Transformer Windings

Prepared By: Bist Ritu M.

Topics

1. Introduction.2. Types of transformer windings.3. Output equation- emf per turn of

winding4. Selection of type of winding.5. Position of windings relative to core.6. Resistance & leakage reactance of

winding.

Introduction

Main purpose of a winding is to convert the energy on the primary side to a different voltage level on the secondary side.

They are broadly classified as circular & rectangular.

Generally circular windings are used for all the types of transformers invariably.

Rectangular windings involve wastage of space & under s/c conditions it gets deformed. Hence is used for very small capacity transformers.

Further classifications are explained here after.

Type & arrangement of windings depends on some

factors:

1. Current rating,2. Short circuit strength,3. Temperature rise,4. Impedance,5. Surge voltage, &6. Transport facilities.

Types Of Windings

1. Cylindrical Windings2. Helical Windings3. Cross-over Windings4. Disc & Continuous Disc Windings5. Aluminium Foil Windings6. Sandwich Windings

Cylindrical windings

Cylindrical windings are layered type and use either rectangular or round conductors.

Conductors are wound on the flat side with their longer sides parallel to the core axis.

The layered winding may have conductors wound in one, two or more layers and is, therefore, accordingly called the one, two or multi-layered winding.

Cylindrical windings employing rectangular conductors are used mainly as low voltage windings upto 6.6 kv for kva ratings upto 600-750.

• Cylindrical windings using round conductors are multi-layered.

• They are wound on a solid paper bakelite cylinder.

• In order to improve cooling conditions of the inner layer, these are often wound on vertical strips forming an oil duct, between winding & insulating cylinder.

• Mainly used for hv windings with voltages 6.6, 11 & 33 kv for ratings upto 600-100 kva.

Helical Windings

Helical windings are mostly used for l.v. coils of medium & high capacity transformers with currents as high as say 2000 A.

Here axial spacing strips are provided for duct formation to improve oil circulation for better cooling.

The ducts are formed by spacers placed all the way round the periphery of the cylinder at regular intervals as shown in the previous figure.

Transposition of helical winding

For uniform division of current between the parallel conductors, they are transposed so that each conductor may take each possible position.

Cross-over windings

• Suitable for current rating not exceeding about 25 A.

• Mostly used for h.v. windings of low rating transformers where number of turns may be large but conductors are of small cross sections with double cotton covering or paper covering.

• The whole cross-over winding is divided into a number of coils depending upon the voltage rating.

• Each coil is wound over a former usually U pieces, with several number of layers & many number of turns per layer.

• The coil ends, one from inside and other from outside are joined to other similar coils in series.

• Various coils are separated by blocks of insulating material to allow free oil circulation.

Continuous Disc Windings

A Continuous disc winding consists of several discs & in each disc there are radially wound strip conductors so that there may be several layers in each disc.

Layers are wound spirally from inside outwards.

The discs are separated from each other by press board sectors.

The winding is named “Continuous Disc” as it is wound with a continuous conductor without a single joint.

These are reliable & strong and therefore are widely used both as l.v. & h.v. coils in large capacity transformers.

Aluminium foil Windings Aluminium in place of

copper can be used as a material for conductors in any of the windings.

Aluminium, when used as a single section has many disadvantages compared to copper.

Hence are employed in foil windings, because it can be rolled to thinner & more flexible sheets than copper.

Sandwich Windings• Most commonly used for

shell type transformer.• H.v & l.v. windings are

subdivided into large number of sections. Each h.v. section lies between two l.v. sections.

• The end l.v. sections contain half the number of turns of a normal l.v. section.

• Larger the number of subdivisions, better is the coupling between the l.v. & h.v. windings and least will be the leakage reactance thereby improving the voltage regulation.

Windings used in core type transformer

Continuously transposed conductor windings

Selection of windings

Position of winding related to the core

Resistance of windings

Leakage reactance of the winding

Method Of Installing Transformer winding coils

A particular method of installing transformer winding coils is by directly leading out winding outlets from a winding frame and then the transformer structure is formed using such method.

Thank you