Write selection criteria of distribution transformer with any four points.

Write selection criteria of distribution transformer with any four points.
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Answer :

Selection Criteria for distribution transformer: i) Load requirements that decides kVA Rating ii) Required Tappings iii) Vector group. iv) Winding Impedances v) Termination Arrangement. vi) Cooling system vii) Nature of load viii) Ambient/ Environment conditions ix) Voltage ratings x) Nature of service required xi) Tariff applicable etc. 

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Answer :

Selection Criteria for Distribution Transformer: 

i) Ratings - The kVA ratings should comply with IS:2026 ( Part l )-1977*. The noload secondary voltage should be 433 volts for transformers to be used in 415 V system. Voltage should be normally in accordance with IS:585-1962 except for special reasons when other values may be used. 

ii) Taps -The transformers of these ratings are normally provided with off-circuit taps on HV side except in special cases when on-load tap changers are specified. The standard range for off-circuit taps which are provided on HV side should be of 2.5 percent and of 5.0 percent. In case of on-load tap changers, the taps may be in steps of 1.25 percent with 16 steps. The positive and negative taps shall be specified to suit the system conditions in which the transformer is to be operated. 

iii) Connection Symbol - The two winding transformers should be preferably connected in delta/star in accordance with IS:2026 (Part 4)-1977s. The exact connection symbol (Dyn11or Dyn1) is to be specified depending upon requirements of parallel operation. 

iv) Impedance - Consideration shall be given in the selection of impedance for the standard available rating of the switchgear on the secondary side and associated voltage drops. 

v) Termination Arrangement - The HV and LV terminals may be bare outdoor bushings, cable boxes or bus trunking depending upon the method of installation. Wherever compound filled cable boxes are used, it is preferable to specify disconnecting chamber between transformer terminals and cable box to facilitate disconnection of transformer terminals without disturbing the cable connections (see also IS:9147-1979). In case of extruded insulation cables with connections in air, a separate disconnecting chamber is not necessary. vi) Cooling - The transformers covered in this group are generally ONAN, AN
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