How to Choose the size of the Conductor?

May fry everything. Voltage potential at 66000 volts is six times greater that the 11Kv system is rated. The insulation properies of the 11Kv system may not with stand the 66Kv potential.

Higher voltages are generally used for power transport over longer distances. ie: the lower the current the lower the losses. You don't really state what these distances are in your question so I will relate it back to voltages and distances used around our plant. Our whole refinery is approx 2km square and power is reticulated at 11.5kV to various substations within this area. Size of the cable at 11kV relates back to the expected max current draw and distance from source of each transformer. The transformers are generally 2 or 5MVA. We also have an overhead powerline at 66kV with approx 10MVA load with 35mm2 Al cables. Distance is about 60km. We also have a remote pumping station at 11.5kV about 5km away with 35mm2 Al cables. The load is about 50kVA. I hope this helps you a little.

The whole thing depends upon whether u have a 1 KV feeder near by.otherwise u may need a transformer of 66/11 and minimum for this rating could be 2.5 MVA.The cost transformer + 11KV cable and associated system vs 66kv cable and associated system may give u a clear idea.I feel 66kv cable may be still expensive.On 11KV ur current will be approx 25A.Why cant u think of supply at required voltage for this power like 380 0r 440V whatever is the standard in lv voltage in your country.

The current will obviously no more be 4.37A, as the KV drops from 66, to 11, it will become 26A. and corresp power loss 36 times (6 square)

The selection is techno- economic -

Usually if you are a power producer, output is at 11KV, then you step up to 66KV and step down to reqd voltage

Cost = cost of HV insulator, transformer, saving = saving in loss (I2R), voltage drop

Decision is made accordingly. As the Current and distance increases, the I2R and Voltage drop more than compensates for the other expenditure and we prefer higher and higher voltages.

Also if Input, output etc either or both require 66KV, the equation is simpler.

regards

I am sorry but if you do not know the difference between 66000 and 11kV distribution systems or which one to use, then you really should not be laying cables yourselves!