Q/P ratio _____________ in HVDC & __________ in EHVAC as the distance of the transmission increases.

Question

Examoogle · Accepted Answer

Option D: remains inchanged, increases

Explanation & Concepts:
In HVDC transmission, the Q/P ratio (Reactive power to Active power) remains constant because HVDC converters operate independently of line length regarding reactive power demand at the receiving end. In contrast, EHVAC lines have inherent shunt capacitance and series inductance that increase reactive power requirements with distance, causing the Q/P ratio to increase as the transmission length grows.

Working Principle: In EHVAC, the charging current $I_c = V \omega C$ increases with line length, contributing to reactive power generation/absorption. HVDC systems use converter stations (rectifiers and inverters) that typically consume reactive power proportional to the real power $P$ handled, keeping their ratio ($Q/P$) stable regardless of the DC line length.

Key Formulas: $Q_{AC} \approx V^2 \omega C \cdot l$ — Reactive power due to line capacitance (increases with length $l$); $\frac{Q}{P} = 	an(\phi)$ — Basic ratio definition

Key Points: HVDC converter stations require reactive power compensation but it is independent of DC line length.; EHVAC line capacitance causes the charging current to increase linearly with length, impacting the Q/P ratio.; Long-distance EHVAC requires shunt reactors to compensate for the increase in reactive power.; The Q/P ratio is a critical parameter for voltage regulation and stability in power systems.

Advantages: HVDC enables efficient long-distance power transfer without distance-based reactive power accumulation.; HVDC lines provide better control over active power flow compared to EHVAC.

Disadvantages: EHVAC systems suffer from Ferranti effect at long distances due to charging current.; HVDC converter stations are expensive compared to AC substations.

Applications: Long-distance bulk power transmission.; Asynchronous interconnection of power grids.

Additional Info: The term Q/P ratio essentially describes the reactive power support needed per unit of active power transferred.. Option D was chosen to correct the typo 'inchanged' to 'unchanged' while maintaining the correct physical principle.

Answer: D is correct — The Q/P ratio remains unchanged in HVDC because converter reactive demand is independent of line length, whereas it increases in EHVAC due to line capacitance effects.

Exam Tip: Always remember that in EHVAC, the charging current $I_c$ is proportional to voltage and length, which is why shunt compensation becomes mandatory for long lines.

Examoogle · Answer

Option A: increase, decrease

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Option B: decrease, increase

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Option C: remains unchanged, remain unchanged

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