why distribution transformers are rated exactly at 25 KVA and 63 KVA and 100 KVA?
It's a matter of standardization in engineering. As example, according IEC 60076-1 for the transformers with rated apparent power under 10MVA, the preferred values are based on the R10 series (R=Renard) , respectively: 10, 12.5, 16, 20, 25, 31.5, 40, 50, 63, 80, 100, and multiples of 10^n. The numbers cover a decade (order of magnitude) in 10 steps and they represent a geometric series with factor 1.26 (obtained from 10^0.1; finally the values are rounded).
It's a matter of standardization in engineering. As example, according IEC 60076-1 for the transformers with rated apparent power under 10MVA, the preferred values are based on the R10 series (R=Renard) , respectively: 10, 12.5, 16, 20, 25, 31.5, 40, 50, 63, 80, 100, and multiples of 10^n. The numbers cover a decade (order of magnitude) in 10 steps and they represent a geometric series with factor 1.26 (obtained from 10^0.1; finally the values are rounded).
Yes, products are standardised so they can be plugged and put into operation easily, with the existing components of the Electrical System.
Extract of IEC 60076-1:2011
5.1.2 Preferred values of rated power
For transformers up to 20 MVA, values of rated power should preferably be taken from the R10 series given in ISO 3:1973, Preferred numbers – series of preferred numbers:
(...100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1 000, etc.) kVA.
NOTE National practices may be different.
I agree with Dr. Abhijit Mandal.
It depends on IEC standard but different countries will have a range of rated power.
In my country:
1 phase: 25kVA, 37,5kVA, 75kVA, 100kVA
3 phase: 100kVA, 250kVA, 320kVA, 400kVA, 560kVA, 630kVA, 750kVA, 1000kVA, etc
Different countries around the world operates different systems depending on the country's adopted standards.
Ghana my country operates AC system and have adopted British Standard
transformers are rated, 50kVA, 100kVA ect.
Dear professor
Its design as per requirement. Even it can be designed to any value. Usually distribution of power require lower kva rating of transformer
Dr. Maruthi GS
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