The choice of transmission systems and nominal voltages for a transmission line is from HV AC (up to 220 kV) EHV AC (400 kV - 750 kV) UHVAC (above 760 kV AC) depending on technical and economic considerations .
Advantages:
• High-power transfer of AC lines P α V2.
• Line losses decrease with increasing transmission voltage and improved power factor for the same power transfer.
• Bulk power transfer from a large group of generation stations to the main transmission network.
Disadvantages:
• Higher voltage provides less current, less I2R losses.
• Short-circuit levels: in the case of very long lines of more than 500 km, intermediate switching substations are required to install by-pass reactors for compensation.
• Right of way: in some cases of large cities, industrial locations, it is impossible to acquire the right of way for EHV AC lines.
• Line Isolation: The creep age distance (leakage distance) determined on the basis of the required impulse with the level of the carrier.
• Crown: The critical value of the voltage depends on the pressure, temperature, humidity and level of air pollution.