What is called internal overvoltage

Internal overvoltage is the harmful overvoltage caused by the sudden change of power system state due to operation, accident or improper coordination of grid parameters.
Atmospheric overvoltage is also called external overvoltage, is due to the direct lightning strike caused by direct lightning overvoltage or lightning strike in the vicinity of the equipment, the induction lightning overvoltage produced on the equipment.
Internal overvoltage and atmospheric overvoltage are high, may cause flashover of insulation weak points, cause insulation damage of electrical equipment, or even burn down.