System Fault Current

Simply because it's the size on the market. Manufacturer make a standard range, if you want bespoke equipment then you will have to pay for it.

What is the problem with higher rated equipment if it's relatively cheap?

Dear Ganesan,

Since your so called VCB will be on the bus connected to the transformer. The most imp consideration is Fault MVA and fault current of the bus. Now, this bus must be connected to diff equipments having their own %imp, so hereby increasing the fault current and fault MVA on the bus.

therefore, VCB is selected as per fault KA of the bus. not the equipment connected to it.

You do not select circuit breaker switchgear based on the steady state fault level, you must enure its capable of coping with transient fault levels which it may require to break.

I want to give a simple method to estimate maximum short circuit current with a 3-phase fault on secondary of the transformer.

Isc( Symmetric Short Circuit Current) = secondary current of the Transformer divided by percentage impedance.

In this case secondary current is 15MVA/(1.732x11KV)= 787.3Amps

Hence Maximum Isc = 787.3/(10/100) = 7.87KA.

I do not know how you got 10.3KA. even if you have big Induction motor connected on the same Bus of transformer.

The best and accurate method is draw the whole network with their per unit impedances. Apply the Thevenin theorem then you will get accurate short circuit current.

Now the question is why to go standard VCB. Custom made VCB will be very costly and needs more time to construct. So it is better to use nearest higher side standard rating of VCB.

Yes! Availability is the issue.