Explain the criteria of selection of power transformer.

Explain the criteria of selection of power transformer.
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2 Answers

Answer :

Criteria of selection of power transformer: 

i) Ratings: The no load secondary voltage should be specified 5% above the nominal voltage in order to partially compensate for regulation of transformer. If the transformers need to operate in parallel with each other, then their voltage ratios should be adjusted as per guidelines given in IS:10028(Part-II)-1981 

ii) Taps: On load tap changers on HV side should be specified. For On load tap changer should be 16taps of 1.25percent. If the oil circuit taps are to be specified, they should be in the range of +-2.5% and +-5% and should be provided on the HV side of transformer. 

iii)Connection Symbol:  For power transformers upto 66kV high voltage side(HV) rating, the most preferred connections are delta/star(Dyn) and star/star(YNyn) If the voltage rating is higher, then star/star(YNyn) and star/delta(YNd) connections should be preferred. The section of group is made on the basis of requirement of parallel operation with other transformers 

iv)Impedance: The value of transformer impedance is decided by considering the secondary fault levels and the associated voltage dips. For deciding the precise value of transformer impedance by referring IS:2026(Part-I)-1977. If the transformer is to be operated in parallel then the impedance be selected as per the guidelines given IS:10028(Part-II)-1981. 

v)Termination Arrangement: The HV & LV terminals may be one of following three types depending upon on the method of installation: Bare outside bushing Cable boxes Bus trunking The types of bushings that should be specified are: Upto 33kV : Porcelain bushings 66kV & above: Oil filled condenser type bushings. 

vi)Cooling

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Answer :

Selection Criteria for Power Transformer: 

1) Ratings - The kVA ratings should comply with IS:10028 (Part l)-1985. The noload secondary voltage should be 5 % more than nominal voltage to compensate the transformer regulation partly. The transformer required to be operated in parallel, the voltage ratio should be selected in accordance with guidelines given in 12.0.1 & 12.0.1.1 of IS:10028 (Part l)-1985 


2) Taps - On-Load tap changers on HV side should be specified, wherever system conditions warrant. In case of OLTC, total number of taps should be 16 in steps of 1.25 %. The standard range for off-circuit taps which are provided should be in range of + 2.5 percent and + 5 percent. 


3) Connection Symbol - The preferred connections for two winding transformers should be preferably connected in delta/star (Dyn) and star/star (YNyn). For higher voltage connections star/star (YNyn) or star/delta (YNd) may be preferred accordance with IS:10028 (Part l)-1985. 4) Impedance -The transformer impedance is decided taking into consideration the secondary fault levels and voltage dip. The typical values are given in table 3 of IS:2026. 


5) 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. 


6) Cooling - The transformers covered in this group are generally ONAN, ONAN/ONAF, ONAN/ONAF/OFAF.

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