For 11kV Cable Sizing

Here's some IEE rules. Don't know if they will help. You should really hire a competent electrician to do this type of work. Asking strangers is asking for trouble! IEE Rules of Conduct - Institution of Electrical Engineers rules for the ...

I wonder if this is why they choose to be anonymous?

I may have to resurrect my "do not respond to anonymous posters" policy.

Look at Okonite Cable

As in forever?!?

I might be simple, but arcing at any voltages at the substation, except of a switch gear, and even there is bad manners. NO, arcing is not good, generally, but poor design.

it's simple: this depends on the thickness of the coating above the copper (or alu wire). That coating can hold a given short-circuit, generally for 0.5 seconds (sometimes, may be less, 0.3 second) That value may be found in the description of cable manufacturer. For 11 kV, it's just like the calculation of an LV cable (such as 440 V). That value is the lone to take in account. After that, you have to set your relay in coordination with the other end of line (in taking in account the possible busbar protection of 11 kV swgr)The distance of 3 km is quite important and gives a capacitive resultant in your calculation. This resultant is something smoothing the short-circuit time. Just set your relay with the time given by the cable manufacturer and all things will be sure, because that distance will smoothes the effect of short-circuit inside the cable (in spite of 0.5 second, the real time of short-circuit will be longer and the real value of Isc shorter). if your link may be changed very often (for instance, every 5 years), you leave the value of setting as i described it. If you don't change your cable very often (i.e. every ten years), divide that value by 2 or 3, in order to take in acount the ageeing of cable link. Don't forget the longer energisation time due to the length of your link. Take it in account if you have some electrical motors to start, downstream to your 11 kV link.

I suppose that you know how to lay down your cable. But one thing is important, the values given above are for a cable laid between slices of 20 cm (so 40 cm) of a good sieved sand of a good quality, under 60 cm of sieved ground without stones. The way of laying the cable is very important for the quality of short-circuit resistance and the impedance of the cable.

Good luck, brzk.