Correct option is A
The basis of insulation design in modern Extra High Voltage (EHV) lines is switching voltage.
Switching over-voltages are the main factor in the design of EHV transmission lines. These over-voltages can be caused by:
· Energizing transmission lines and cables
· Switching high voltage reactors
· Switching transformers that are loaded by a reactor on tertiary winding
· Fault initiation and clearing
· Switching of a transformer at no load
The insulation level of a transmission line is based on the maximum switching surge overvoltage to the ground.
