Correct Answer

Concept & PrincipleElectricalPower System

Option A

Increases

Quick Summary: The charging MVAR of a transmission line is directly proportional to the frequency and the square of the line voltage. As the supply frequency increases, the capacitive reactance of the line decreases, leading to a higher charging current and consequently an increase in charging MVAR.

💡 Explanation

The charging MVAR of a transmission line is directly proportional to the frequency and the square of the line voltage. As the supply frequency increases, the capacitive reactance of the line decreases, leading to a higher charging current and consequently an increase in charging MVAR.

🔢 Key Formulas

$Q_c = V^2 \cdot 2\pi f C$ — Charging reactive power formula

$X_c = \frac{1}{2\pi f C}$ — Capacitive reactance

⚙️ Working Principle

A transmission line acts like a capacitor due to the potential difference between conductors and the ground (or between conductors themselves). The charging current is given by $I_c = V \cdot \omega C$ , where $\omega = 2\pi f$ . Since the reactive power $Q_c = V \cdot I_c$ , substituting $I_c$ yields $Q_c = V^2 \cdot 2\pi f C$ . Thus, $Q_c \propto f$ . Increasing frequency $f$ directly increases the charging reactive power.

📌 Key Points

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Charging MVAR is the reactive power generated by the line capacitance.
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It is significant in long EHV (Extra High Voltage) transmission lines.
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Higher frequency reduces the capacitive reactance ( $X_c = 1/\omega C$ ), allowing more current to flow through the shunt capacitance.

✅ Advantages

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Provides voltage support at light load conditions.
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Can help in maintaining voltage profile in lightly loaded long lines.

❌ Disadvantages / Limitations

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Causes Ferranti effect at light loads (voltage rise at receiving end).
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Increases the total current the line must carry, reducing effective transmission capacity.

🛠️ Applications / Uses

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Used in planning reactive power compensation schemes.
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Essential for stability analysis in power systems.

📄 Additional Information

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The charging MVAR effect is most pronounced in underground cables compared to overhead lines due to higher capacitance.
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Option B (Decreases) is incorrect because an increase in frequency decreases the impedance offered by the shunt capacitance, resulting in higher current flow.

📊 Diagram / Illustration

✅

A is correct — The charging MVAR is directly proportional to the supply frequency, hence it increases as the frequency increases.

Core Concepts Used

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Shunt Capacitance Reactive Power Frequency Dependence

💡 EXAM TIP

Always remember the relation $Q = V^2 \omega C$ for transmission lines; it helps in understanding both voltage stability and Ferranti effect concepts.

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