Wire sizes in the interior of transformers?

Question

High power transformers are very common, for obvious reasons.  Much unlike homemade transformers that I've ever built, only out of tiny enameled #26 wire, that wouldn't power anything practical.

I've tried using wire as large as #6 wire to wrap a homemade transformer, as what would be needed to connect externally to 10 kVA transformers, but that simply isn't practical.  The wire is simply too stiff to wrap it in tight windings.

Consider the example of a 225 kVA three phase transformer, that carries about 300 Amps of current on each of its secondaries that are 277 Volts.  Externally, I'd need to connect with 400 kcmil wire, that is almost an inch in total diameter.  What exactly do they do in order to build transformers at this high power?

Do they parallel a whole bunch of tiny windings?
Do they use a cooling fluid, so that tiny wire can have a much greater ampacity?
Or do they also use big wire, but the way it is constructed somehow relieves the strain resulting from wrapping it?  Heat treating the metal, perhaps.

Newton1Law · Accepted Answer

You might want to review the source below to help you understand how power transformers operate. I'll try to explain how they are able to use smaller wire gauge to serve the same load you have to use larger wire gauge for.

You recognized that if you can cool a conductor, then it may call more current. Thus two wires that are the same size may have different current carrying capacity based on the temperature of the wire.  So if I can keep the wire cool enough, there is no limit to the amount of current that is allowed to flow in a particular wire.

I could use small conductor to supply a very large current load if I supercool the conductor. This is exactly what is happening when we use super conductivity techniques. (see http://en.m.wikipedia.org/wiki/Superconductivity).

In power transformers that are 10 kVA to about 10 MVA, cooling design is a major factor allowing use to use smaller conductors in the transformer to serve a load that requires much larger wire in free air.  The cooling is performed by circulating oil through and around the conductors and then removing the heat from the oil by some type of external radiator.

Cooling like this is so effective that many power transformers have multiple ratings based on the cooling capability of the external cooling devices.  For instance the core and coils of a triple rated power transformer is 10/20/25 MVA.  The two additional ratings of 20 and 25 MVA is based on two additional sets of fans helping to cool the oil flowing through the radiators.

Hope this helps,

Newton1Law

Anonymous · Answer

The transformers are wound using machinery that can bend the heavy wire as required. Parallel wires are used, but they are not usually "tiny," but smaller that what would be needed for single wires. Transformers are wound with flat strap-like conductors rather than round wires. Liquid cooling allows the maximum current in a given size wire.

PS

Not everything described is used for every large transformer. Construction techniques vary. There may be other techniques and designs used also.

PS
Not everything described is used for every large transformer. Construction techniques vary. There may be other techniques and designs used also.

cosmo · Answer

High-power transformers are often filled with a heat-conducting liquid.  PCBs were originally used for this purpose.

? · Answer

Wire must be chosen to that the current density does not cause them to overheat.

Trending News

Wire sizes in the interior of transformers?

4 Answers