TRANSFORMER CONSTRUCTION

Transformer construction consists of the followings: 

   - (a) The Tank or  Enclosure and associated   

           accessories or fittings;

   - (b) The Core and Winding  assembly;

   - (c) The Insulating and Cooling medium.

•         The Insulating and Cooling Medium  

        - a.  Air; 

        - b. Gas (Nitrogen or SF₆ gas);

        - c. Mineral (Transformer) Insulating Oil;

        - d. Synthetic Nonflammable Insulating    

              Liquid (Askarel);

        - e. Water (External Cooling)

. Type of Circulation 

     - a. N = Natural   : Self-Cooled, Natural Convection

     - b. F = Forced   :  Forced Circulation : Pumps

                                 :  Forced Cooling : Fans

•         Standard Type Designations 

   1. Air Natural Cooling       - - -   AN

   2. Air Forced Cooling       - - -   AF

   3. Gas Natural Cooling    - - -   GNS

   4. Gas Forced Cooling    - - -   GPS

  (The Gas could either be SF₆ gas or Nitrogen)

   5. Oil-immersed natural cooling  - - -  ONAN

   6. Oil-immersed water cooling    - - -  ONWN

   7. Oil-immersed forced-air cooling - - ONAF

   8. Oil-immersed forced-oil water cooling - - OFWN

   9. Oil-immersed forced-oil forced-air cooling - -OFAF

 10. Oil-immersed forced-oil natural cooling - - - OFAN

•         11.  Nonflammable liquid-immersed natural    

          cooling - - LNAN

   12.  Nonflammable liquid-immersed forced-air

          cooling - - LNAF

   13.  Nonflammable liquid-immersed forced-

          liquid water cooling - - LFWN

   14.  Nonflammable liquid-immersed forced-

          liquid forced-air cooling - - LFAF

   15.  Nonflammable liquid-immersed forced-

          liquid natural cooling - - LFAN

•         Transformer Circuits : Types

•         1. The Magnetic Circuits:

   - a. The Core Type: has one magnetic circuit  

   linking the ‘high’ and ‘low’ voltage windings.

   This circuit forms a ‘core’ through the coils.

   Cheaper than the Shell type and is used extensively in Power Distribution transformers.

   - b. The Shell Type: Has two or more magnetic circuits in the form of a ‘shell’ around the coils.

   Naturally requires more iron for the ‘extra-cores’ and costlier to construct than the Core-type. Used only in special applications and in small sizes.

   - c. The Torroidal Type: which is a ‘core-less’ circuit, with the ‘high’ and ‘low’ laying side by side and on top of one another. Very cheap and Compact in size, limited in usage to Accessory transformers.

•         2. The Electrical Circuits : 

•         There are basically two types of windings from a geometrical aspect, namely:

    - a. Layer (Barrel) Type  and

    - b. Disc Type.

    However manufacturers have enlarged these categories to make for easier identification as follows:

    - a. Layer (Barrel) type:

          - -(i)   Single-layered;

          - -(ii)   Multi-layered;

          - -(iii)  Helical winding;

          - -(iv)  Multi-section winding.

•         In both the single-layered and multi-layered design, each layer extends the full length of the winding and there are no spaces between the individual turns.

•         The Helical winding is similar to a layered-type winding with the addition of the spacers between turns. When higher currents are required, a double helix is used, consisting of two separate sets of conductors separated by radial spacers, but wound in parallel on the same cylinder.

    The Multi-section is a combination of the layer and helical design.             

    The winding is multi-layered with each layer divided into a number of sections. Secondly, each section is separated by spacers.

    By winding in sections, the voltage between layers is reduced so that the winding is suitable for higher voltages than the multi-layer winding.

•         - b. The Disc Type: 

          - -(i)   Disc and Continuous Disc Winding;

          - -(ii)   Interleaved Disc winding;

          - - (iii) Sheet (Strip) Windings;

          - -(iv) Pancake Winding.

•         The Disc winding is made up of several discs of rectangular conductors, i.e. each disc is a number of turns radially. A pair of discs can be wound on a special form in such a manner that no centre connection (crossover) is required. The full winding is then made by brazing together the outside connections between pairs of discs. To avoid brazing, a continuous disc winding is formed.

•         The Interleaved disc winding is not continuously wound. It is made up of pairs of discs which are joined to the next pair by a brazed connection.

•         Sheet (Strip) windings are generally made to have more than one section, each section being composed of several layers.

•         The ‘Pancake’ coil is used in large shell-type high voltage transformers. Pancake coils are usually wound with square or rectangular conductors, one or more conductors in parallel, depending upon the current. This winding exposes a large amount of conductor surface to the cooling medium.