Fault Level & Fault Current

The same as anywhere else. The Prospective Short-circuit Current [PSC] is the supply voltage divided by the earth loop impedance. So in order to calculate it, you need to measure the earth loop impedance. Fortunately, specialised testers to do it are available these days, and they are much better than the ones we used in 1942.

"Determining the minimum size of the earthing grid conductors is necessary to ensure that the earthing grid will be able to withstand the maximum earth fault current. Like a normal power cable under fault, the earthing grid conductors experience an adiabatic short circuit temperature rise. However unlike a fault on a normal cable, where the limiting temperature is that which would cause permanent damage to the cable's insulation, the temperature limit for earthing grid conductors is the melting point of the conductor. In other words, during the worst case earth fault, we don't want the earthing grid conductors to start melting!"

Other references

http://www.openelectrical.org/wiki/index.php?title=Earthing_Calculation

http://www.eng.uwi.tt/depts/elec/staff/alvin/ee35t/notes/Substation-Design.html

A Simplified Method for Calculating the Substation Grounding..

MV/LV transformer substations: theory and examples of short-..

Usually we use a simplified method to estimate the Fault Current (Is). i.e.: rated current devided by impedance.

Example, if the source (upstream) is from a 2000kVA 11kV/433V Z=5% Transformer, Is at 433V side to be 2000/433/1.732/0.05 = 53kA.

The actual Is will slightly less than this value.

You didn't take into account the fault level at the primary of the transformer

First calculate the rated current of the Transformer. Ir= KVAr/(1.73xKVr), where r stands for "rated". Now Ifl= Ir/%Z, where Ifl stands for "fault level" . You may calculate the individual fault level for as many as, transformers installed in your substation and add them all to get the fault level of your substation.

Rated current of a transformer = I_r = KVA / 1.73x kv

MVA _{fault level}= rated MVA x100 / percentage Impedence

I _fault= MVA _{fault level}/ 1.73 x KV

you can calculate individual MVA fault level of each transformer installed in the substation and then add all MVA fault levels to get the overall fault level of your substation.

The fault level at the point of connection could be obtained from the utility. Then add up all impedances in series from that point up to any point in the system downstream and calculate the fault level at that point. Contribution by induction motors to be considered but will other transformers parallel to the selected transformer have any impact?. Illustrate by a single line diagram with arrows showing flow of fault current.

Mr.pnaban are you electrical engr.? If so can you tell how 3rd harmonics are mitigated in tertiary windings of transformer? just tell me the physical effect and not a mathematical solutions.

In a single line diagram if you indicate the path of fault current, the doubts will be cleared. Also if you show the positive,negative and zero sequence components of the current flow at the time of fault it will give a more clear picture.