Additional Transformer Capacity

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tyler1215

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I have a Power Transformer that is forecasted to reach its maximum MVA rating this summer. My Tech crew pulled some cooling fans off of an older transformer we had sitting around, and installed the fans on the PWT that's at capacity. I need to report the new MVA rating to our power provider and reliability council. How can I calculate the new MVA ratings to report?
 
tyler1215 said:
I have a Power Transformer that is forecasted to reach its maximum MVA rating this summer. My Tech crew pulled some cooling fans off of an older transformer we had sitting around, and installed the fans on the PWT that's at capacity. I need to report the new MVA rating to our power provider and reliability council. How can I calculate the new MVA ratings to report?
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It is interesting that you have avoided providing any information about the transformer in question. Do you have a picture of he nameplate from the transformer? I would rather not waist my time asking you a bunch of question when you could just as easy take s picture of the NP from the transformer which will with almost all certainty provide that information.
 
Templdl is correct. You did not provide any information. The transformer may not have a fanned rating. Older transformers
did not have the insulation to warrant a fanned rating or 65C rating. The following is a general guide for your information.
Ratings:
ONAN = Oil - Nat circ/ Air - Nat circ
ONAF = Oil - Nat circ/ Air - Forced circ

The transformer has an ONAN rating at 55C rise and an ONAN rating at 65C rise, as well as an ONAF rating at each temperature rise. Your transformer may not have a 65C rating or an FA rating. Check with MFG.

Using a 1000 kva transformer:
Base rating 55C = 1000 kVA ONAN or OA)
Base rating 65C = 1120 kVA (+12% of 55C OA rating
Forced air (FA) rating 55C = 1250 kVA (+25% of OA rating)
Forced air (FA) rating 65C = 1400 kVA (+12% of 55C FA rating)
The FA ratings are with one bank of fans. You may be able to add an additional bank of fans
to obtain a higher rating.

tyler1215 : Please respond to this post.
 
My apologizes for not providing detail information regarding the transformer. I wanted to know if calculating the additional capacity was possible, instead of having someone do it for me (teach a man to fish...).

I am attaching a picture of the name plate. The details for the transformer are below:

OA/FA
MVA 7.5/9.375
HV 67000
LV 24940Y/14400
LV Neutral
Impedance 7.42% at 67000 - 24940

Existing fan size: 3-fans, 24" blade housing

New Fans: 3-fans, 1750RPM, 16" blade housing

If you have any questions, feel free to ask. I would be interested in some papers that have explored this subject, as well.
 

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The OA/FA rating says the unit was designed to have fans. The nameplate says the fan cooled rating is "9.375".
 
templdl said:
Name plates do explain a lot don't they Jim. The fans on the existing unit should have been obvious one would think.
00
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So, given that there is already a fan cooled rating, and if it is already being used to that rating, is there an increase in rating for adding more fans? Or would UL/NEC just say that it will run cooler with the same max load?

Tapatalk!
 
jim dungar said:
The OA/FA rating says the unit was designed to have fans. The nameplate says the fan cooled rating is "9.375".
Click to expand...

Oh, I overlooked the fact that it has an OA/FA rating and not OA /FFA. FA should indicate that it is equipped with fan originally and FFA indicates that the transformer is designed with the capacity to add fans thus "future fan" FFA should have been used. .
 
GoldDigger said:
So, given that there is already a fan cooled rating, and if it is already being used to that rating, is there an increase in rating for adding more fans? Or would UL/NEC just say that it will run cooler with the same max load?

Tapatalk!
Click to expand...
Strangely enough the temperature rise of the transformer is missing from the NP. If it liquid it most likely have been 65 or 55degC or 55/65degC.
Remember that the transformer design must consider any addition capacity because addition capacity equates to higher current. By adding addition fan it may increase the capacity but the current carrying conductors and windings may not be rated for the higher current.
There also could be an outside chance that should the transformer be rated simple 55degC rise thatthat it my be run at 65degC if, and only if the transformer manufacturer aferms that the transformer's design is capable of handling the extra capacity. But to do so would be contrary to what is stated on the NP.
 
If this already has fans, you will need another power transformer or to redistribute load to other substations that have allowed capacity for it. (I say allowed because certain reliability standards may dictate your neighboring substations or even this one have reserve capacity at peak load)



Also, you cant just add any fans. FA ratings are calculated around a certain amount of air flow. Fans are often done as a kit and controlled via appropriate methods. Keep in mind that unlike a pole pig these larger transformers cant handle sustained overloads as well for that matter.
 
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