Effect of Transformer Capacity on Cooling Method.

Why are you killing us?

Your capacity for drinking alchohol will be more in cool atmosphere than hot conditions.

ONAN = OIL NATURAL, AIR NATURAL. ONAF = OIL NATURAL, AIR FORCED. NATURAL means oil or air cooling by convection or radiation which going to happen anywhere something is hot. Forced means motor-driven air blast fan or even a chimney. The transformer loses more heat for the same oil temperature if more air is going through the cooler due to the fan - it can give more output, despite the increased losses. OF means OIL FORCED, usually an electric pump. A windmill coupled to a pump in the transformer would still be OF!!

well, thats about cooling. But how , the capacity increased?...if you dont know, please dont waste our time.

It was written in the first question - "give me an idea how the capacity changes by changing the cooling method". I wrote - "The transformer loses more heat for the same oil temperature if more air is going through the cooler due to the fan - it can give more output, despite the increased losses." An idea was requested, I gave an idea. I could not know the meaning of ONAN and ONAF were understood by the questioner, except as "something to do with cooling". "How" the capacity is increased is by fans or pumps to (improve cooling). "Why" is because transformer oil temperature is kept "to standard" despite the increased transformer losses at a greater capacity (output current). You wrote "..if you dont know, please dont waste our time". If you take "not" out of "do not" it makes "If you do know, please waste our time". So was the great effort by the other posts to explain losses, temperature rise etc "wasting our time??

The capacity/rating of any electrical equipment is maximum capacity or load on which it can operate for expected life. It may be noted that for given operating condition the maximum temperature of equipment is dependent on it's load

For the transformer in question, the ONAN capacity is 35 MVA meaning the heat generated by losses at 35 MVA would be sufficient to reach maximum operating temperature with ONAN cooling.

With cooling improved ONAF, the transformer certainly would be able to carry more load resulting in higher loss. This rating in present case is 50 MVA for transformer of 35MVA ONAN.

It is interesting to note that for two transformers 35MVA/50MVA and 50MVA ONAN transformer, iron core and copper winding in both the case are designed for 50 MVA and saving is first transformer is on account of cost on insulation media. in other words less oil would be required ONAF transformer to desipate same amount of heat.

Current flowing through any conductor generates heat (I2R). And, as far as, damages to windings are concerned, it is only due to temperature. If you are able to remove the heat generated faster, you may afford to generate more heat. That is what happens in your ONAN/ONAF Transformer. In ONAN Cooling, the coling is Oil Natural Air Natural and hence the rate of heat dissipation is slower. In ONAF (Oil Natural, Air Forced) you are able to remove more heat as you are forcing Air by use of fans. Now, you can afford to "overload" the same transformer windings as heta dissipation is better now. Clear?

These threads also be further help:

http://cr4.globalspec.com/thread/43826/Disadvantages-of-ONAN-ONAF-against-ONAN-Transformer

http://cr4.globalspec.com/thread/61811/ONAF-Cooling

- MS

The ONAN has a maximum power output of 35MVA i.e the maximum output power it can produce for it's load at maximum winding and oil temperature without commanding a oil or winding temperature alarm and/or trip. However with ONAF in the form of fans on the transformer fins, the capacity is increased to 50MVA simply because the winding and oil temparature at the at the same power output of 35MVA is lower. If you test the transformer temperature at 35MVA ONAN on maximum load, you will see that it is just about the same as that of the 50MVA ONAF but with a greater load-HENCE GREATER CAPACITY.