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Power Distribution Transformers
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Power Distribution Transformers:

Core is built up from mitred low loss cold rolled, grain oriented steel laminations. Individual stampings are coated on both sides with an insulation material which is both oil and temperature resistant.

The Core limbs are securely bound with heavy duty fibrous glass tapes and the yokes are firmly clamped between steel channels. All cores have mitred joints. Top & bottom clamps are secured to each other by means of tie rods which serve the dual purpose of securing the windings in place and of transferring the load from the bottom to the top clamps when the core and windings are lifted. This prevents tensile stresses being set up in the core legs which could adversely affect the iron losses.

Transformer windings are designed to meet three fundamental requirements such as mechanical, thermal and electrical. They are cylindrical in shape and are assembled concentrically.

The low tension windings are normally of the helical type single or multilayer, using flat copper strips, depending upon the size and current rating of the transformer, for higher rating transformers, low tension windings can be continuous disc type. The high tension windings consist of paper covered round conductors. For higher KVA ratings continuous disc windings are used, for lower ratings cross-over type windings are used.

The windings are rigidly supported by pre-compressed press board, dovetailed strips and spacers. Interlayer cooling ducts are provided to ensure that the temperature gradient between windings and oil are maintained and hence the hot spot temperature is minimized and a, high life expectancy is achieved.

Insulation between layers and turns is based upon the impulse test level of the voltage class of the winding as specified in IS 2026. Completed Core coil assemblies are dried and pressed to desired dimensions prior to impregnation with transformer oil under vacuum.

Tapping and phase leads on H.V. side are run in multi paper covered flexible conductors in Bakelite tubes, the phase leads being separated from the tapping leads by adequate barriers. These tubes are rigidly braced to ensure that clearances between leads and tank sides are maintained when the transformer is fixed in position. Low voltage leads, being of larger cross section have sufficiently strong supports provided wherever necessary.


The principal component of insulation used in a transformer is Pressboard sheet, usually machines into strip, circular and spacer forms. All insulation items are pre-shrunk by heating and compressing. The final heating and compressing at assembly stage brings the insulation material to its finished thicknesses as desired in design. Apart from this, for medium and higher ratings of transformers, pre-compressed board, permali rings etc. are used for better mechanical strength and higher insulation capacity. The coolant oil used complies with IS-335/IEC-296, and every consignment received at works is tested before permitting it for entry into storage tanks.

In order to prevent deformation when subjected to short circuit forces, solid block end insulation, backed by substantial supporting frames, is utilized. The axial end thrust under fault conditions, is minimized by ensuring that design dimensions are closely adhered for both low voltage and high voltage windings during manufacture. Transformers designed and constructed in this way are capable of withstanding the effects of short circuit testing.

For the range of transformers covered in this catalogue, off-circuit tappings are provided on the high tension windings at ± 2.5% and ± 5% on high voltage for variation of higher voltages. Other tapping ranges can be supplied on request. Tap selection is effected by means of an off circuit tapping switch, operated by an external handle. Tapping leads are brought away from the coils by means of paper insulated conductors. They are designed in such a manner that the ampere turn balance is maintained throughout the tapping range. On special demand from customers, tap changing with on-load tap changing switch manufactured.

All bushings are of high quality porcelain or cast Epoxy complying to ISS 2099 / IEC-137.

All tanks are made from low carbon mild steel plates which are electrically welded. The design of the tank is usually of rectangular shape, the thickness of plates used in sides, top cover and base conform to customer specifications. The core and coils are fixed to the tank in such a way that damage will not result during transport. All tanks are oil pressure tested to a pressure of 0.4 atmospheres plus the weight of oil contained. The interior of the tank is painted with oil & heat resistant paint. Zinc chromate red oxide primer is applied to the exterior, before applying two coats of weather proof outdoor type synthetic enamel paint of shade IS: 631. Transformer tank and all other fabricated items are thoroughly cleaned before painting.

The following tests will be carried out on all Transformers. The numbers shown do not necessarily indicate the sequence in which the tests will be carried out. All tests will be in accordance with the. Latest revision of IS 2026.

               1. Measurement of resistances of all windings of each unit.
               2. Ratio tests at all tap positions.
               3. Polarity and Vector group tests at normal tap.
               4. No load loss at rated voltage, rated frequency at normal tap.
               5. Excitation current measurements at rated voltage, rated frequency at normal tap.
               6. Impedance and load loss measurement at rated current, at normal tap of each unit.
               7. Separate Source Voltage withstand test.
               8. Induced over voltage withstand test.
               9. Measurement of insulation resistance.
               10. Dielectric capacity of the transformer oil.

The design and manufacture of our transformers are subject to constant improvement and review, and the particulars given in this Catalogue may vary in details with the equipment supplied.

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