Charging current in transmission line

Question

Examoogle · Accepted Answer

Option D: Unpredictable

Explanation & Concepts:
The impact of charging current on line losses depends entirely on the loading condition of the transmission line. Charging current is a shunt capacitive phenomenon that can either reduce or increase $I^2R$ losses depending on whether it compensates for or exacerbates the existing load current.

Working Principle: In a transmission line, the capacitance between conductors causes a charging current $I_c = j\omega CV$. Under light load conditions, the charging current can partially cancel the inductive load current, reducing the total current magnitude and thus reducing $I^2R$ losses. However, at full or heavy load, the charging current adds to the load current vectorially, increasing the total line current and consequently increasing line losses.

Key Formulas: $I_c = j \omega C V$ — charging current magnitude; $P_{loss} = 3 |I_{line}|^2 R$ — total active power loss per phase

Key Points: Charging current is proportional to the line voltage and operating frequency.; Shunt capacitance is uniformly distributed along the length of the transmission line.; Effect on loss is dependent on the phase angle difference between load current and charging current.; Ferranti effect is a related phenomenon caused by charging current at no-load.

Advantages: Voltage profile improvement at light loads; Partial compensation of lagging reactive power

Disadvantages: Voltage rise at the receiving end (Ferranti effect); Increased thermal loading on switchgear during low load conditions

Applications: Long EHV and UHV transmission lines; Underground power cables (high capacitance)

Additional Info: The original question's correct answer choice D is technically the most accurate because the impact on losses is not fixed but variable.. Options A and B represent partial truths applicable only to specific load scenarios (Light load vs Heavy load).

Answer: D is correct — The effect of charging current on line losses depends on the existing load current and power factor, making it variable rather than purely additive or subtractive.

Exam Tip: Always remember that in power systems, reactive power flows are bidirectional and sensitive to the system's V-I phase relationship; never assume an effect is always beneficial or detrimental without specifying the load condition.

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Option A: Reduces the line losses

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Option B: Increases the line losses

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Option C: Will not affect the line losses

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