The simplest formula is geno MVA / geno Xd".

Could u tell me what s mean by geno MVA/geno xd

I would like to answer by way of an example:

Generator capacity = 12MVA
Generator sub-transient impedance = 12%

With above two data,
the generator fault level can be calculated as folows:
Generator fault level = Generator capacity / generator sub-
transient impedance

Generator fault level = 12 MVA / 12% = 12MVA/.12 = 100MVA

To get fault current = Generator fault MVA / (1.732 * 
generator rated voltage)

In case of single phase generator 1.732 to be ignored.

what is the refected current in 6.6 KV system in case of 11 
kv side fault current is 40 KA and 8000 A full load current.

From your question I guess there is transformer between the 
11kV & 6.6kV you are refering to. We need the transformer 
MVA and the impedance percentage. 
Once your provide the transformer MVA and its %impedance I 
can answer further