Which of the following factors of the cable restrict the power transfer capacity of the HVDC link?

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Examoogle · Accepted Answer

Option C: Resistance offered by the line

Explanation & Concepts:
In an HVDC link, the power transfer capability is primarily limited by the thermal capacity of the conductors and the associated voltage drop caused by the line resistance. Unlike HVAC systems, where inductive and capacitive reactances limit power transfer via stability and charging current constraints, DC systems lack reactive power concerns for steady-state stability.

Working Principle: The DC power transfer is governed by the relation $P = V_d I_d$. The line resistance $R$ causes a voltage drop $\Delta V = I_d R$ and power dissipation $P_{loss} = I_d^2 R$. To maintain voltage regulation and stay within thermal limits of the cable insulation, the current is restricted, thereby capping the total power throughput.

Key Formulas: $P = V_d \cdot I_d$ — DC Power equation; $P_{loss} = I_d^2 \cdot R$ — Conduction loss constraint

Key Points: HVDC lines have no skin effect, making resistance constant across the cross-section.; Thermal limit of the cable insulation is the primary constraint for DC underground cables.; Inductance and Capacitance do not impact steady-state power transfer in DC, unlike AC transmission.; Voltage drop is linearly proportional to line resistance.

Advantages: Lower transmission losses compared to AC for long distances.; No reactive power compensation required for the line itself.

Disadvantages: High cost of converter stations.; Limited power capacity due to cable thermal insulation limits.

Applications: Long-distance bulk power transmission.; Submarine cable interconnections.

Additional Info: In HVDC, the 'stability limit' which plagues AC systems does not exist because there is no phase angle difference to maintain between the two ends of the line.. Option A and B are irrelevant for HVDC because $X_L$ depends on $f$ (which is $0$ Hz) and $X_C$ does not draw continuous charging current under steady DC conditions.

Answer: C is correct — The resistance of the line is the primary factor limiting power transfer due to thermal constraints and associated voltage drops in DC circuits.

Exam Tip: Remember that in DC systems, $f=0$, so $X_L = 2\pi fL = 0$ and $X_C = \frac{1}{2\pi fC} = \infty$; this simplifies transmission analysis to purely resistive models.

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Option A: Inductive reactance offered by the line

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Option B: Capacitive Reactance offered by the line

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Option D: None of these

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