Electrical Engineering ⇒ Topic : Power transformers (IS : 2026.-1962)

Power transformers (IS : 2026.-1962):

Standard ratings (for 3-phase transformers): 25, 40, 63, 100, 125, 160, 200, 250, 315,400, 500, 630, 822, 1000, 1250, 1600, 2000, 2500, 3125, 4000, 6300, 8000, 10000, 12500,16000, 20000, 25000, 31500, 40000, 50000, 63000 and 80000 kVaA.

Standard ratings (for 1-singlel phase transformers): 1, 2, 5, 10, 16 and 25 kVA.

Above 25 kVA, the standard rating for single phase transformers shall be one-third of the value given for 3-phase transformers.

Tappings : The standard tapping ranges are 21/2% ± 5%. The changing is carried out by means of an externally operated off-circuit switch capable of being locked in position.Limits of temperature rise :

(i) Windings (measured by resistance) 55°C (ON, OB, OW cooling)

                                                              60°C (OF1V, OFB cooling)

                                                              65°C (OFW cooling)

(ii) Oil (measured by thermometer in top oil) all types .... 45°C.

(iii) Cores core shall be desgined so that the temperature rise on any part of the external surface does not exceed that of the winding.

Impedance : 4.5% at 75°C, for transformers of rating upto an including 100 kVA, 11kV and 4.75% for transformers of rating above 100 kVA including 1000 kVA, 11 kV