There are two forms of transformer windings. One is a rectangular form and another is a circular form. In spite of that, transformer windings can also be either concentric or sandwiches. In a single-phase core type transformer, we generally use concentric winding. On the other hand, in a single phase shell type transformer, we commonly use sandwich windings.
For optimizing the insulation cost of the transformer, we normally place the low voltage winding on the core in a core type transformer. Obviously we insert required insulation between the core and the low voltage winding. Then we place the high voltage winding on the low voltage winding with proper insulation.
For the same reason, that is for optimizing the insulation cost, we place the low voltage winding on the outside portion in a shell type transformer.
The shape of the concentric winding is cylindrical. In the larger power transformer, we insert axial spacing strips between the turns of the coil to facilitate the proper circulation of the oil to the windings. Also, we use bakelite tubes to insulate the high voltage winding from the low voltage winding.
The shape of the sandwich winding is like an assembly of discs. The low voltage winding is subdivided into a number of sections. Also, the high voltage winding is subdivided into a number of sections. Each of the sections forms a disc-like shape. Then we fit these discs alternatively one by one on the core limbs. There is a space between the turns in each disc by inserting insulation strips to facilitate the proper circulation of the oil. There is also a gap between two adjacent discs for the same reason.
The main advantage of sandwich winding is, it reduces the flux leakage. In this construction of winding, a high voltage section is always surrounded by two low voltage sections. In order to maintain the criteria and also to maintain the symmetry of the distribution the number of turns of low voltage disc at both ends is half of that of the inner low voltage discs.