standards / manuals / guidelines for small hydro development - AHEC

The transformer may be operated without danger an any particular tapping at the ratedkVA provided that the voltage does not vary by more than + 10% of the voltagecorresponding to the tapping.The transformer should be suitable for continuous operation with a frequency variation of+3 % from normal 50 Hz. Combined voltage and frequency variation should not exceedthe rated V/f ratio by 10%.8.3 Voltage RatioThe high voltage rating should be suitable for the voltage of the transmission system towhich it will be connected. The low voltage rating should be suitable for the generatorvoltage (if unit connected) or generator bus. Generator transformers are generallyprovided with Off-circuit taps on HV side for HV variation from +2.5 to – 7.5 % in stepsof 2.5 %. On load taps if provided should have tapping range of +5% to – 10 % in stepsof 1.25%.For interconnecting 2 transmission voltage system say 66 kV and 132 kV size, autotransformerswith standard ratings as per IS should be provided. For interconnectingauto-transformers, use of either regulating transformer or on-load tap changer may bemade. Interconnecting transformer are generally star-star connected with tertiary deltawinding for flow of 3 rd harmonic current.8.4 Duty Under fault ConditionGenerator transformer should be designed for exceptional circumstances arising due tosudden disconnection of the load and should be capable of operating at approximately 25% above normal rated voltage for a period not exceeding one minute and 40 % abovenormal rated voltage for a period of 5 seconds. All transformers above 5 MVA should beprovided with overfluxing protection device.8.5 Electrical ConnectionsTransformers shall be connected in accordance with IS vector symbol specified inordering schedule of the requirements. Vector symbol specified for generatortransformers is normally YND11.Auto connected and star/star connected transformers shall have delta connectedstabilizing windings if specified in the order. Two leads from one open corner of the deltaconnection shall be brought out to separate bushings. Links shall be provided for joiningtogether the two terminals so as to complete the delta connection and earthing it externalto the tank.3-6