BY equation ,
transmitted power, p= ?3VIcosŲ
circulating current, I = (P/?3 VcosŲ)
(1) For a amount of power transmission at higher voltage ,
If power is constant and voltage increases then current decreases.
Dimension of conductor and weight depend on current density, therefore lower conductor sizes would have lower weight and cost.
(2) Distance between two tower depends on conductor weight , therefore lower weight would have long span between two tower and higher weight would have short span between two tower .
therfore , lower weight would have cost advantage due to reduction of a number of tower or pole.
(3) Design and strength of tower should be according to weight and tensile strength of conductor because in limit of elasticity , force stress is directly proportional to the strain.
therefore , young modulus would have a ratio of force stress and strain.
Y= Force stress / strain (newton per meter^2)
According to young modulus , cost of higher young modulus tower will be higher than lower young modulus tower in proportional to tensile strength .
(4) Line losses = I^2 * R (in watt)
For higher voltage transmission line, line losses depend on circulating current and conductor resistance.
Therefore , lower current would have lower losses and cost and higher current would have higher losses and cost.