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Monday, 18 May 2015

Transformer KVA rating and Impedance

Transformer

LV, 415V, Current Rating of Transformer = KVA x 1.3
Short Circuit Current of T.C /Generator = Current Rating / % Impedance
No Load Current of Transformer =< 2% of Transformer Rated current
Capacitor Current (Ic) = KVAR / 1.732xVolt (Phase-Phase)
The diversity they would apply to apartments is around 60%
Maximum HT (11kV) connected load will be around 4.5MVA per circuit.
Earth pits per transformer: 2Nos. for body and 2Nos. for neutral earthing.
Clearances, approx.1000mm around TC allow for transformer movement for replacement.
transformer KVA  rating and impedance

Single Phase Transformer   
5KVA, 10KVA, 15KVA, 25KVA, 37.5KVA, 50KVA, 75KVA, 100KVA, 167KVA, 250KVA, 333KVA, 500KVA, 833KVA, 1.25KVA, 1.66KVA, 2.5KVA, 3.33KVA, 5.0KVA, 6.6KVA, 8.3KVA, 10.0KVA, 12.5KVA, 16.6KVA, 20.8KVA, 25.0KVA, 33.33KVA.
Three Phase Transformer
3KVA, 5KVA, 9KVA, 15KVA, 30KVA, 45KVA, 75KVA, 112.5KVA, 150KVA, 225KVA, 300KVA, 500KVA, 750KVA, 1MVA, 1.5MVA, 2MVA, 2.5MVA, 3.7MVA, 5MVA, 7.5MVA, 10MVA, 12MVA, 15MVA, 20MVA, 25MVA, 30MVA, 37.5MVA, 50MVA, 60MVA, 75MVA, 100MVA
Note:
- Transformer up to 25 KVA can be mounted direct on pole.
 - Transformer from 25 KVA to 250KVA can be mounted either on “H” frame of plinth.
 - Transformer above 250 KVA can be mounted plinth only.

Impedance of transformer – As per IS 2026)
>MVA                                                % Impedance
< 1 MVA                                5%
1 MVA to 2.5 MVA               6%
2.5 MVA to 5 MVA               7%
5 MVA to 7 MVA                  8%
7 MVA to 12 MVA                9%
12 MVA to 30 MVA              10%

> 30 MVA                              12.5%
further reading: a practical guide to cable installation and tool box talk

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