Transformer Windings, Terminals, Tapping and Bushings

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Transformer Windings

The windings of the wires or coils on the leg of the core of the transformers are called transformers winding. The fundamental requirements for transformer windings are:

  • Cost of initialisation. The availability of the copper and the efficiency of the transformer should be economically affordable.
  • The heating conditions should me checked and considered for the durability and reliability of the function of the transformer.
  • The winding should be stable. It should not be affected easily by mechanical forces or short circuits.
  • The windings should not be easily affected by voltage fluctuations.

Different types of windings:

Concentric windings: Core type transformers use this type of winding.The two limbs of the core are wound with primary and secondary coils. Usually the low voltage winding is placed near the core, while the heavy voltage winding is places on the outside. Three subdivision of this winding are:

  • Cross-over windings:

It is suitable for small transformers with heavy voltage winding. It can be used for currents up to 20A. The conductors are lightly insulated, either by cotton or paper. The length of the complete winding is calculated by the number of coils connected in series.

  • Helical winding:

These winding are compatible for large transformers with low voltage usages. The rectangular strips of conductor are wound in the shape of a helix. With addition of extra insulation, this winding can also be used for high voltage services.

  • Continuous disc winding:

Flat types of conductors are used. Either single strip or multiple strips placed parallel can be used to wound spirally from inside to outwards. This is a robust type of winding.

Sandwich coils: Sandwich coils windings are used in shell type transformers. It can be used for both low voltage and high voltage supplies. The windings can be divided; hence the leakage can be controlled for this type of winding.

Terminals and Leads

The shape and size of leads are to be considered for transformers that are used in high voltage supplies.The dielectric stress and corona should be avoided. These are formed due to the corners and bends. The connections of the windings are made by insulated copper rods.

  • For air cooled transformers, the windings are directly connected to the busbars.
  • And for oil cooled transformers, the windings are connected to insulated bushings.


The change in the transformation ratio can change the network voltage. The process of changing the transformation ratio is called tapping. The winding is changed by changing the number of turns. The change in turns can be done in two ways:

  • Off load tap change; the tapping is done when the transformer is out of the circuit.
  • On load tap change; the tapping can be done when the transformer is connected to the circuit of have a load.

It is effective when tapping is done on high voltage windings. The variation can be done by large change in the number of turns. Tough it is to obtain a low voltage variation as; each turn can vary the voltage by a large amount.

The two ways by which tapping can be provided on a transformer are:

  • Phase ends.
  • Neutral points or in the middle of the windings.

When tapping is done on phase ends, the number of bushing insulators can be reduced. It helps in case of limited cover space. But, when tapping is done mid winding, the insulation is small. Neutral tapping is done in high voltage transformers.


The process of bringing out the terminals of the winding and connecting to the external circuits by providing insulation is known as Bushing. For low voltage transformers, the bushing is provided by porcelain insulation to the conductor at the connection point. For high voltages, large bushing is considered to be provided. It is achieved by large porcelain insulators for voltages as high as 33hV.


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