Technical Specifications of Outdoor Distribution Transformer 11KV/0.433KV | IS 1180

What is Distribution Transformer

A distribution transformer or service transformer is a transformer that provides the final voltage transformation in the electric power distribution system, stepping down the voltage used in the distribution lines to the level used by the customer. If it is mounted on steel tubular poles, it is called pole mounted. If it is mounted on concrete pads, it is called Pad mounted Distribution Transformer. Generally it steps down voltage from 11KV to 0.433KV.

Technical Specification As per IS 1180 (Part – 1): 2014

  1. The standard capacity rating shall be 16KVA, 25KVA, 63KVA, 100KVA and 250KVA.
  2. The transformer shall be suitable for outdoor installation with three phases, 50Hz, 11KV system. It should be suitable for service with fluctuations in supply voltage upto \pm12.5%.
  3. The vector should be Dyn-11.
  4. The cooling of the transformer should be Oil Natural Air Natural (ONAN).
  5. Anticipated continuous loading of winding shall be 110% of rated current. Anticipated unbalanced loading shall be around 10%.
  6. Audible sound levels at rated voltage and frequency shall be as below
KVA RatingAudible Sound Levels
  1. The core shall be stack/ wound types of M3 grade (0.23mm) or HIB grade or higher, cold rolled grain oriented (CRGO) annealed steel lamination. The maximum flux density any part of the core and yoke at rated voltage and frequency shall be such that flux density with 12.5% combined voltage and frequency variation from rated voltage and frequency shall not exceed 1.9 Tesla.
  2. No load current shall not exceed 3% of full load current and will be measured by energizing the transformer at 433 Volts, 50Hz on the secondary. Increase of voltage of 433 volts by 12.5% shall not increase the load current by 6% (maximum) of full load current.
  3. HV and LV windings shall be wound from super enamel covered/Double paper covered, aluminium conductor/foil for all ratings.
  4. Inter layer insulation shall be Nomex/Epoxy dotted Kraft Paper. Proper bonding of inter layer insulation with the conductor shall be ensured. Test for bonding strength shall be connected.
  5. Current density of HV and LV winding should not be more than 1.6 Ampere per sqmm for aluminium conductor.
  6. No Taps are required for Transformer upto 100KVA rating Transformer.
  7. The specific resistance of transformer oil shall not be less than 2.5\times 10^{12}ohm-cm at 270 C when tested as per IS:6103.
  8. The maximum total losses at 50% and 100% load conditions at rated voltage and frequency and at 750 C shall be as below.
S.No. Rating (KVA)Max. Load Loss at 50% Loading (Watts)Max. Load loss at 100% Loading (Watts)
  1. The value of impedance of the specified rating transformers at 750 C shall be 4.5%
  2. For all Distribution transformers rating the oil temperature rise measured by thermometer should not rise 350 C and The winding temperature rise measured by resistance method should not rise above 400 C.
  3. The top cover of the tank shall be slightly sloping to drain rain water.
  4. The Tank should be made of mild steel of below mentioned thickness. (as per IS 1852 )
S.No Tank upto 25KVAAbove 25KVA to 100KVAAbove 100KVA
1. Top and Bottom plates3.15mm5.0 mm6 mm
2.Side wall Plates2.5mm3.15 mm4 mm
  1. For distribution Transformers of all ratings the plain tank shall be capable of withstanding a pressure of 80kPa and a vacuum of 250mm of mercury for 30 minutes. The permanent deflection of flat plates, after pressure/vacuum has been released shall not exceed the values given below:
Length of Flat PlatePermanent deflection
Upto and Including 750 mm5.0 mm
751 mm to 1250 mm6.5 mm
1251 mm to 1750 mm8 .0 mm
Above 1751 mm9.0 mm
  1. There shall be no leakage at any point when transformer is subjected to an air pressure of 35kPa above atmospheric pressure maintained inside the plain tank for 10 minute.
  2. The tank with corrugations shall be tested for leakage test at a pressure of 15kPa above atmospheric pressure at the top of the tank for 6 hours. there shall be no leakage at any point.
  3. The volume of the breather shall be suitable for 500 gms of silica gel as per IS 3401 for all transformer ratings.
  4. The capacity of conservator tank shall be such as to contain 10% of the oil (in Tank). Normally 3 % quantity of oil shall be contained in the conservator.
  5. The inside diameter of the pipe connecting the conservator to the main tank should be 25 to 50 mm and it should be protected in the conservator tank so that its end is at least 20mm above the bottom of the conservator.
  6. The bushing rods and nuts shall be made of brass material 12 mm diameters for HT and 12 mm diameter for LT bushing upto 250 KVA.
  7. For 11KV (HV Side) , 12 KV class bushings and for 0.433 KV (LV Side) , 1.0 KV class bushing shall be used.
  8. Minimum external phase to phase and phase to earth clearances of bushing terminals shall be as follows.
VoltagePhase to PhasePhase to Earth
11KV (HV)255mm140mm
  1. Routine Test conducted on all units as per IS 2026 (Part – I) \\\;1. Measurement of winding resistance test \\\;2. Measurement of voltage ratio and check of phase displacement.\\\;3. Measurement of Short Circuit Impedance and Load Loss at 50% and 100% Load.\\\;4. Measurement of no load loss and current. \\\;5. Measurement of insulation resistance.\\\;6. Induced over voltage withstand test.\\\;7. Separate source voltage withstand test. \\\;8. Pressure test. \\\;9. Oil leakage Test.
  2. Type Tests to be conducted on one unit are as given below \\\;1. Temperature rise test [IS 2026 (Part – 2)] \\\;2. Lighting Impulse Voltage tets with chopped wave [IS 2026 (Part-3)]. Basic Insulation Level for 11KV shall be 75 kVp .\\\;3.Short circuit Withstand Test [IS 2026 (Part – 5)].\\\;4.Pressure test [IS 1180 (Part – 1)].
About AJAZ UL HAQ 136 Articles
AJAZ UL HAQ is an Assistant Electrical Engineer and has 10.5 Years of Experience in operation & maintenance, erection, Design of Transmission and Distribution Electrical System, Internal Electrification of Buildings, Electrical Substations. He has worked on Projects - Integrated Power development Scheme (IPDS), Prime Minister's Development Package (PMDP) and Re-structured Accelerated Power Development and Reforms Programme (RAPDRP).