Issues of EHV-AC power transmission

EHV-AC transmission (about 300–765 kV) carries large power efficiently, but it brings several technical problems that must be solved in design.

Main issues

1. Corona: high surface gradient causes corona loss, radio interference (RI), TV interference and audible noise. Bundled conductors are used to reduce it.
2. Switching overvoltages: energising, re-closing and fault clearing create surges of 2–3 pu, which govern the insulation design. Closing resistors and arresters are needed.
3. Lightning overvoltages and insulation coordination.
4. Ferranti effect and reactive power: large line charging causes high receiving-end voltage at light load; shunt reactors are required.
5. Stability limits: long lines need series compensation, which may cause subsynchronous resonance.
6. Electrostatic and magnetic field effects under the line on people, animals and vehicles; induction in nearby lines and pipelines.
7. Large clearances and right-of-way, heavy towers and big insulator strings.
8. Line design: conductor selection, bundle spacing, sag and tension, and environmental impact.