Increasing MVA Without Fans 7

[Mbrooke]

Is there such a thing as a transformer's MVA being increased past the ONAN rating without fans- such as making the radiators much larger?

 

[prc]

Yes; possible.

 

[waross]

Other than re-rating the temperature rise, fans are probably the easiest and cheapest.
You get a lot of redundancy as well.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

@PRC: Tell me what you know!

 

[cuky2000]

In hydro plants, a power transformer often is cooled with a water heat exchanger.

 

[Mbrooke]

I've heard of that- its a neat idea.

 

[crshears]

And it works very well indeed.

Added thought via edit: absent an existing cooling tower or cooling water system already present for other purposes, however, you'd just be incurring needless extra expense and complication...

 

[prc]

As per IEEE, transformers are designed for ONAN rating and then upgraded for ONAF by putting up fans.In IEC world, transformer is designed for ONAF rating and de-rated with out fan cooling. Definitely fan cooling is economical than adding more or bigger radiators.But better contact OEM before such over loading. It is not only cooler,but many other parts of transformers are to be checked whether designed for such overloads.

 

[Mbrooke]

Any idea by how much larger radiators help?

 

[electricuwe]

Larger radiator won't help much. Design of natural cooling between winding and oil as well as between oil and ambient have to match each other.

 

[crshears]

I agree with electricuwe; larger rads will only be of limited help since they are subject to the law of diminishing returns, since a lower rad bottom temp will only increase natural oil flow very slightly.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[DM61850]

Cheapest method would be to use thermal couples and IEEE equations to determine how much capacity the transformer really has. There are utilities that do dynamic loading calculations. There are co-ops that run their transformers harder on cold days without any calcs. The nature of the load can let you ride it harder as well. A transformer has a lot of oil that has a lot of thermal intertia. If you are just trying to run out a peak, you might not have to do anything more than just be aware that you can't handle a certain loading for longer than X hours. If you contact the manufacture, they probably rated the transformer really conservative with like 110 F ambient temp. Some utilities undersize their transformers for their peak and live with loss of life due to overloading it because according to their money calculations, that is the cheapest method and a lot of times transformers are changed out to growing load and not because they reached their end of life.

 

[ScottyUK]

Mbrooke,

You want to avoid fans, are you also keen to avoid forced oil circulation using pumps?

 

[Mbrooke]

From noise perspective I'm ok with forced oil pumps. However the industry is moving away from them due to oil contamination. If someone can offer pumps that won't do so I'll take them.

 

[bacon4life]

If noise is your issue with fans, have you looked into low noise fans? We purchased an ONAF transformer with a sound level of only 50 dB for a substation very closed to a residential area. By comparison, similarly sized transformers seem to run 57 dB to 65 dB on the ONAN rating. If you need just a small increase rather than the typical the typical 33/66% increase, mounting just a few very low speed fans may be feasible.

 

[Mbrooke]

Any good low noise fans out there which you might recommend?

 

[prc]

1) Fan cooling also increases cooling only by reducing the bottom oil temperature going back in to tank. Fan cooling is recommended from cost angle. There are users who ask for 100 MVA units only with ONAN cooling.
2)Yes.ONAF is to be preferred over ODAF (oil pump &fans).My factory regularly supply 500MVA 400 kV 3 phase autos &1500 MVA 765 kV Auto Banks with only ONAN/ONAF cooling.
3) Special low noise transformer cooling fans are made by almost all reputed makers. If you mention the area where you are looking, I can suggest makes. But better leave these to transformer makers who can do professional job
4) DM 61850,sorry to say, your idea is not safe. Any wet, old transformer, if overloaded can result in failure by bubble formation from winding.

 

[Mbrooke]

Those 100MVA ONAN only units- how are they designed differently if it all? Are they basically a 100 ONAN / 140MVA ONAF unit without fans? Or an 65MVA core designed around a system that can cool without fans?

 

[prc]

As I mentioned earlier, in IEC world we design units for max rating. In fact, there is no difference in active part between a 100 MVA ONAN unit or 100 MVA ONAF unit. With ONAF unit, we will provide less number of radiators and put fan to increase heat transfer from radiators to air. This will bring down bottom oil temperature further, increasing thermal head for cooling inside oil.

 

[Mbrooke]

Does the 100MVA ONAN have a smaller core than the 100MVA ONAF is my question? And if so by how much typically?