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Transmission of power by AC cable is impossible beyond
40 km
400 km
200 km
1000 km
40 km
Transmission of power by AC underground cables is limited to approximately 40 km due to the high charging current resulting from the significant shunt capacitance of the cable insulation. As the cable length increases, this capacitive charging current dominates the conductor capacity, leaving negligible capacity for useful power transfer.
Transmission of power by AC underground cables is limited to approximately 40 km due to the high charging current resulting from the significant shunt capacitance of the cable insulation. As the cable length increases, this capacitive charging current dominates the conductor capacity, leaving negligible capacity for useful power transfer.
ICтАЛ=V├Ч╧ЙCl тАФ Charging current in an AC cable
C=ln(D/d)2╧А╧╡0тАЛ╧╡rтАЛтАЛ тАФ Capacitance per unit length of a cable
Unlike overhead lines where shunt capacitance is low, underground cables are constructed with conductors placed in close proximity to the metallic sheath, separated by insulation, forming a capacitor. The charging current ICтАЛ=V├Ч2╧АfC├Чl increases linearly with length l and voltage V. When the length exceeds a critical limit (approx. 40 km), the charging current consumes the entire thermal rating of the cable, making power transmission ineffective.
Underground cables have much higher shunt capacitance than overhead lines.
Charging current represents a reactive power flow that occupies the conductor's thermal capacity.
High-voltage AC (HVAC) transmission via cable becomes uneconomical beyond 40-50 km.
For distances exceeding 40-50 km, HVDC (High Voltage Direct Current) transmission is preferred as it does not suffer from charging current effects.
No electromagnetic interference with external systems.
Suitable for high-density urban areas and submarine crossings.
Higher security against weather and environmental hazards.
Extremely high initial installation cost.
Limited power transfer distance due to charging current.
Difficulty in fault location and maintenance.
Urban power distribution.
Under-sea power links.
Connection between power plants and city substations through densely populated zones.
The charging current effect is negligible in overhead lines for short to medium distances because the spacing between conductors is large, resulting in very low capacitance.
Options B, C, and D are incorrect as they represent distances where the charging current would exceed the total current carrying capacity (ampacity) of the cable conductor.
A is correct тАФ AC power transmission in underground cables is limited to about 40 km due to excessive capacitive charging current.
Always remember that for long-distance subsea power transmission, we use HVDC to eliminate the limitation imposed by the charging current (reactive power flow).