Types of Transformer Windings: Cylindrical, Helical, Crossover & Disc Winding

Core and Windings of Three Phase Core Type Transformer

There are different types of windings used for different kinds of applications and arrangements. Windings are the conductors wrapped as helical, disc, cylindrical or crossover coils. Those turns produce ampere-turns. The core carries flux to the other winding so the two sides have different voltages. Mainly there are two types of transformer:

1. Core type transformer
2. Shell type transformer

In a core type machine the primary and secondary windings sit on the outer limbs. In a shell type machine the windings sit on the inner (central) limb.

Core type transformers usually use concentric windings. The low voltage winding sits nearer the core. Windings can be interlaced to reduce leakage reactance. Choice of winding depends on current rating, short-circuit force, temperature rise, impedance, surge voltage and transport limits.

Types of Winding used for Core Type Transformer

Cylindrical Windings

These windings are layered type and uses a rectangular or round conductor shown in Fig.(a) and (b). The conductors are wound on flat sides shown in Fig.(c) and wound on the rib side in Fig.(d).

Uses of Cylindrical Windings

Cylindrical windings are often the low voltage winding. Textbook examples use them up to about 6.6 kV, 600-750 kVA and 10 to 600 A. Those figures are typical shop ranges, not IEC limits.

Cylindrical windings are often used in multi-layer form. Rectangular conductors suit two-layer coils because the lead-outs are easy to bring out. Oil ducts separate the layers so oil can circulate.

In multi-layered cylindrical windings, circular conductors are wound on vertical strips to open oil ducts. Textbook examples go up to about 33 kV, 800 kVA and 80 A. Bare conductor diameter is often kept to about 4 mm in that class.

Helical Windings

We use helical windings on low voltage, high capacity transformers, where the current is higher and the turn count is lower. The output of the transformer in textbook examples is about 160 – 1000 kVA from 0.23-15 kV. Strip area is often kept at least 75-100 mm square for strength. The number of strips in parallel in one conductor is often limited to about 16.

There are three types:

• Single Helical Winding
• Double Helical Winding
• Disc-Helical Winding

Single Helical Windings consist of winding in an axial direction along a screw line with an inclination. There is only one layer of turns in each winding. The advantage of Double Helical Winding is that it reduces eddy current loss in conductors. This is on account of the reduced number of parallel conductors situated in the radial direction.

In Disc-Helical Windings, parallel strips are placed side by side in a radial direction to cover the entire radial depth of the winding.

Multi-layer Helical Winding

We use it commonly for high voltage ratings for 110 kV and above. These types of winding consist of several cylindrical layers concentrically wound and connected in series.

We make the outer layers shorter than the inner layers to distribute capacitance uniformly. These windings primarily improve the surge behavior of transformers.

Crossover Winding

These windings are used in high voltage windings of small transformers. The conductors are paper-covered round wires or strips. The windings are divided into several coils to reduce voltage between adjacent layers. In one common shop practice the coils are axially separated by 0.5 to 1 mm, with voltage between adjacent coils kept within about 800 to 1000 V.

The inside end of a coil is connected to the output side end of the adjacent one as shown in the above figure. The actual axial length of each coil is about 50 mm while the spacing between two coils is about 6 mm to accommodate blocks of insulating material.

The width of the coil is 25 to 50 mm. The crossover winding has a higher strength than cylindrical winding under normal conditions. However, the crossover has lower impulse strength than the cylindrical one. This type also has higher labor costs.

Disc and Continuous Disc Winding

Primarily used for a high capacity transformer. The winding consists of a number of flat coils or discs in series or parallel. The coils are formed with rectangular strips wound spirally from the center outwards in the radial direction as shown in the figure below.

The conductors can be a single strip or multiple strips in a parallel wound on the flat side. This makes robust construction for this type of windings. Discs are separated from each other with press-board sectors attached to vertical stripes.

The vertical and horizontal spacers provide radial and axial ducts for oil to reach each turn. Textbook conductor area is about 4 to 50 mm square and current about 12 – 600 A.

The minimum width of the oil duct is often given as 6 mm for 35 kV. Disc and continuous windings give good axial mechanical strength and are relatively cheap to wind in quantity.

Windings for Shell Type Transformer

Sandwich Type Winding

Allow easy control over the reactance the nearer two coils are together on
the same magnetic axis, the greater is the proportion of mutual flux and the less is the leakage flux.

Leakage can be reduced by subdividing the low and high voltage sections. The end low voltage sections, known as half coils, contain half the turns of the normal low voltage sections.

In order to balance the magnetomotive forces of adjacent sections, each normal section whether high or low voltage carries the same number of ampere-turns. The higher the degree of subdivision, the smaller is the reactance.

Advantages of Shell Type Windings in Transformers

The advantages of shell-type windings include:

• High short-circuit withstand capability
• High mechanical strength
• High dielectric strength
• Close control of leakage magnetic flux
• Good cooling paths
• Flexible design
• Compact size
• A proven design when the factory is set up for shell form