HVDC transmission lines are __________ as compared to HVAC lines.

Question

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Option C: more expensive for short distances

Explanation & Concepts:
HVDC transmission is more expensive for short distances compared to HVAC because of the high cost of terminal equipment like converters, rectifiers, and inverters. While the line costs for HVDC are lower due to needing fewer conductors, these savings only offset the high capital expenditure of converter stations after a specific distance known as the 'break-even distance'.

Working Principle: In an HVDC system, AC power from the source is converted to DC using thyristor or VSC-based rectifier stations, transmitted over a two-conductor line, and then converted back to AC via an inverter station at the receiving end. The high capital cost of these stations remains relatively constant regardless of the line length. Conversely, HVAC lines have cheaper terminal equipment (transformers) but higher line costs due to skin effect, proximity effect, and charging currents.

Key Formulas: $C_{Total} = C_{Terminals} + C_{Line} 	imes L$ — Total cost function where $L$ is distance; $P_{Loss} = I^2 R$ — Power loss in conductors

Key Points: HVDC converter stations involve complex power electronics equipment.; The break-even distance is typically between 500 km to 800 km for overhead lines.; HVDC is more economical for very long-distance bulk power transfer.; HVAC suffers from reactive power compensation requirements for long lines.

Advantages: No skin effect, resulting in full utilization of conductor cross-section.; Lower transmission line costs (only 2 conductors needed).; No reactive power compensation needed for the line itself.

Disadvantages: High cost of converter stations (rectifiers and inverters).; Generation of harmonics requiring expensive filtering.; Difficulty in tapping the DC line for intermediate power extraction.

Applications: Long-distance bulk power transmission.; Submarine cable transmission.; Asynchronous interconnection of two AC grids.

Additional Info: For distances shorter than the break-even point, HVAC is preferred due to low terminal costs.. Option B is incorrect because HVDC becomes cheaper, not more expensive, for very long distances.

Answer: C is correct — HVDC is more expensive for short distances due to the high capital investment required for converter stations at both ends.

Exam Tip: Remember: HVDC saves on line costs (copper/aluminum) but spends on station costs; it is the inverse of the HVAC economic model.

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Option A: difficult to erect

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Option B: more expensive for long distances

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Option D: less expensive for short distances

Feature	HVAC	HVDC
Terminal Cost	Low (Transformers)	High (Converters)
Line Cost	High	Low

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