Sizing Switchgear According to SC Current

The power company should be able to give you the source impedance of their network feeder to your facility (i.e. at the point of common coupling), which I presume in your case is the 22kV side of your transformer.

You can use this source impedance to calculate all the short circuit levels in your system, including at the 22kV side of your transformer. Often they'll give you the SC capability of the source in MVA, but you can easily convert that to an impedance value.

If it is difficult to ask them for the source impedance, may I change the question by asking them the impedance of transformer and ASCR line?

Sorry, I got one mistake with 110/22kV transformer. Its capacity shall be 64MVA.

You must get the 110kV Source fault level, the percentage impedance of the 64MVA Trf. & the type, size & length of the OH Conductor.

The only correct way of estimating fault levels in your plant is to get the short circuit levels at the point of common coupling from the power supply utility. We will also need single line to ground fault levels apart from the 3 phase to ground fault levels, if we want to determine short circuit levels for all types of faults.

Ignoring source impedance from the utility will result in conservatively higher short circuit levels and corresponding breaker sizes [and costs]

use the formula A = (I * sqrt of t )/k

A= Cross aection area of cable or cunductor

I=Short ckt current in KA

t = withstand time in Sec

k= Constant for Al is 0.094

I am sorry to say that I disagree with this formula. This is a thermal withstand rating of the equipment equation.

While selecting the breaker, the important point is the highest making and breaking capability of the breaker. This requires short circuit calculations.