A step-down transformer's primary is high voltage and secondary is low voltage, based on voltage configuration or load we select which winding we do fast.
In air-cooled Transformers output voltage is high and load current is low. So in air cooled low voltage winding in side and high voltage winding in out side of the coil . So we can reduce high voltage short problems and we can easily provide coil insulation . reduce short problem. It is cooled in oil.
In step-down transformers we can do high voltage inside of the coil and low voltage and high current of the coil is outside. Because we can better coled in outer side . and reduced to temp problems.
AnswerThe terms 'primary' and 'secondary' do
not relate to a transformer's voltages, but to how its windings are connected. The primary winding is connected to the supply and the secondary winding is connected to the load. So, for a step-up transformer, the secondary winding is the higher-voltage winding while, for a step-down transformer, the secondary winding is the lower-voltage winding.
Your question, therefore, should ask whether the higher- or the lower-voltage winding is the 'inside' winding (i.e. the winding closest to the core), and the answer is that it is the lower-voltage windings are innermost and the higher-voltage windings that are outermost.
