Two offers for the same kVA rating can differ a great deal inside the tank. Knowing the main parts helps you read a datasheet and ask the right questions.
Core
The core carries the magnetic flux. It is built from thin, coated silicon steel laminations. The core type, where the windings surround the limbs, is used for almost all distribution and power transformers because it is simple to build and insulate. The grade of steel and the quality of cutting and stacking decide most of the no-load loss.
Windings
The primary winding connects to the supply and the secondary to the load. The low-voltage winding has fewer turns of heavier conductor; the high-voltage winding has more turns of lighter conductor. Conductor material (copper or aluminium) and winding design decide most of the load loss and the ability to withstand short circuits.
Tank
On oil-immersed units the tank supports the active part, holds the oil and gives off heat. A conventional tank is simple and strong but the active part must be lifted out for inspection; a bell-type tank lets the cover be lifted away instead, which helps on sites without a heavy crane.
Bushings
Bushings take the leads through the tank wall or cover while insulating them from the earthed tank. Porcelain bushings are standard on distribution units; condenser bushings are used at higher voltages.
Tap changer
The tap changer alters the number of turns in use so the output voltage can be held near its rated value. An off-circuit changer is set with the transformer switched off; an on-load changer adjusts while it carries load.
Other fittings
Oil, the breather, gas relay, pressure relief device, thermometers and radiators complete the unit. Ask for the fitting list with each offer so you compare like with like.
Our factory makes cores and coils in house; see cores and coils if you assemble transformers yourself.