(SEM VII) THEORY EXAMINATION 2024-25 HVDC & AC TRANSMISSION

KEE072 – HVDC & AC TRANSMISSION

Complete Solved Question Paper

SECTION A

(Attempt all questions – 2 × 10 = 20 marks)

Q1(a) Power transfer capacity: EHV AC vs DC

EHV AC: Limited by stability, reactive power, and skin effect

HVDC: Higher power transfer for same conductor size, no reactive power
HVDC has higher power transfer capacity

Q1(b) Role of bundled conductors

Reduces corona loss                                  Reduces radio interference

Lowers surface voltage gradient                Increases power handling capability

Q1(c) Impact of switching surges

Cause overvoltages                                    Stress insulation

Can damage equipment                            Important in EHV system design

Q1(d) Radio interference in EHV AC

Radio interference is caused by corona discharge producing electromagnetic noise, which affects radio and communication signals near transmission lines.

Q1(e) Role of creepage distance

Distance along insulator surface                Prevents surface flashover

Important under polluted conditions        Improves insulation reliability

Q1(f) Impact of pollution on EHV lines

Increases leakage current                          Causes flashover

Reduces insulation strength                      Increases maintenance cost

Q1(g) Firing angle

Firing angle (α) is the delay angle at which thyristors are triggered in an HVDC converter.

Q1(h) Types of DC links

Monopolar                                               Bipolar                                          Homopolar

Q1(i) AC vs DC filters

Q1(j) Role of smoothing reactor

Reduces DC current ripple                                     Improves current continuity

Protects converters                                                Limits fault current

SECTION B

(Attempt any three – 10 × 3 = 30 marks)

Q2(a) Surface voltage gradient & corona

High surface voltage gradient initiates corona      Causes power loss, noise, RI

Reduced by bundled conductors and larger diameter conductors

Q2(b) Ferro-resonance effects

Causes overvoltage                                                Overheating of transformers

Insulation failure                                                     System instability

Q2(c) Efficiency of impulse generator

Depends on:                                                            Circuit configuration

Number of stages                                                    Charging voltage

Component losses                                                    Proper design improves output efficiency.

Q2(d) 6-pulse & 12-pulse converters

6-pulse: Simple, higher harmonics                          12-pulse: Lower harmonics, better efficiency
Used to improve power quality.

Q2(e) Role of surge arresters

Protect equipment from overvoltage                       Divert surge current to ground

Prevent insulation damage                                       Essential for HVDC protection

SECTION C

Q3(a) Corona discharge in EHV systems

Corona is partial discharge around conductors due to ionization of air.

Effects:                                                                     Power loss

Audible noise                                                            Radio interference

Ozone formation                                                       Reduced efficiency

Q3(b) UHVAC vs lower voltage systems

Advantages:                                                             Higher power transfer

Reduced losses                                                          Fewer lines required

Disadvantages:                                                        High initial cost

Environmental impact                                               Complex insulation design

Q4(a) Properties of corona pulses

Short duration pulses                                                High frequency

Cause RI and noise                                                    Affect line performance

Q4(b) Reducing switching overvoltages

Controlled switching                                                 Surge arresters

Pre-insertion resistors                                               Proper insulation coordination

Q5(a) Voltage multiplier circuit

Uses diodes and capacitors                                      Converts AC to high DC voltage

Used in HV testing                                                    Example: Cockcroft–Walton circuit

Q5(b) Impedance matching in divider

Ensures accurate voltage measurement                   Prevents signal distortion

Improves frequency response                                  Essential for HV measurement accuracy

Q6(a) Rectifier & inverter in DC link

Rectifier: Converts AC to DC, controls power input

Inverter: Converts DC to AC, controls power output
Power flow controlled by firing angle.

Q6(b) Overlap angle

Time during which two thyristors conduct               Caused by source inductance

Reduces output DC voltage                                      Affects converter performance

Q7(a) MTDC systems

Types:                                                                       Radial MTDC

Ring MTDC                                                                Mesh MTDC

Used to connect multiple terminals on DC network.

Q7(b) Commutation failure

Causes:                                                                     AC voltage dip

Faults                                                                         Weak AC system

Effects:                                                                     Loss of power transfer

Increased current stress                                            System instability