Short Circuit Rating 2

[pukar12345]

Hi all,

I saw the short circuit rating of a distribution system as following

1. for 27.6/16.0 kV 16kA
2. for 13.8/8.0 kV 34kA
3. for 2.4/4.16kV 56kA


Why short circuit rating is higher when system voltage is small compare to higher system voltage ?

Is not higher short circuit rating in higher voltage system ?

I will appreciate for reply.

 

[burnt2x]

For the same MVA, if the secondary voltage is low, the FLA (VA/(1.732 X V))is higher. Since Isc is more or less equal to FLA/Zpu, then Isc will be greater:
Example: 10 MVA trafo; %Z = 5.79% or 0.0579 pu. Vs1 = 2400; and Vs2 = 4160:
Isc1 = 10,000/(1.732 X 2400 X 0.0579) = 41.5 kA.
But Isc2= 10,000/(1.732 X 4160 X 0.0579) = 23.97 kA.