Before discussing the details of transformer windings further it is necessary to look in some detail at winding conductors.

Power station transformers use copper windings almost exclusively and the detailed requirements for them are set down in BS1432. During the late 1960s, there was a very sharp rise in the price of copper and some manufacturers attempted some experimental use of aluminium, but its higher resistivity greatly complicates the designer's problems. It necessitates a greater cross-section of conductor and a larger number of cooling ducts; these in turn cause a large increase in winding size, which increases the size of the core. In addition, the increased winding size increases impedance, requiring a further increase in frame size to counteract this so the iron losses are increased still further, risking a runaway situation. Unlike other areas of power station plant (e.g., cables) it is exceedingly unlikely that aluminium will ever become a serious rival to copper for transformers.

Winding conductors are normally rectangular in section. They have a far better winding space factor compared with circular conductors and are used exclusively, except for the very small transformers where both circular conductors and, on occasions, foil windings are used. These will be discussed further in Section 2.4 of this chapter which deals with auxiliary transformers.

Winding strands must be insulated from each other within a winding conductor and, of course, each conductor must be insulated from its neighbour. This is achieved by wrapping the strands helically with paper strip, and at least two layers are used, so that the outer layer overlaps the butt joints in the layer below. The edges of the copper strip are radiused in order to assist in paper covering. This also ensures that, where strands are required to cross each other at an angle, there will be less ‘scissor action’ tending to cut into the insulation. Where conditions demand it, many layers of conductor insulation can be applied and the limit to this is determined by the need to maintain a covered cross-section which can be built up into a stable winding. This demands that, particularly when they are to have a thick covering of insulation, winding conductors should have a fairly flat section, so that each can be stably wound on top of the conductor below. This usually means that the axial dimension of the strand must be at least twice, and preferably two and a half times, the radial dimension.