As we have seen in the construction of the transformer, it has two main parts; Transformer core and transformer winding. Transformer winding is divided into two parts; primary winding and secondary winding.
The function of the transformer is to convert the voltage level of electrical power. According to that, one winding is connected at the high voltage side and the other winding is connected at a low voltage side. Therefore, it is also classified as high voltage (HV) winding and low voltage (LV) winding.
Requirements of Transformer Winding
Transformer winding should follow the below requirements,
- The initial cost of the transformer winding must be as low as possible. And also, the maintenance and service cost of the winding is less.
- It must be fulfilled the standard requirement for the heating condition. If the winding is not allowing the higher temperature, the life of the transformer is decreased drastically.
- Concerning the force appearing, the winding should be mechanically stable when a sudden short circuit occurs in the transformer.
- The winding should have the necessary electrical strength to overcome overvoltage.
According to the insulation point of view, the placement of the HV and LV winding is very important. If the HV winding is placed near the transformer core, two layers of insulation are necessary to insulate from both; core and the LV winding.
If the HV winding is placed outer side and the LV winding is placed near to the core, only one layer of HV insulation is required. And this insulation layer is placed between HV and LV winding.
In practically, all transformers coils are wound on a form, dipped in insulating varnish, and then assembled with the core.
It is the easiest way to build the core with the legs of the rectangular cross-section. Therefore, it is the most economical type of transformer when the coils are formed in a rectangular shape.
In a small transformer, rectangular concentric coils are used. But in the case of the large unit, under short circuit conditions, the high repulsion forces generated between the primary and secondary coils. This tends to “round out” the flat sides of the outer coil. If this happens, it resulting in damage to the coil insulation. And in the worst case, it makes the transformer unserviceable.
To avoid this situation in a large unit of a transformer, Cylindrical concentric coils are used. According to their arrangement of the HV and LV winding, it can be either a concentric or a sandwich.
Cylindrical or Concentric winding is employed in the core type of transformer. Sandwich winding is employed in shell type of transformer.
On account of the easier insulation facilities, The LV winding is placed nearer to the core in the case of concentric winding and on the outside position in the case of Sandwich winding. The insulation space between LV and HV winding also serves to facilitate cooling.
There are four types of concentric winding.
- Cylindrical winding
- Helical winding
- Cross-over winding
- Continuous disc winding
These windings are layered type and use either rectangular or round conductors. If the cross-section of a turn does not exceed 8 to 10 mm2 the cylindrical winding is made multi-layer of the round conductor.
For a larger turn cross-section, the winding is made of a rectangular conductor, usually double layer.
The winding is made up of turns with helically wound with the turns close to each other. Therefore, the height of the winding is equal to the height of the layer.
Such windings are employed for LV coils of medium and a high capacity transformer. this type of winding is used where the number of turns is small but the current is high. For example, low voltage windings of medium and high capacity transformers.
It requires to use of a conductor consisting of a row of parallel rectangular conductors arranged in one radial direction of the winding flatwise and close to each other.
For a more uniform distribution of current between the parallel conductors, they are transposed. So, each conductor may take each position.
For currents not exceeding 20A, cross over winding are suitable and are very largely employed. It is used for HV winding of low rating transformers where the number of turns may be large but conductors have a small circular section with double cotton covering or paper covering.
Depending upon the voltage rating, the entire winding is divided into several coils. Each coil is wound on a former (usually U piece) with several layers of several turns per layer.
The one end of the coil from inside and the other end of the coil from outside, are joined to other similar coils in series, spaced with blocks of insulating material to allow free circulation of oil.
Continuous Disc Winding
The disc coils, as their name suggests, consist of some flat coils or discs connected in series or parallel.
The coils are formed with rectangular strips wound spirally from the centre outwards in the radial direction. The conductor used is in such lengths are sufficient for complete winding or section of winding between tapings.
The conductor can be a single strip or number of strips in parallel, wound on the flat side. Continuous disc windings are reliable and strong. Hence, it is widely employed as LV and HV windings in large rating transformers.
Sandwich windings are more common with shell type core construction. They permit easy control over the short circuit impedance of the transformer and leakage reactance.
Leakage reactance is reduced by subdividing the LV and HV winding into a large number of sections or coils and arranging alternatively the HV and LV section near the yoke.
By bringing HV and LV coils close on the same magnetic axis, the leakage is reduced and the mutual flux is increased. By increasing the numbers of sandwiched coils, the reactance can be substantially reduced.
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