HVDC Transmission MCQ [Free PDF] – 15 Objective Questions with Answers

A line-commutated converter is fundamentally controlled through its firing angle, giving one principal control degree of freedom per converter bridge for steady-state DC voltage/current regulation. Unlike VSC converters, an LCC cannot independently control active and reactive power.

Modern efficient line-commutated HVDC converters use series-connected high-power thyristor valves. Thyristors can be triggered at a controlled firing angle and naturally commutate off using the AC system voltage.

A ±200 kV bipolar HVDC system has one pole at +200 kV and the other at −200 kV with respect to ground. Therefore the pole-to-pole transmission voltage is 200 − (−200) = 400 kV.

Line-commutated HVDC uses thyristors and depends on the AC system voltage for commutation, so adequate short-circuit strength and reactive-power support are important. AC voltage disturbances can produce commutation failure, particularly at the inverter. Key relation: LCC reactive demand is a significant fraction of transmitted power and is compensated locally.

Exam focus: Strong AC system and commutation margin matter for LCC. Common trap: LCC cannot normally operate a passive AC network without a voltage source.

Line-commutated HVDC uses thyristors and depends on the AC system voltage for commutation, so adequate short-circuit strength and reactive-power support are important. AC voltage disturbances can produce commutation failure, particularly at the inverter. Key relation: LCC reactive demand is a significant fraction of transmitted power and is compensated locally.

Exam focus: Strong AC system and commutation margin matter for LCC. Common trap: LCC cannot normally operate a passive AC network without a voltage source.

Line-commutated HVDC uses thyristors and depends on the AC system voltage for commutation, so adequate short-circuit strength and reactive-power support are important. AC voltage disturbances can produce commutation failure, particularly at the inverter. Key relation: LCC reactive demand is a significant fraction of transmitted power and is compensated locally.

Exam focus: Strong AC system and commutation margin matter for LCC. Common trap: LCC cannot normally operate a passive AC network without a voltage source.

HVDC converts AC to DC for transmission and back to AC at the receiving end. Long lines/cables avoid charging current and skin effect and can have lower losses, but converter stations are costly and LCC schemes consume reactive power. Key relation: Typical chain: AC generation → rectifier → DC link → inverter → AC grid/distribution.

Exam focus: HVDC advantage is in the link, not DC generation/distribution in conventional grids. Common trap: Do not claim HVDC eliminates converter terminal equipment.

“HVDC system” in power-system exams normally refers to a high-voltage DC transmission link embedded between AC systems. The conversion terminals are what allow two AC systems—including asynchronous systems—to exchange power over a DC link. Key relation: Typical chain: AC generation/grid → AC/DC converter → HVDC line/cable → DC/AC converter → AC transmission/distribution grid.

Exam focus: HVDC changes the transmission medium, not the conventional AC nature of generation and distribution grids. Common trap: Reading “HVDC” as if the entire power system from generator to consumer were DC.

HVDC converts AC to DC for transmission and back to AC at the receiving end. HVDC can interconnect asynchronous AC systems. Key relation: Typical chain: AC generation → rectifier → DC link → inverter → AC grid/distribution.

Exam focus: HVDC advantage is in the link, not DC generation/distribution in conventional grids. Common trap: Do not claim HVDC eliminates converter terminal equipment.

Line-commutated HVDC uses thyristors and depends on the AC system voltage for commutation, so adequate short-circuit strength and reactive-power support are important. AC voltage disturbances can produce commutation failure, particularly at the inverter. Key relation: LCC reactive demand is a significant fraction of transmitted power and is compensated locally.

Exam focus: Strong AC system and commutation margin matter for LCC. Common trap: LCC cannot normally operate a passive AC network without a voltage source.

In a parallel MTDC system all terminals operate at essentially the same DC voltage, so power rating is mainly reflected in current rating rather than valve-voltage rating. In a series MTDC system the same current flows through all terminals, so individual terminal power is mainly reflected in its voltage contribution rather than valve-current rating. Thus both printed statements are false, giving option D.

HVDC converts AC to DC for transmission and back to AC at the receiving end. Long lines/cables avoid charging current and skin effect and can have lower losses, but converter stations are costly and LCC schemes consume reactive power. Key relation: Typical chain: AC generation → rectifier → DC link → inverter → AC grid/distribution.

Exam focus: HVDC advantage is in the link, not DC generation/distribution in conventional grids. Common trap: Do not claim HVDC eliminates converter terminal equipment.

Because all terminals in a series MTDC link carry the same DC current, the natural coordinated strategy is to control a common current and regulate each terminal power through its DC voltage. The strategy described in option B is therefore used for series MTDC operation.

Voltage instability in an MTDC/HVDC network is strongly influenced by which terminal regulates DC voltage and by whether that terminal is operating as rectifier or inverter; it is not universally “more severe at the VST irrespective of mode.” The correct truth combination in the source is option B.

The key distinction is commutation. LCC relies on the AC system for commutation and consumes reactive power; VSC uses self-commutating devices and synthesized AC voltage, giving flexible P/Q control and better weak-grid capability. Key relation: Conceptually, active power is strongly related to converter voltage-angle difference, while reactive power is strongly related to voltage-magnitude difference (subject to network/converter limits).

Exam focus: VSC keyword pair: self-commutated + independent/flexible P-Q control. Common trap: Transferring the “strong AC system required” limitation from LCC to VSC.