Transmission

Some knowledge about Electric Power Transmission:

(1) Transmission conductor size ranges from 12mm2 to 750mm2, thicker wires lead to small increase in the capacity limit (or current-carrying capacity)of transmission lines, due to skin effect.

(2) Transmission voltage levels:
EHV: >230KV
Transmission: >110KV
Subtransmission: 33-66KV
Distribution: <33KV
Comparing with the voltage of power generating plants: 2.3KV-30KV

(3) Long distance transmission cost is around $0.005-$0.02/KWh in US. Compare to the production cost of generating units: $0.01-$0.025/KWh, and retail rate: $0.1/KWh (from wikipedia)

(4) Which factor sets the thermal limit for a transmission line:
 short line: The thermal limit is set by the resistance or heating limit of conductors;
 intermediate line:  voltage drop is the one that determines the power limit. P=v1*v2*sin(delta)/x. You don't want the voltage drop too much (v2 small), or it is not suitable for load (usually no more than 5% voltage drop). The voltage drop is defined as the magnitude of the difference of two phasors (|v1-v2|). 
long line: it is the angle delta or system stability that determines the power limit. When the angle approaches 90 degrees, the system approaches its stability margin, which is undesirable for the operation.