Low voltage windings are installed close to the transformer steel laminate core with the high voltage windings placed on the exterior. Windings are installed concentrically around the core limbs, the inner shell designates the layer of windings closest to the core, there is then a mid-shell and/or outer shell depending upon if a regulating winding is present. Each winding has approximately the same height around the limb; this reduces losses and lowers the potential short circuit stresses.
Windings are constructed of aluminium or copper depending upon the availability of materials and cost. Copper winding transformers are dimensionally smaller than their aluminium counterpart winding transformers (assuming the name plate ratings are the same).
Windings are usually cold rolled during fabrication; this gives the windings a higher mechanical strength and higher resistance to deformation during short circuit conditions. In addition to cold rolling, windings are sometimes annealed prior to the insulating material being applied. Each strand is smooth and without sharp corners, this reduces dielectric losses.
Each conductor consists of strands, it is these rectangular shaped strands that are used in parallel to form the part referred to as a conductor; paralleled conductors then form windings. The rectangular shape of the strands is used in order to utilise space within the window effectively. It is not uncommon to use circular strands for smaller transformers, but even then the sides are often rolled flat in order to utilise the space more effectively.
The main consideration for larger transformers is quite often not the thermal or eddy current losses, it is the winding’s ability to withstand short circuit forces i.e. the winding’s mechanical strength.
Each strand is insulated using thin (typically < 0.25 mm) paper or lacquer. Paper insulating material is wrapped around each strand until the desired paper thickness is obtained. The core is separated from the inner windings using pressboard, the same barrier technique is again used in order to separate the windings from each other.
Winding Configuration Types
There are four main types of windings:
A regulating winding may be used dependent upon the type of winding configuration used. The regulating winding connects the transformer windings with the tap changer and is installed in the outermost winding shell.