86% of nameplate, IIRC. At 50 deg C, it is 70%.



David Castor

http://www.cvoes.com

 

[bdn2004]

thanks dpc,

Any thoughts on the other part of my question...would you think it would be wise to place additional load on this existing transformer?

 

[dpc]

I would be OK with loading up to the nameplate kVA rating subject to any required derating due to high ambient. You don't want the entire operation to be held hostage by worries about one transformer.

I'm interpreting your post to mean that the transformer is outdoors. The 24 hr average outdoor ambient will almost certainly never exceed 40 deg C, so you should be safe with the 86% level.

You don't mention the actual age of the transformer, but I would also recommend that you take regular oil samples and get test reports - noting any significant changes that might indicate trouble.

Another consideration is what the impact of a transformer failure would be - lost revenue, downtime, etc. Even brand new transformers can fail, so it's always good to have a plan to live without it for a while.

David Castor

http://www.cvoes.com

 

[bdn2004]

dpc,

Thanks for the reply. The transformer is 30+ years old and it is outdoors. It is regularly maintained and shows no signs of deterioration from recent testing.

You do however raise a point I didn't think about. This is a single point of failure. This goes down, the whole operation goes down. That's why we are replacing the stuff it feeds.

But I would think a transformer this size should be fairly easy to get in an emergency. At least I'm guessing this is why there is no redundancy built in to the power system.

 

[dpc]

If the primary voltage is common, a replacement unit shouldn't be too hard to locate, if you will accept a used unit. A portable 480V generator in that size is also a possibility as a temporary measure.

David Castor

http://www.cvoes.com