Transmission Lines

Losses are $I^2R$, so for the same power, higher voltage means lower current and much lower loss. Indian transmission uses 132, 220, 400 and 765 kV AC, plus HVDC links (±500 and ±800 kV).

Lines are modelled by parameters per kilometre: R, L, C (and G). Models by length:

• Short (< 80 km) — series R and L only.
• Medium (80–250 km) — nominal π or T model.
• Long (> 250 km) — distributed parameters.

Key quantities: surge impedance $Z_c = \sqrt{L/C}$ (≈ 400 Ω for overhead lines) and the surge impedance loading $SIL = V^2/Z_c$. Phenomena: Ferranti effect (receiving-end voltage rises on a lightly loaded long line), corona and skin effect. Bundled conductors reduce corona and reactance.