A, lighting voltage
B. radio interference
C. skin effect
D. switching over voltage
Answer: D. switching over voltage
Explanation:
The operation of switching devices can join or separate parts of a power system. After a closing operation, transient currents will flow through the system, while after an opening operation a transient recovery voltage will appear across the terminals of the interrupting device.
Overvoltage refers to the phenomenon that the rms value of the AC voltage rises at the power frequency, exceeding 10% of the rated value, and the long-term voltage fluctuation phenomenon lasts longer than 1 minute; the occurrence of overvoltage is usually the result of load switching.
At transmission line voltages up to 230 kV the insulation level is decided by the requirement of protection against lightning.
In EHV, switching surges cause higher over voltages than lightning and therefore mainly responsible for insulation level decision.