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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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The charging current drawn by the cable
Lags behind the voltage by 90°
Leads the voltage by 90°
Leads the voltage by 180°
None of above
Leads the voltage by 90°
An underground cable acts as a capacitor because the core (conductor) and the sheath (ground) are separated by a dielectric insulation. In any pure capacitor, the current leads the applied voltage by exactly 90°. Therefore, the charging current drawn by a cable due to its inherent shunt capacitance leads the system voltage by 90°.
An underground cable acts as a capacitor because the core (conductor) and the sheath (ground) are separated by a dielectric insulation. In any pure capacitor, the current leads the applied voltage by exactly 90°. Therefore, the charging current drawn by a cable due to its inherent shunt capacitance leads the system voltage by 90°.
Ic=V⋅ωC=V⋅2πfC — Charging current formula
Xc=2πfC1 — Capacitive reactance of the cable
The cable behaves as a parallel plate capacitor where the core is one plate, the metallic sheath/earth is the other, and the insulation is the dielectric medium. When AC voltage is applied, the charging current Ic flows to charge and discharge this capacitance. Since Ic=XcV=V⋅jωC, the current leads the voltage by a phase angle of 90°.
Underground cables have high shunt capacitance compared to overhead lines.
Charging current is constant as long as the supply voltage and frequency remain stable.
Charging current can cause voltage rise in lightly loaded transmission lines (Ferranti effect).
Charging current exists even when the load at the receiving end is zero.
Useful for voltage regulation in long lines (Ferranti effect).
Does not contribute to ohmic heating loss like real power currents.
Increases the total current rating requirement for the cable.
Can lead to excessive dielectric stress in insulation.
High-voltage underground transmission systems.
Submarine power cables.
The charging current is directly proportional to the frequency and the capacitance of the cable.
Option A is incorrect as lagging current is a characteristic of inductive circuits (like motors or transformers).
Option C is incorrect as a 180° phase shift implies the current is in exact opposition to the voltage, which is not characteristic of simple reactive components.
B is correct — The charging current of a cable leads the voltage by 90° because the cable functions as a pure capacitor.
Always remember that in cables, shunt capacitance is the dominant parameter, whereas in overhead transmission lines, series inductance is usually dominant.