About this previous year paper
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
| AC Filters | DC Filters |
|---|---|
| Reduce harmonics on AC side | Reduce DC ripple |
| Installed at converter station | Installed in DC line |
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