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Examoogle тАв User тАв info@examoogle.com тАв EE-2024-8821
Chapter 1 of 12 тАв Page 1 of 248ЁЯФТ Protected PDF тАв Watermarked
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The operating voltage and consequently the electric stress on the dielectric of solid type cable is increased from a low value, the dielectric power factor remains almost unchanged up to a certain value of the stress beyond which it increases very rapidly. This is due to increase in
Resistivity of dielectric material
Ionization of voids present in the dielectric
Core to core capacitance of the cable
Core to earth capacitance of the cable
Ionization of voids present in the dielectric
In solid-type impregnated paper cables, the dielectric power factor remains stable at low electric stress. However, as the voltage exceeds a certain critical threshold, air-filled voids within the insulation begin to ionize, leading to a sharp increase in dielectric loss and power factor.
In solid-type impregnated paper cables, the dielectric power factor remains stable at low electric stress. However, as the voltage exceeds a certain critical threshold, air-filled voids within the insulation begin to ionize, leading to a sharp increase in dielectric loss and power factor.
PdтАЛ=V2╧ЙCtan╬┤ тАФ Dielectric power loss in a cable
tan╬┤=IcтАЛIrтАЛтАЛ тАФ Dielectric power factor (loss tangent)
As the voltage stress reaches the ionization potential of the air/gas trapped in the insulation voids, gas discharge occurs within these pockets. This ionization produces conductive paths and energy dissipation in the form of heat and localized partial discharges, which manifest as a rapid rise in the dielectric loss tangent (tan╬┤).
Solid type cables use mass-impregnated paper as the primary insulation.
Voids are formed due to the thermal expansion and contraction cycles of the impregnating oil.
Once ionization starts, the chemical breakdown of the dielectric accelerates due to ozone and nitrous acid formation.
Ionization is the primary cause of dielectric deterioration in medium voltage underground cables.
Cost-effective for lower voltage levels
Well-understood manufacturing process
Susceptible to ionization at higher voltage stresses
Limited by thermal cycles causing void formation
Low voltage power distribution networks
Underground transmission systems up to 33 kV
The phenomenon described is commonly known as 'Partial Discharge' (PD).
Option A is incorrect because resistivity generally decreases with temperature/stress, not causing a sharp increase in power factor.
Options C and D are incorrect because cable capacitances are geometric constants determined by cable dimensions and permittivity, not dependent on stress levels in this manner.
B is correct тАФ The rapid increase in dielectric power factor beyond a critical stress level is caused by the ionization of air-filled voids within the insulation.
Always remember that in power systems, any sudden non-linear jump in dielectric loss is almost always attributable to gas ionization in voids or moisture ingress.