This specification covers design, engineering, manufacture, assembly, stage testing, inspection and testing before supply and delivery at site of oil immersed, naturally cooled 3 phase 11kV/433V and 33kV/433V distribution transformer for outdoor use.

These are some of the details of the design and construction of equipment. However the equipment conform in all respects to high standards of engineering, design and workmanship and are capable of performing in continuous commercial operation, in a manner acceptable to the purchaser.

The transformer and accessories are designed to facilitate operation, inspection, maintenance and repairs. The design incorporates every precaution and provision for the safety of equipment as well as staff engaged in operation and maintenance of equipment.

All outdoor apparatus, including bushing insulators with their mountings, are designed so as to avoid any accumulation of water.


The standard distribution transformer ratings are 3-phase, 25kVA, 50kVA, 100kVA, 200kVA, 315kVA, 500kVA and 1000kVA in both 33kV/0.433kV and 11 kV/0.433kV line voltages.


The materials conform in all respects to the relevant International Standards, with latest amendments thereof unless otherwise specified herein.


The Distribution Transformers supplied against this Specification are  suitable for satisfactory continuous operation under the following climatic conditions as per IS 2026 (Part - I).

  1. Maximum ambient air temperature: 50 degrees Celsius
  2. Minimum ambient air temperature: -5 degrees Celsius
  3. Maximum average daily ambient air temperature: 40 degrees Celsius
  4. Maximum yearly weighted average ambient temperature: 32 degrees Celsius
  5. Maximum altitude above mean sea level (Metres): To be specified by the user


  1. The climatic conditions specified above are indicative and are changed by the user as per requirements.
  2. The equipment is generally for use in moderately hot and humid tropical climate, conducive to rust and fungus growth unless otherwise specified.


The transformers are suitable for outdoor installation with three phase, 50 Hz, 11kV and 33kV system in which the neutral is effectively earthed and they are suitable for service with fluctuations in supply voltage up to plus 12.5% to minus 12.5%.

The transformers conform to the following specific parameters:

Transformer Specification

Audible sound levels (decibels) at rated voltage and frequency for liquid immersed distribution transformers are as per NEMA Standards of Audible sound levels.


CORE MATERIAL – This shall be CRGO

  1. The core is stack / wound type of high grade cold rolled grain oriented annealed steel lamination having low loss and good grain properties, coated with hot oil proof insulation, bolted together and to the frames firmly to prevent vibration or noise. The core is stress relieved by annealing under inert atmosphere if required. The complete design of core ensures permanency of the core loss with continuous working of the transformers. The value of the maximum flux density allowed in the design and grade of lamination used is stated in the offer.
  2. No-load current don’t exceed 3% of full load current and are measured by energizing the transformer at 433 volts, 50 Hz on the secondary. Increase of voltage of 433 volts by 12.5% doesn’t increase the no-load current by 6% (maximum) of full load current.


a. Material:

  • LV winding is such that neutral formation is at the top.
  • The winding construction of single HV coil wound over LV coil is preferable.
  • Inter layer insulation is Nomex /Epoxy dotted Kraft Paper.
  • Proper bonding of inter layer insulation with the conductor is ensured. Test for bonding strength is conducted.
  • Dimensions of winding coils are very critical. Dimensional tolerances for winding coils are within limits as specified in Guaranteed Technical Particulars.
  • Current density for HV and LV winding is not be more than 2.8Ampere per sq mm for copper and 1.6Ampere per sq mm forAluminium Conductor.
  • The core/coil assembly is securely held in position to avoid any movement under short circuit conditions.
  • Joints in the winding are avoided. However, if jointing is necessary the joints are properly brazed and the resistance of the joints is less than that of parent conductor. In case of foil windings, welding of leads to foil is done within the winding.

2. TAPS:

  • OFF- circuit tap changing switch five positions (i.e. -5%,-2.5%, Normal, +2.5%, +5%) with locking arrangement + 5% to -5% in step of 2.5% Off load Tap changer. 43
  • Tap changing is carried out by means of an externally operated self-position switch and when the transformer is in de-energized condition. Switch position No.1 corresponds to the maximum plus tapping. Each tap change results in variation of 2.5% in voltage. Provision are made for locking the taping switch handle in position. Suitable aluminium anodised plate are fixed for tap changing switch to know the position number of tap.

3. OIL :

  • The insulating oil complies with the requirements of IS 335 or BS 148. The specific resistance of the oil is not be less than 2.5 X10 12 ohm-cm at 27 0C when tested as per IS 6103.
  • Oil is filtered and tested for break down voltage (BDV) and moisture content before filling.
  • The oil is filled under vacuum where necessary.
  • The design and all materials and processes used in the manufacture of the transformer, is such as to reduce to a minimum the risk of the development of acidity in the oil.




We guarantee individually the no-load loss and load loss without any positive tolerance. We also guarantee the total losses at 50% and 100% load condition (at rated voltage and frequency and at 75 0C).


There are tolerances however displayed on the label for the percentage loading values.


  • The percentage impedance at 75 º C. is also always indicated in the particulars.
  • Temperature rise: The temperature rise over ambient is within the stated values.
  • Top oil temperature rise is measured by thermometer.
  • Winding temperature rise is measured by resistance method.
  • The transformer is capable of giving continuous rated output without exceeding the specified temperature rise.


  1. Electrical grade insulation epoxy dotted Kraft Paper/Nomex and pressboard of standard make or any other superior material is used.
  2. All spacers, axial wedges / runners used in windings are made of pre-compressed Pressboard-solid, conforming to type B 3.1 of IEC 641-3-2. In case of cross-over coil winding of HV all spacers are properly sheared and dovetail punched to ensure proper locking. All axial wedges / runners are properly milled to dovetail shape so that they pass through the designed spacers freely. Insulation shearing, cutting, milling and punching operations are carried out in such a way, that there aren’t  any burr and dimensional variations.


  1. The internal clearance of tank is such, that it shall facilitate easy lifting of core with coils from the tank without dismantling LV bushings.
  2. All joints of tank and fittings are oil tight and no bulging occurs during service.
  3. Inside of tank is painted with varnish/hot oil resistant paint.
  4. The top cover of the tank is slightly sloping to drain rain water.
  5. The tank plate and the lifting lugs are of such strength that the complete transformer filled with oil may be lifted by means of lifting shackle.
  6. All welding operations are as per the relevant ASME standards
  7. The transformer tank is of robust construction rectangular/octagonal/round/ elliptical in shape and is built up of electrically tested welded mild steel plates of thickness of 6 mm for the bottom and top and not less than 4.0 mm sheets for the sides for distribution transformers.
  8. The corners shall be fully welded at the corners from inside and outside of the tank to withstand a pressure of 0.8 kg/cm2 for 30 minutes.


  1. We also offer corrugated tanks for transformers of all ratings.
  2. The transformer tanks are of robust construction corrugated in shape and built of tested sheets.
  3.  Corrugation panel is used for cooling and therefore transformer is capable of giving continuous rated output without exceeding the specified temperature rise.
  4. Tanks with corrugations are tested for leakage test at a pressure of 0.25kg/ sq cm measured at the top of the tank.
  5. The transformers with corrugation should be provided with a pallet for transportation, the dimensions of which should be more than the length and width of the transformer tank with corrugations.


  1. The conservators are provided on transformer tank where need be.
  2. When a conservator is provided, oil gauge and the plain or dehydrating breathing device are fitted to the conservator which is also provided with a drain plug and a filling hole [32 mm (1¼”)] normal size thread with cover. In addition, the cover of the main tank is provided with an air release plug.
  3. The dehydrating agent is silica gel. The moisture absorption is indicated by a change in the colour of the silica gel crystals which should be easily visible from a distance. Volume of breather is suitable for 1 kg.
  4. The capacity of a conservator tank is designed keeping in view the total quantity of oil and its contraction and expansion due to temperature variations. The total volume of conservator is  such as to contain10% quantity of the oil. Normally 3% quantity the oil shall be contained in the conservator.
  5. The cover of main tank is provided with an air release plug to enable air trapped within to be released, unless the conservator is so located as to eliminate the possibility of air being trapped within the main tank.



  1. All paints, when applied in a normal full coat, are free from runs, sags, wrinkles, patchiness, brush marks or other defects.
  2. All primers are well marked into the surface, particularly in areas where painting is evident and the first priming coat is applied as soon as possible after cleaning. The paint is applied by airless spray according to manufacturer’s recommendations. However, where ever airless spray is not possible, conventional spray be used with prior approval of purchaser.


  1. After all machining, forming and welding has been completed, all steel work surfaces are  thoroughly cleaned of rust, scale, welding slag or spatter and other contamination prior to any painting.
  2. As soon as all items have been cleaned and within four hours of the subsequent drying, they shall be given suitable anti-corrosion protection.


Following are the types of paint which may be suitably used for the items to be painted at shop and supply of matching paint to site: Heat resistant paint (Hot oil proof) for inside surface

For external surfaces one coat of thermo setting powder paint or one coat of epoxy primer followed by two coats of synthetic enamel/polyurethene base paint.


  • The bushings conform to the relevant standards specified and shall be of 50 outdoor type. The bushing rods and nuts are made of brass material 12 mm diameter for both HT and LT bushings. The bushings shall be fixed to the transformers on side with straight pockets and in the same plane or the top cover for transformer.
  • For transformers of 11 kV, 17.5 kV class bushings and for 0.433 kV, 1.1 kV class bushings shall be used.
  • Bushing are of porcelain/epoxy material. Polymer insulator bushings conforming with relevant IEC can also be used.
  • Bushings of plain shades as per IS 3347 shall be mounted on the side of the Tank and not on top cover.


  • All the equipment offered are fully type tested as per the relevant standards including the additional type tests. The type test have been conducted on a transformer of same design during the last five years at the time of bidding. The bidder shall furnish two sets of type test reports along with the offer. Offers without type test reports will be treated as non-responsive.
  • The test certificates for all routine and type tests for the transformers and also for the bushings and transformer oil are available


  1. Ratio, polarity, phase sequence and vector group.
  2. No Load current and losses at service voltage and normal frequency.
  3. Load losses at rated current and normal frequency.
  4. Impedance voltage test.
  5. Resistance of windings at each tap, cold (at or near the test bed temperature).
  6. Insulation resistance.
  7. Induced over voltage withstand test.
  8. Separate source voltage withstand test.
  9. Neutral current measurement-The value of zero sequence current in the neutral of the star winding is not be more than 2% of the full load current.
  10. Oil samples (one sample per lot) to comply to the required breakdown voltage
  11. Measurement of no load losses and magnetizing current at rated frequency and 90%, 100% and 110% rated voltage.
  12. Pressure and vacuum test for checking the deflection.


  1. Temperature rise test for determining the maximum temperature rise after continuous full load run. The ambient temperature and time of test is stated where required.
  2. Impulse voltage test is 95 kV peak instead of 75 kV 30.3 Short circuit withstand test: Thermal and dynamic ability.
  3. Magnetic Balance Test.
  4. Noise-level measurement. 55
  5. Transformer tank is subjected to specified vacuum. The tank designed for vacuum shall be tested at an internal pressure of 0.35 kg per sq cm absolute (250 mm of Hg) for one hour.
  6. Transformer tank together with its radiator and other fittings are subjected to pressure corresponding to twice the normal pressure or 0.35 kg / which ever is lower, measured at the base of the tank and maintained for an hour. The permanent deflection of the flat plates after the excess pressure has been released, shall not exceed the figures for vacuum test.
  7. Short Circuit Test and Impulse Voltage Withstand Tests: ECS supplies transformers designed and successfully tested for short circuit and impulse test.
  8. Type test certificates for the tests carried out on prototype of same specifications are submitted along with the transformer


In respect of raw material such as core stampings, winding conductors, insulating paper and oil, ECS uses materials manufactured/supplied by standard manufacturers and furnishes the manufacturers’ test certificate as well as the proof of purchase from these manufacturers where required