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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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Insulation coordination for UHV lines is done based on
Lightning surges
Lightning surges and switching surges
Switching surges
None of above
Switching surges
Insulation coordination for Ultra High Voltage (UHV) transmission lines (typically above 400 kV) is primarily dictated by internal switching surges rather than lightning surges. As voltage levels increase, the dielectric strength of the insulation against lightning stays relatively constant, but the magnitude of switching overvoltages becomes the limiting factor for air-gap clearance requirements.
Insulation coordination for Ultra High Voltage (UHV) transmission lines (typically above 400 kV) is primarily dictated by internal switching surges rather than lightning surges. As voltage levels increase, the dielectric strength of the insulation against lightning stays relatively constant, but the magnitude of switching overvoltages becomes the limiting factor for air-gap clearance requirements.
Vsw≈(2.0−3.0)Vph — typical range for switching surge magnitude relative to phase voltage
d=k⋅Vswn — relationship where gap length d is determined by switching surge voltage Vsw
In UHV systems, the ratio of switching surge magnitude to lightning impulse strength is such that switching transients dominate the insulation design. Switching surges are generated by operations like circuit breaker switching, fault initiation, or clearing, resulting in slow-front waves that require larger phase-to-ground and phase-to-phase clearances.
Switching surges are 'slow-front' transients (duration ≈ 250/2500 μs).
Lightning surges are 'fast-front' transients (duration ≈ 1.2/50 μs).
For EHV and UHV, the air-gap insulation strength is significantly lower for switching surges than for lightning impulses.
Cost optimization in UHV transmission relies on controlling switching surges using Pre-Insertion Resistors (PIR).
Optimized tower design and reduced transmission line costs
Precise protection coordination for UHV systems
Requires high-cost switching surge control equipment
Complex dielectric testing procedures
765 kV and 1200 kV transmission line design
Substation insulation design and surge arrester rating
Standard insulation levels are determined by the 'critical flashover voltage' (CFO) for switching surges.
Option A is incorrect because lightning surges are the primary design factor for low-voltage distribution systems, not UHV transmission systems.
Option B is incorrect because, while both exist, switching surges form the dominant constraint for UHV.
C is correct — Switching surges determine the air-gap clearance requirements for UHV lines because the insulation withstand strength for slow-front surges is significantly lower than for fast-front lightning impulses.
Always remember: Lightning governs low voltage (Distribution), while switching governs ultra-high voltage (Transmission).