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.
Wire must be chosen to that the current density does not cause them to overheat.
High-power transformers are often filled with a heat-conducting liquid. PCBs were originally used for this purpose.
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.
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.
