Need for EHV and UHV lines

EHV (about 300–765 kV) and UHV (1000 kV AC and above, ±800 kV DC) lines are needed because the power a line can carry rises with the square of voltage, while losses and the cost per MW fall as voltage rises.

Justification

1. Power capacity rises as $V^2$:

$$P = \frac{V_sV_r}{X}\sin\delta \propto V^2$$

The natural (surge impedance) loading is $SIL = \frac{V^2}{Z_c}$. Taking $Z_c \approx 400\ \Omega$:

So one 765 kV line carries as much as about twelve 220 kV lines.

2. Lower losses: for power $P$ the current is $I = \frac{P}{\sqrt{3}V\cos\phi}$, so the loss $3I^2R \propto \frac{1}{V^2}$. Doubling the voltage cuts losses to a quarter.
3. Less right-of-way per MW: fewer lines are needed, which saves land, forest and resettlement cost, very important in hilly areas like Nepal.
4. Lower cost per MW-km: although towers and insulation cost more, the cost per unit of power carried is lower.
5. Better stability over long distance: a higher voltage gives a larger power margin for the same angle and, with bundled conductors, lower reactance.
6. Remote large generation: big hydro, coal or renewable plants are far from load centres (e.g. Himalayan hydro to Indian plains, Chinese west-to-east transfer), needing bulk transfer over hundreds of km.
7. Interconnection of grids and power trading need strong high-capacity links.

Costs to accept

Corona loss, radio and audible noise, switching overvoltage, larger clearances and insulation, and high field effects must be controlled using bundled conductors, closing resistors, arresters and proper design. Even with these, EHV/UHV is the economical way to move large blocks of power over long distances.