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Underground system cannot be operated above
220 kV
66 kV
33 kV
11 kV
66 kV
Underground power cable systems are typically limited to 66 kV due to the technical challenges associated with insulation, capacitance, and heat dissipation. Beyond this voltage level, the dielectric stress on the cable insulation becomes prohibitive, and the charging current of the cable becomes excessively high.
Underground power cable systems are typically limited to 66 kV due to the technical challenges associated with insulation, capacitance, and heat dissipation. Beyond this voltage level, the dielectric stress on the cable insulation becomes prohibitive, and the charging current of the cable becomes excessively high.
IcтАЛ=2╧АfCV тАФ charging current increases linearly with voltage and capacitance
gmaxтАЛ=rln(R/r)VтАЛ тАФ maximum dielectric stress in a cable
The primary limitation is the dielectric strength of the insulation material (e.g., XLPE). High-voltage cables act as large capacitors due to the proximity of the conductor and metallic sheath. At voltages above 66 kV, the charging current IcтАЛ=╧ЙCV becomes significant, reducing the effective power transmission capacity. Furthermore, cooling underground cables is difficult, leading to heat buildup that degrades insulation life prematurely.
Underground cables have high capacitance per unit length compared to overhead lines.
Dielectric losses increase significantly at higher voltage levels.
Heat dissipation is constrained by the soil thermal resistivity.
66 kV is the conventional limit for cost-effective underground transmission in many older power system designs.
No interference with communication lines
Less vulnerable to lightning and weather
Enhanced aesthetics in urban areas
High installation cost
Difficult fault detection and repair
High charging currents limiting transmission length
Urban power distribution
River and lake crossings
Airport surroundings
While 66 kV is the traditional answer for this classic MCQ, modern EHV/UHV XLPE technology now allows underground cables to operate at 220 kV and 400 kV using advanced cooling and insulation techniques.
The charging current is a consequence of the cable's construction: a central conductor, a dielectric, and an earthed sheath acting as two plates of a capacitor.
B is correct тАФ 66 kV is the standard traditional upper limit for underground transmission systems due to insulation and capacitive charging current constraints.
Always remember that underground cables act as massive capacitors; the resulting charging current is why their transmission length is shorter than that of overhead lines of equivalent voltage.