Transformer & Distribution - Online Article

Transformer is one of the most vital electrical machinery. The development of the present day power system is very much attributable to the large number and types of transformer that are in operation in the system, such as, generator transformers, step-up transformers, step-down transformers, interlinking transformers, power transformers & distribution transformers etc. It is important that utmost care is taken in the design, manufacture, testing, installation, and maintenance of transformers.

About Transformers

The transformer helps in converting low voltage into high voltage or visa-versa and accordingly the transformer is termed step - up or step-down. The transformer works on the principle of electro-magnetic induction. Such phenomena can take place in a static device, only, if the magnetic flux is continually varying. It is therefore clear that static transformers can only be used with alternating currents only. When an alternating EMF is applied to the primary winding of a transformer with the secondary winding open circuited, a small current flows in the primary winding which serves to magnetize the core and to feed the iron losses of the transformer.

As primary and secondary windings are wound on the same core, the magnetizing flux is the same for both the windings. The magnetizing flux corresponds to the magnetizing current in the primary and the number of turns of the primary winding. Primary and secondary windings are wound on the same core. Hence, the induced voltage per turn is the same for both primary and secondary winding. Also the absolute value of induced voltage in the primary and secondary windings is proportional to the number of turns in the respective windings.

Transformer Winding

Transformer winding is the first step in the manufacturing process. The transformer winding shop contains numerous wooden formers on which the winding insulators are placed. The winding consists of the copper wires, flat rectangular and paper insulator. They are placed on formers closed to each other bounded by insulator laps. The dimensions of the former are made to exactly suit to the dimensions of the core.

Depending on the power capacity and other requirements the coil is made in different sizes.

The wooden formers are connected to motors, which use to run at desired speed and in required time the placement of winding is done.

Different types of Winding

  • Disc Winding: -
    • Plane disc winding (11 KVA-132 KVA)
    • Interleaved disc winding (220 KVA)
      • Full Interleaved Winding
      • Partial Interleaved Winding
    • Intersheild Winding
  • Spiral Winding: -
    • Simple Spiral Winding
    • Multistart Interwound Winding
    • Crossover Winding
  • Hellical Winding: -
    • Simple Hellical Winding
    • Interwound Hellical Winding (220 KVA)     
  • Standard Winding: - In this the winding is done continuously from one end to another end without any break.
  • Non-Standard Winding: - This type of winding starts from one end and ends in the center. Then the formers are rotated axially through 180 degree, and again winding is done from center to other end.

Equipment used in Winding Shop

  • Winding machine with turn counter.
  • Reel/drum stand.
  • Conductor tensioning device.
  • Winding formers.
  • Non-metallic mallets.

Core Building Shop

The process of fabricating the core is carried on simultaneously as the winding and the insulation jobs are being manufactured.

The material used for core manufacture is C.R.G.O steel i.e. Cold Rolled Grain Oriented Steel. These are laminated to reduce the eddy current losses. These steels have electrical grains, which are disturbed on rough handling. These steels come in different grades such as m1y, m3 etc. M1y is commonly used in GEC ALSTOM. They are (0.25-0.30) m.m thick. These steels come in rolled form and there is a separate shop, which used in C.N.C (Computer Numerically Controlled) machine to cut this in required form.

These steels are arranged in the form of stepped core, the numbers of steps are designed by design specification. Cores are built in horizontal portion and they are placed in vertical portion with the help of cranes.

Assembly Shop

The wounded coil and core, after being manufactured separately are brought to assembly section. In this section the wounded coil are placed on the core and respective connection are made. They are connected to O.L.T.C (On Load Tap Changer). After that transformer is sent to TANKING SHOP.

Tanking Shop

In this section the job is placed in auto clave for drying and then they are taken out for the pressing and cooling. It is then retanked and then placed in autoclave for final dry out and oil filling. Once the body is filled with oil it is sent for second servicing and oil filtering. The final fitting is done and the complete unit is sent to TESTING SECTION.

Sequential list of activities in theTanking Shop

  • Trail tanking
  • Tank preparation
  • Servicing
  • Coil clamping
  • Lid fitting
  • Internally mounted O.L.T.C fitting
  • Externally mounted O.L.T.C fitting
  • 245/145 KV bush fitting
  • 72.5/52/47/36/17.5 KV bush fitting
  • Turret fitting
  • Oil box fitting
  • Cable box fitting
  • Earthing arrangement fitting
  • Pressure fitting
  • Conservator and pipe work opining
  • H.T lead transport support fitting
  • R and D plate punching
  • Gasket cutting
  • Non-return device assembly
  • Gave driving
  • Oil work and filter plant operation
  • Thermo Pac and vacuum plant operation

Fabrication Shop

In this shop, two important parts of the transformer are manufactured.

Radiator

These act as a cooling device in the transformer. By increasing the surface area of the whole transformer body, they make heat dissipation faster and thereby maintain the transformer optimum temperature.

Manufacture

The whole process of manufacturing of radiators is carried out in full automatic way. The whole process is broken down in a sequence -

  1. Arrival of steel switch.
  2. Cutting of sheets in required sizes.
  3. Welding into required sheet.
  4. Attachment of pipes of appropriate sizes.
  5. Final testing.

Main Transformer Body

In the manufacture of the main body automation is not involved. After the arrival of sheets they are being cut into required sizes and shapes. After this the sheets are bended inside and outside the main transformer body and then varnished the is then tested at a pressure of about 12 pounds per sq. inch and fabrication is assumed to be complete if there is no pressure drop in an hour.

Testing of Transformer

Testing is an important activity in the manufacture of any equipment while certain preliminary test are carried out at different stages of manufacture provides an effective tool which assures quality and confirmation to design calculation the final test on fully assembled equipment guarantee the suitability of the equipment for satisfactory performance in service. Thus the tests performed on a transformer may be classified as shown in Flow-chart no. 2. The details of the tests have been discussed here under:

Prelimniary Tests

These are carried out at different stages before the transformer is put in the tank. They help in detecting faults at an early stage.

  • Core insulation test.
  • Core loss test.
  • Ratio test.

Final Test

These test are done on a completely assembled transformer. They may be further classified under following:

  • Routine test.
  • Measurement of winding resistance.
  • Voltage ratio test.
  • Measurement of impedance voltage.
  • No load loss and current test.
  • Dielectric test.

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