Manuel Bolotinha explains the various types of power transformer windings and their applications.

Power Transformer Windings
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The power transformer windings consist of current-carrying conductors wound around the sections of the core, and these must be properly insulated, supported and cooled to withstand operational and test conditions.

Copper and aluminium are the primary materials used as conductors in power-transformer windings.

While aluminium is lighter and generally less expensive than copper, a larger cross section of aluminium conductor must be used to carry a current with similar performance as copper.

Copper has higher mechanical strength and is used almost exclusively in all but the smaller size ranges, where aluminium conductors may be perfectly acceptable.

In cases where extreme forces are encountered, materials such as silver-bearing copper can be used for even greater strength.

The conductors used in power transformers are typically stranded with a rectangular cross section, although some transformers at the lowest ratings may use sheet or foil conductors.

Multiple strands can be wound in parallel and joined together at the ends of the winding, in which case it is necessary to transpose the strands at various points throughout the winding to prevent circulating currents around the loop(s) created by joining the strands at the ends.

Individual strands may be subjected to differences in the flux field due to their respective positions within the winding, which create differences in voltages between the strands and drive circulating currents through the conductor loops.

Proper transposition of the strands cancels out these voltage differences and eliminates or greatly reduces the circulating currents.

The choice of the type winding is largely determined by the rating of the winding. Some of the common types of windings are:

  • Cross over type
  • Spiral type
  • Helical type, and
  • Continuous disc type.

Cross over type windings are suitable for currents not exceeding about 20 A.

They comprise of circular cross section and are used for HV windings in small transformers in the distribution range.

A number of such coils are joined in series, spaced with blocks which provide insulation as well as duct for cooling.


Strip conductors are wound closely in the axial direction without any radial ducts between turns. Spiral coils are normally wound on a Bakelite or pressboard cylinder.


Though normally the conductors are wound on the flat side, sometimes they are wound on the edge. However, the thickness of the conductor should be sufficient compared to its width, so that the winding remains twist-free.


Figure 4 shows a double layer spiral coil where an oil duct separates the two layers. For such a coil, both the start and the finish leads lie at one end of the coil and may at times prove to be advantageous for making the terminal gear.

Normally it is not necessary to provide ant transposition between the parallel conductors of a spiral winding as the lengths and the embracing of leakage flux are almost identical.

Helical type windings are used in low voltage windings of small and medium sized transformers.

The simplest helix coil consists of a single layer, formed by turns lying directly side by side, extending over the axial winding length. Each turn may comprise a single conductor or a number of conductors in parallel, the helix at each end of the coil being supported by a suitably shaped edge block to give adequate mechanical strength in an axial direction.

A number of conductors are used in parallel to form one turn. The turns are wound in a helix along the axial direction and each turn is separated from the next by a duct.


Helical coils may be single layer or double layer or multi-layer, if the number of turns is more.

Unless transposed, the conductors within a coil do not have the same length and same flux embracing and therefore have unequal impedance, resulting in eddy losses due to circulating current between the conductors in parallel.

To reduce these eddy losses, the helical windings are provided with transposition of the conductors which equalize the impedances of the parallel conductors.

Continuous disc type windings are used for voltages above 36 KV.

These coils consist of a number of sections placed in the axial direction, with ducts between them.

Each section is a flat coil, having more than one turn, while each turn itself may comprise one or more conductors (usually not more than four or five), in parallel.

The conductor may be either a single rectangular strip or a number of rectangular strips in parallel, wound on the flat. This reduces considerably the risk of the strip twisting slightly in winding and thereby making an unsatisfactory disc.

The sections are connected in series, but without any joints between them. This is achieved by a special method of winding. It is not necessary to provide a cylindrical former for these coils, as these are self-supporting.

Each disc is mechanically strong and exhibits food withstand of axial forces.

Several types of windings are commonly referred to as “pancake” windings due to the arrangement of conductors into discs.

However, the term most often refers to a coil type that is used almost exclusively in shell-form transformers.


The conductors are wound around a rectangular form, with the widest face of the conductor oriented either horizontally or vertically.

In core-form transformers, the windings are usually arranged concentrically around the core leg.

Most distribution transformers for residential service are built as a shell form, where the secondary winding is split into two sections with the primary winding in between.

This so-called LO-HI-LO configuration results in lower impedance than if the secondary winding is contiguous.

The LO-HI configuration is used where the higher impedance is desired and especially on higher – kVA ratings where higher impedances are mandated by standards to limit short-circuit current. Core-form transformers are always built LO-HI because the two coils must always carry the same currents.

In three-phase transformers, windings may be installed in cores with 3, 4 or 5 legs. 



Manuel Bolotinha

Manuel Bolotinha, MSc, has a Licentiate Degree in Electrical Engineering – Energy and Power Systems (1974), by Instituto Superior Técnico/University of Lisbon (IST/UL), where he was also Assistant Professor, and a Master Degree in Electrical and Computers Engineering (April 2017) by Faculty of Sciences and Technology/Nova University of Lisbon (FCT-UNL).