Switching surges occur on power systems as a result of instantaneous changes in the electrical configuration of the system, and such changes are mainly associated with switching operations and fault events.
These over-voltages generally have crest magnitudes which range from about 1 per unit to 3 pu for phase-to- ground surges and from about 2.0 to 4 pu for phase-to-phase surges (in pu on the phase to ground crest voltage base) with higher values sometimes encountered as a result of a system resonant condition. Wave-shapes vary considerably with rise times ranging from 50 μs to thousands of μs and times to half-value in the range of hundreds of μs to thousands of μs. For insulation testing purposes, a wave-shape having a time to crest of 250 μs with a time to half-value of 2000 μs is often used.
These over-voltages generally have crest magnitudes which range from about 1 per unit to 3 pu for phase-to- ground surges and from about 2.0 to 4 pu for phase-to-phase surges (in pu on the phase to ground crest voltage base) with higher values sometimes encountered as a result of a system resonant condition. Wave-shapes vary considerably with rise times ranging from 50 μs to thousands of μs and times to half-value in the range of hundreds of μs to thousands of μs. For insulation testing purposes, a wave-shape having a time to crest of 250 μs with a time to half-value of 2000 μs is often used.