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Switching over voltage are more hazardous than lightning surges in case of
Low voltage system
33 kV system
EHV and UHV system
All of above
EHV and UHV system
Switching overvoltages become the dominant design constraint in Extra High Voltage (EHV) and Ultra High Voltage (UHV) systems because lightning surge magnitudes do not scale proportionately with system voltage, whereas switching surge magnitudes increase significantly as system voltage rises. Insulation coordination in these systems is primarily governed by the ability to withstand these internally generated transients rather than external atmospheric lightning strikes.
Switching overvoltages become the dominant design constraint in Extra High Voltage (EHV) and Ultra High Voltage (UHV) systems because lightning surge magnitudes do not scale proportionately with system voltage, whereas switching surge magnitudes increase significantly as system voltage rises. Insulation coordination in these systems is primarily governed by the ability to withstand these internally generated transients rather than external atmospheric lightning strikes.
VswitchingтАЛ=kтЛЕVphaseтАЛ тАФ where k is the switching surge factor (typically 2.0 to 3.5)
BILтЙИ3.3├ЧVlineтАЛ тАФ empirical limit for system insulation levels
Switching surges are caused by sudden changes in the network state, such as circuit breaker operations, reclosing of long lines, or fault clearing. In EHV/UHV systems, long lines behave like transmission lines with significant capacitance and inductance, leading to traveling wave phenomena and resonance. The magnitude of these surges is proportional to the operating voltage VopтАЛ, whereas lightning surge insulation levels (Basic Insulation Level) are restricted by the sparkover voltage of air gaps which does not scale linearly at very high voltages.
Lightning surges are independent of the system operating voltage.
Switching surges are proportional to the system operating voltage.
As operating voltage exceeds 300 kV, switching surges become the critical factor for insulation design.
Modern UHV lines use controlled closing resistors to dampen switching overvoltages.
Enables accurate insulation coordination
Reduces cost of excessive line insulation
Requires complex circuit breaker closing mechanisms
Increases design complexity for EHV/UHV stations
Design of 400 kV, 765 kV, and 1200 kV transmission networks
Selection of Surge Arresters for HVDC and HVAC substations
In lower voltage systems (e.g., 33 kV), lightning strikes are the primary cause of insulation failure due to the relatively high BIL compared to the operating voltage.
Option D is incorrect because the relative danger of switching surges is negligible at lower voltages where lightning withstand levels are already very high relative to the operating voltage.
C is correct тАФ Switching overvoltages scale with system voltage and become the limiting factor for insulation design in EHV and UHV systems, whereas lightning surges remain relatively constant.
Remember that insulation is designed for the 'worst case' transient; at low voltages, lightning dominates, but at UHV, the network's own switching behavior dominates.