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 in various forms like helical, disc, cylindrical, crossover which generates MMF that is carried by the core to other windings for having the different levels of voltages. Mainly there are two types of transformer:
In core type, we wrap the primary, and secondary windings on the outside limbs, and in shell type, we place the primary and secondary windings on the inner limbs.
We use concentric type windings in core type transformer. We place low voltage winding near to the core. However, to reduce leakage reactance, windings can be interlaced. Winding for core type depends on many factors like current rating, short circuit withstands capacity, the limit of temperature rise, impedance, surge voltage, transport facilities, etc.
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 low voltage windings used up to 6.6 kV for kVA up to 600-750, and current rating between 10 to 600 A.
We often use cylindrical windings in its multi-layer forms. We use rectangular conductors in two-layered type because it is easy to secure the lead-out ends. Oil ducts separate the layers of the windings this arrangement facilitates the cooling through oil circulation in the winding.
In multi-layered cylindrical windings, we use circular conductors, wound on vertical strips to improve cooling conditions. The arrangement creates oil ducts to facilitate better cooling. We use this types of winding for high voltage ratings up to 33 kV, 800 kVA and current ratings up to 80 A. The maximum diameter we use for a bare conductor is 4 mm.
Helical Windings
We use helical windings low voltage, high capacity transformers, where the current is higher, at the same time windings turns are lesser. The output of the transformer varies from 160 – 1000 kVA from 0.23-15 kV. To secure adequate mechanical strength the cross-sectional area of the strip not made less than 75-100 mm square. The maximum number of strips used in parallel to make up a conductor is 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, the parallel-connected strips are placed side by side in a radial direction to occupy the total radial depth of 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
We use these windings for high voltage windings of small transformers. The conductors are paper covered round wires or strips. The windings are divided into a number of coils in order to reduce the voltage between adjacent layers. These coils are axially separated by a distance of 0.5 to 1 mm. The voltages between adjacent coils should not be more than 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 lover 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 the free circulation of oil which comes in contact with every turn. The area of the conductor varies from 4 to 50 mm square and limits for current are 12 – 600 A.
The minimum width of the oil duct is 6 mm for 35 kV. The advantage of the disc and continuous windings is their greater mechanical axial strength and cheapness.
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 voltages sections. The end low voltages sections contain half the turns of the normal low voltage sections called half coils.
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
- Excellent control of leakage magnetic flux
- Efficient cooling capability
- Flexible design
- Compact size
- Highly Reliable Design
