Calculating transformer cooling efficiency

[cayjay]

Where can I find a method to calculate cooling efficiency for a oil filled transmission transformer? I have the top oil and ambient temperatures and fan/pump status.

Thanks

 

[electricpete]

I don't know exactly what is meant by efficiency.

If you know the current and have the factory data, you could roughly estimate the heat input. It would be the no-load losses plus load losses corrected by current^2. (assumes voltage is comparable... if not you may need to correct no-load losses as well I think proportional to voltage^2?). Also if you're getting fancy might adjust the load losses by 0.4% per degrees C winding temperature to account for resistance change.

So if your transformer is in steady state, the heat being added is the same as the heat being removed.

If you know ambient temperature and oil temperature you could express that as a ratio of btu dissipated per rise above ambient for given pumping configuration. That might roughly be considered cooling effectiveness.

You could calculate an efficiency of pump and fan motors in terms of mechanical output per electrical input but mechanical output is not likely important to you... temperature rise is.





=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[cayjay]

electricpete,

thanks for the note. i am trying to diagnose a problem with the transformer's cooling system.

i am not sure what the industry exactly means by the term "cooling efficiency" (it is used quite frequently - see Google) but I would expect (and what I am really looking for) is the quantitative measure of the transformer cooling system's ability to dissapate heat for a given rise in ambient temperature assuming that the fan and pump motor runs steady state. Given a change in "cooling effciency" would indicate a problem with the cooling system (birds nest, fan motor problem).

bottom line is that this should be a simple problem and i want to use an industry standard way of caculating he value of cooling efficiency.

 

[PowerfulStuff]

Cayjay,
When transformers are built they will normally have a temperature rise test carried out to ensure that they meet specifications.This would give a temperature rise for full load.

For your situation this test can be approximated by setting up a steady load as close to 100% as possible and run until the transformer temperatures reach steady state. Fans and pumps should be run in manual to acheive maximum cooling. Loading should also be recorded.

It should then be possible to compare the rise in this test with the nameplate rise reduced according to the load you were able to acheive.

 

[GamezBeCJ]

Cayjay:

Power transformers usually have two or three ratings. A first power rating named ONAN (Oil as the cooling medium inside the transformer working by natural convection AND air as the outside cooling medium working by natural convection). Second and third stages usually involve the use of fans and oil pumps in the radiators, which in turn would add some cooling capacity either by forcing the medium outside (air) or the medium inside (oil). Those are named ONAF (for forced air) and ODAF (for forced air and oil).
Now, what the manufacturer does is to calculate the losses for the firs MVA (power) rating, i.e. 15000 watts of losses at 100 MVA of power. Then he would calculate the extra losses produced by the increase in power rating, for example, going to 133 MVA the losses would be 26500 instead of 15000 (losses follow a square rule respect to the load current).
The goal is to keep the same temperature rise, say 65ºC, even with increased losses, this are the losses that the cooling system (fans and pumps) have to take care of. In our example the fans would have to remove 11500 watts (26500-15000). The losses in watts should be provided by the transformer manufacturer.
How do you know how many fans you need, there are some references. A transformer fan manufacturer has some information on this available on line (

http://www.krenz.com).

Finally, one way to know if your cooling system is working fine, load the transformer at its maximum capacity, the fans and pumps must be running, if your temperature rise is at or below the rated temperature rise, the cooling system is working ok. (The temperature rise is the substraction: Transformer temperature - Ambien temperature = Temperature rise).
If you loose part of the cooling system (radiators, fans or pumps) the available power rating of the transformer would be diminished.