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Chapter 1 of 12 тАв Page 1 of 248ЁЯФТ Protected PDF тАв Watermarked
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H.V. cables are filled with oil under pressure of gas because
Pressure provides the necessary strength
Pressure provides the necessary voltage bearing capacity
Pressure will avoid the formation of voids
All of these
Pressure will avoid the formation of voids
Extra High Voltage (EHV) cables utilize oil under positive pressure to ensure the dielectric integrity of the insulation system. The pressure prevents the formation of gaseous voids (bubbles) within the oil-impregnated paper insulation, which are primary sites for partial discharge.
Extra High Voltage (EHV) cables utilize oil under positive pressure to ensure the dielectric integrity of the insulation system. The pressure prevents the formation of gaseous voids (bubbles) within the oil-impregnated paper insulation, which are primary sites for partial discharge.
VdтАЛ=EтЛЕd тАФ where VdтАЛ is the breakdown voltage, E is dielectric strength, and d is insulation thickness
PoilтАЛ>PatmтАЛ тАФ represents the requirement for maintaining positive pressure against vacuum formation
When a cable experiences thermal expansion and contraction cycles due to load variations, temperature changes cause the oil volume to fluctuate. Without a pressurized system (like a reservoir), the cooling cycle would lead to the contraction of oil, creating vacuum pockets or gas-filled voids. Since gas has a much lower dielectric strength than oil, these voids would ionize under high electrical stress, leading to corona discharge, insulation degradation, and eventual dielectric breakdown.
Voids contain gas with a much lower dielectric constant and strength compared to insulating oil.
Partial discharge in voids leads to 'treeing' in the insulation, causing permanent damage.
Oil-filled cables allow for higher voltage ratings (up to 500kV and above).
Pressure reservoirs maintain constant internal pressure during thermal load cycling.
Elimination of internal voids and ionization
Superior heat dissipation compared to solid cables
High dielectric strength allowing for EHV applications
Complex maintenance and installation
High cost due to auxiliary equipment (pumps, reservoirs)
Risk of oil leakage into the environment
Submarine power transmission cables
High-capacity underground urban transmission lines
Large EHV substations connections
The process of avoiding gas bubbles is specifically targeted at suppressing 'Ionization' or 'Partial Discharge' (PD).
Option A is incorrect because cable strength is primarily determined by mechanical construction and material properties, not just oil pressure.
Option B is incorrect as pressure does not directly increase the voltage bearing capacity of the conductor, but rather protects the insulation from failure.
C is correct тАФ Pressure will avoid the formation of voids, which are detrimental sites for partial discharge and insulation breakdown in EHV cables.
Always remember that in high-voltage insulation engineering, 'voids' are the primary enemy of long-term cable reliability due to their propensity for partial discharge initiation.