100+ HVDC Transmission MCQs with Answers

High Voltage Direct Current (HVDC) transmission has become an important technology in modern power systems, particularly for large-scale power transfer, long-distance transmission, submarine cable connections, and interconnection of asynchronous power networks. As power systems continue to evolve with the integration of renewable energy sources and increasingly complex transmission networks, a sound understanding of HVDC principles, components, converter technologies, control strategies, and protection mechanisms is essential for electrical engineering students and professionals.

This article presents a comprehensive collection of 100 multiple-choice questions on HVDC transmission, designed to support academic learning, competitive examinations, technical interviews, and self-assessment. The questions cover fundamental concepts as well as important topics such as LCC and VSC technologies, six-pulse and twelve-pulse converters, firing and extinction angles, commutation overlap, harmonics, filters, smoothing reactors, reactive power, HVDC protection, and transmission applications.

Each question is accompanied by the correct answer, a helpful hint, and a concise explanation. This format enables readers not only to identify the correct option but also to understand the underlying concept, making the collection useful for both revision and conceptual preparation.

1). HVDC stands for?

High Voltage Direct Current

High Voltage Double Current

High Variable Direct Current

High Voltage Distributed Current

2). The main advantage of HVDC over HVAC for very long-distance transmission is:

Higher corona loss

Lower transmission losses and improved controllability

Requirement of more conductors

Higher charging current

3). In a conventional HVDC system, the AC-to-DC conversion is performed by:

Inverter

Rectifier

Transformer only

Chopper

4). In an HVDC link, the DC-to-AC conversion is performed by:

Rectifier

Inverter

Cycloconverter

Transformer

5). A conventional line-commutated HVDC converter generally uses:

IGBTs

MOSFETs

Thyristors

GTOs only

6). LCC in HVDC stands for:

Load Controlled Converter

Line Commutated Converter

Line Current Controller

Load Current Converter

7). VSC-HVDC stands for:

Voltage Source Converter HVDC

Variable Supply Converter HVDC

Voltage Switching Controller HVDC

Variable Source Current HVDC

8). Which device is commonly used in modern VSC-HVDC converters?

Thyristor

IGBT

Mercury arc valve

Diode only

9). The principal semiconductor device used in classical LCC-HVDC is:

SCR/Thyristor

IGBT

MOSFET

TRIAC

10). The number of pulses in a standard three-phase bridge converter is:

11). A 12-pulse HVDC converter is normally formed using:

Two six-pulse bridges

Three four-pulse bridges

One 12-phase bridge

D. Two three-pulse bridges

12). The main purpose of a 12-pulse converter is to:

Increase frequency

Reduce harmonics

Eliminate transform

Increase AC voltage only

13). The characteristic AC-side harmonics of an ideal p-pulse converter are:

p, 2p, 3p

np ± 1

n/p

2n + 1 only

14). The dominant characteristic AC harmonics of a six-pulse converter are:

2nd and 4th

3rd and 6th

5th and 7th

8th and 10th

15). The dominant characteristic AC harmonics of a 12-pulse converter are:

3rd and 5th

5th and 7th

11th and 13th

2nd and 4th

16). The main function of smoothing reactors in HVDC is to:

Increase AC frequency

Reduce DC current ripple

Increase converter losses

Produce reactive power

17). A smoothing reactor is connected:

Only across the AC bus

In series with the DC line

In parallel with the converter transformer

Across the DC capacitor only

18). HVDC converters generally consume:

Only active power

Reactive power

No power

Only mechanical power

19). Reactive power compensation in LCC-HVDC is commonly provided by:

Shunt capacitors

Series resistors

DC reactors only

Circuit breakers only

20). An important advantage of VSC-HVDC is:

It always requires a strong AC system

Independent control of active and reactive power

It cannot operate with weak grids

It cannot reverse power

21). VSC-HVDC can supply a passive AC network because:

It does not require an external commutating voltage in the same way as LCC

It uses mechanical switches

It has no semiconductor devices

It uses only transformers

22). LCC-HVDC is particularly suitable for:

Very high-power bulk transmission

Small household supplies

Low-voltage distribution

Battery charging only

23). One major disadvantage of LCC-HVDC is:

Very low power capability

Reactive power consumption

No need for filters

No harmonics

24). A major advantage of HVDC for submarine cables is:

Large charging current

No steady-state capacitive charging current

Increased skin effect

Higher dielectric losses

25). For a long submarine cable, HVDC is often preferred because:

AC cable charging current becomes significant

DC cables require no insulation

AC cannot transmit power

HVDC has zero losses

HVDC Transmission MCQs for Exams

26). The power transmitted in a bipolar HVDC system is approximately:

P=VI

P=2VI

P=V/I

P=I/V

27). In a monopolar HVDC system, the return path may be provided by:

Earth/sea or metallic conductor

Only an AC transformer

Only a capacitor

No return path

28). A bipolar HVDC system normally consists of:

One conductor

Two conductors of opposite polarity

Three AC conductors

Four neutral conductors

29). During normal operation of a symmetrical bipolar HVDC link, earth current is ideally

Maximum

Zero

Equal to pole current

Twice the pole current

30). One important advantage of bipolar HVDC is:

No possibility of reduced-power operation

Better reliability and partial operation during one-pole outage

Higher charging current

Need for three DC conductors

31). The firing angle of an LCC converter is normally denoted by:

β only

γ only

θr

32). Increasing the firing angle of a rectifier generally:

Increases DC voltage

Decreases average DC voltage

Has no effect

Makes voltage infinite

33). For an ideal six-pulse converter, the average DC voltage is proportional to:

sin α

cos α

tan α

α²

34). In an LCC inverter, the extinction angle is commonly represented by:

β only

35). Commutation overlap in an LCC converter is mainly caused by:

Converter transformer leakage reactance

DC resistance only

Cable insulation

Smoothing capacitor

36). The overlap angle in an LCC converter is denoted by:

37). During commutation overlap?

Only one valve conducts

Two valves may conduct simultaneously in the same bridge

All valves are off

The DC current becomes zero

38). Increasing commutation overlap generally:

Increases the average DC voltage

Reduces the average DC voltage

Eliminates reactive power

Eliminates harmonics

39). The extinction angle γ in an inverter should be:

Zero under all conditions

Maintained above a minimum value

Negative

Equal to 180°C.

40). Commutation failure is particularly associated with:

VSC-HVDC

LCC-HVDC

DC batteries

Synchronous motors only

41). A major cause of commutation failure is:

AC voltage depression

Excessive DC insulation

Low cable capacitance

High-frequency switching in VSC

42). The DC current in an HVDC link is mainly controlled by:

Converter controls

Transformer oil

Transmission tower height

Insulator color

43). In a typical LCC-HVDC system, the rectifier commonly controls:

DC current

AC frequency only

Cable temperature only

Reactive power only

44). In an LCC-HVDC system, the inverter commonly controls:

Extinction angle

Transformer turns ratio only

DC resistance

Cable capacitance

45). The current margin between rectifier and inverter current controllers is used to:

Increase harmonics

Prevent control conflict

Increase transformer losses

Eliminate insulation

46). The power flow in an HVDC link can be reversed by:

Changing the polarity/current direction as appropriate

Changing only the AC frequency to zero

Removing the transformer

Opening all valves permanently

47). VSC-HVDC generally allows power reversal by:

Reversing DC voltage polarity only

Reversing DC current direction

Removing the converter

Changing transformer frequency

48). A major advantage of VSC-HVDC for offshore wind farms is:

Need for a strong receiving AC grid

Ability to connect to weak or isolated AC networks

High reactive power consumption

Requirement of mechanical commutation

49). The primary purpose of AC harmonic filters at an LCC converter station is to:

Reduce harmonics injected into the AC system

Increase DC current

Increase transformer frequency

Block all active power

50). DC filters in HVDC systems are used mainly to:

Reduce AC voltage

Reduce harmonics on the DC side

Increase reactive power consumption

Increase converter overlap

HVDC Transmission MCQs for Exams

51). HVDC converter transformers provide:

Voltage transformation and electrical isolation

Only frequency conversion

Mechanical power conversion

DC generation directly

52). The transformer phase shift used in a typical 12-pulse converter is:

10°

15°

30°

90°

53). In an HVDC station, the valve hall contains:

Converter valves

Transmission towers

AC generators

Circuit breakers only

54). HVDC converter valves require cooling because:

They generate heat due to conduction and switching losses

They generate no losses

They operate only at room temperature

Cooling increases harmonics

55). Which cooling method is commonly used for high-power HVDC converter valves?

Water cooling

Coal cooling

Mechanical cooling by gears

Vacuum only

56). The DC line of an HVDC system can be:

Overhead, underground, or submarine

Only overhead

Only underground

Only submarine

57). One important economic advantage of HVDC is:

Lower corridor requirement for comparable power transfer

Need for more conductors

Higher charging current

No converter stations

58). For the same transmitted power and voltage level, HVDC can generally use:

Fewer conductors than HVAC

More phase conductors

Six AC conductors

No conductors

59). HVDC transmission is especially advantageous for?

Very short-distance low-power distribution

Long-distance bulk power transmission

Domestic wiring

Low-voltage lighting

60). The distance at which HVDC becomes economically preferable to HVAC is called:

Critical distance or break-even distance

Corona distance

Insulation distance

Commutation distance

61). One major disadvantage of HVDC is:

Converter stations are expensive and complex

It cannot transmit bulk power

It always has higher line losses than AC

It cannot use cables

62). The HVDC converter station converts:

AC to DC or DC to AC

AC to AC only

DC to DC only

Mechanical to electrical energy only

63). The DC line resistance causes:

I2R losses

No losses

Only reactive losses

Mechanical losses

64). For a fixed transmitted power, increasing transmission voltage generally:

Increases current

Reduces current

Has no effect on current

Makes current infinite

65). If current is reduced to half while resistance remains constant, I2R loss becomes:

Twice

Four times

Half

One-fourth

66). Corona in HVDC transmission lines is associated with:

Ionization of air around the conductor

Transformer oil breakdown only

Converter firing control

DC cable capacitance onl

67). HVDC transmission lines may produce:

Audible noise and radio interference due to corona

Only mechanical noise

No environmental effects

Only transformer noise

68). The earth electrode in an HVDC system is used mainly for:

Providing a ground return path under certain operating conditions

Increasing AC frequency

Generating reactive power

Increasing transformer voltage

69). Earth return operation in HVDC may cause:

Environmental and electrochemical concerns

Zero current

Increased AC frequency

Elimination of all losses

70). Metallic return in HVDC means:

A dedicated conductor is used as the return path

The transformer becomes the return path

No return path is needed

AC current flows through the DC conductor

71). HVDC circuit breakers are generally more difficult to design than AC circuit breakers because:

DC current has no natural current zero

DC has no voltage

AC has no current

DC uses only capacitors

72). A major function of an HVDC circuit breaker is to:

Interrupt DC fault current

Increase converter losses

Produce harmonics

Generate AC frequency

73). DC faults in an HVDC line can cause:

Rapid rise of current

No change in current

Only frequency change

Only reactive power change

74). A major protection requirement for HVDC systems is:

Fast fault detection and isolation

Slow fault clearing

No fault detection

Manual operation only

75). Differential protection works by comparing:

Currents at different ends of a protected section

Transformer temperatures only

Frequencies only

Conductor colors

HVDC Transmission MCQs for Exams

76). A traveling-wave protection method primarily uses:

High-frequency fault-generated waves

Transformer oil temperature

Mechanical vibration

Reactive power only

77). The per-unit system in HVDC studies is useful because:

It simplifies calculations and comparison of quantities

It eliminates all losses

It removes harmonics

It eliminates transformers

78). The main purpose of a converter transformer tap changer is to:

Adjust AC voltage applied to the converter

Increase DC line resistance

Remove harmonics completely

Change the transmission medium

79). In an LCC converter, increasing AC system voltage generally:

Increases available DC voltage

Always decreases DC voltage

Has no effect

Eliminates current

80). The ideal no-load DC voltage of a six-pulse bridge is commonly represented as:

Vd0

Id0

Pd0

Xd0

81). The ideal average DC voltage of a six-pulse bridge is:

Vd=Vd0sin⁡α

Vd=Vd0cos⁡α

Vd=Vd0tan⁡α

Vd=Vd0/cos⁡α

82). At α = 0° for an ideal controlled rectifier, the average DC voltage is:

Zero

Maximum positive value

Negative maximum

Infinite

83). At α = 90° for an ideal six-pulse converter:

Vd=Vd0

Vd=0

Vd=−Vd0

Vd=2Vd0

84). If Vd0=100 kV and α = 60°, the ideal average DC voltage is:

100 kV

86.6 kV

50 kV

25 kV

85). If an HVDC link transmits 1000 MW at a DC voltage of 500 kV, the DC current is:

500 A

1000 A

2000 A

5000 A

86). If DC current is 2000 A and line resistance is 5 Ω, the line loss is

5 MW

10 MW

20 MW

40 MW

87). A 12-pulse converter compared with a six-pulse converter generally has:

More low-order harmonics

Reduced characteristic harmonics

No harmonics under any condition

Higher 5th harmonic

88). The AC filters in an LCC-HVDC station can also provide:

Reactive power compensation

Mechanical torque

DC insulation

Transmission towers

89). The reactive power consumption of an LCC converter increases generally with:

Converter loading and firing/extinction conditions

Zero current only

Reduced voltage only

No operating parameters

90). VSC-HVDC has an important advantage over LCC-HVDC in terms of:

Reactive power control

Requirement for commutation voltage

Harmonic generation only

Transformer weight only

91). A major application of HVDC is:

Interconnection of asynchronous AC systems

Domestic lighting

Small motor control

Household heating

92). If two AC systems are interconnected through HVDC, their frequencies:

Must always be identical

Can be different

Must be zero

Must be mechanically synchronized

93). HVDC can help improve power-system stability because:

Power flow can be rapidly controlled

It eliminates generators

It increases system frequency automatically

It prevents all faults

94). The main purpose of a DC smoothing reactor during a DC fault is also to:

Limit the rate of rise of fault current

Increase fault current

Increase AC frequency

Eliminate insulation

95). In VSC-HVDC, the DC link commonly contains:

A capacitor

Only an inductor

Only a resistor

A mechanical commutator

96). In a VSC, the switching frequency is generally:

Much higher than the AC fundamental frequency

Always equal to zero

Lower than DC frequency

Equal to mechanical speed

97). PWM in VSC-HVDC stands for:

Pulse Width Modulation

Power Wave Management

Pulse Watt Measurement

Power Width Modulator

98). A modular multilevel converter (MMC) is widely used in:

javascript:void(0)Modern VSC-HVDC systems

Mechanical relays

Conventional DC motors only

Small household transformers only

99). One important advantage of MMC-HVDC is:

High-quality multilevel output waveform

Requirement for only one semiconductor switch

No control system

No energy storage in submodules

100). Which statement best summarizes the major advantage of HVDC transmission?

It is always cheaper regardless of distance

It provides efficient long-distance bulk transmission with controllable power flow

It eliminates all converter stations

It has no losses

A strong understanding of HVDC transmission requires more than memorizing definitions and formulas; it involves connecting converter operation, power-flow control, system performance, and protection concepts into a coherent understanding of the technology. The 100 MCQs presented in this article provide a structured opportunity to evaluate that understanding across a broad range of HVDC topics.

The inclusion of answers, hints, and explanations makes the question set particularly useful for identifying knowledge gaps and reinforcing important technical principles. Learners can use these questions as a revision tool before examinations, while practicing engineers and professionals can use them to refresh essential HVDC concepts.

Regular practice with conceptual and numerical questions can improve problem-solving ability, strengthen technical confidence, and provide a solid foundation for studying advanced HVDC applications and modern converter-based transmission systems.