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Chapter 1 of 12 тАв Page 1 of 248ЁЯФТ Protected PDF тАв Watermarked
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The cables should not be operated too hot otherwise
Expansion of oil may cause sheath to bust
The oil may loose its viscosity and it may start drawing off from higher levels
Rapid increase in dielectric losses with the temperature may cause thermal instability
All of above
All of above
Operating underground cables at excessive temperatures compromises both physical integrity and electrical insulation properties. High temperatures lead to thermal expansion of oil-filled insulation and a reduction in dielectric strength, collectively risking cable failure.
Operating underground cables at excessive temperatures compromises both physical integrity and electrical insulation properties. High temperatures lead to thermal expansion of oil-filled insulation and a reduction in dielectric strength, collectively risking cable failure.
PdтАЛ=V2╧ЙCtan╬┤ тАФ Dielectric power loss in cable insulation
╬ФV=V0тАЛ╬▓╬ФT тАФ Volumetric thermal expansion of oil
Excessive heat causes volumetric expansion of insulating oil, which exerts mechanical stress on the cable sheath, potentially leading to rupture. Simultaneously, viscosity reduction leads to oil migration (draining away from high points), creating air voids that cause partial discharge. Furthermore, dielectric losses increase exponentially with temperature (PdтАЛ=V2╧ЙCtan╬┤), creating a positive feedback loop known as thermal instability, which can lead to rapid insulation breakdown.
Thermal runaway occurs when dielectric losses grow faster than the cable's heat dissipation capability.
Oil-filled cables rely on maintaining specific viscosity to prevent void formation.
Maximum operating temperature is strictly limited by the insulation material's thermal class (e.g., XLPE vs Paper).
Preventing sheath rupture
Maintaining stable dielectric strength
Requires derating of current capacity
Higher cooling costs for high-voltage installations
High-voltage underground transmission
Submarine cable power distribution
Thermal stability is a critical factor in the Ampacity rating of cables.
Option B refers to the 'drainage' effect where low-viscosity oil leaves high points in sloped installations.
D is correct тАФ Excessive heat causes physical damage via pressure, dielectric degradation via loss increase, and insulation void formation due to oil viscosity reduction.
Always relate thermal limits to the 'Losses' in power systems; in cables, heat is a function of I2R (ohmic) and dielectric losses, both of which must be controlled to prevent insulation degradation.