BEE FREE BEE Bachelor of Electrical Engineering Power Systems and Machines Questions and Answers 2 — Questions and Answers
Question 1: What is the primary purpose of a buck-boost transformer in a power distribution system?
- To regulate voltage levels within a narrow range (Correct answer)
- To convert AC to DC power
- To isolate two circuits galvanically
- To increase the power factor of the system
Correct answer: To regulate voltage levels within a narrow range
A buck-boost transformer adjusts voltage up or down by a small percentage to maintain acceptable voltage levels in distribution feeders.
Question 2: In a three-phase synchronous generator, what happens when the field excitation is increased beyond the value needed for unity power factor?
- The generator operates at leading power factor
- The generator operates at lagging power factor
- The generator supplies reactive power to the grid (Correct answer)
- The generator absorbs reactive power from the grid
Correct answer: The generator supplies reactive power to the grid
Over-excitation causes the synchronous generator to supply reactive power (VAR) to the grid, appearing as a lagging power factor source.
Question 3: Which type of DC motor is best suited for electric traction applications requiring high starting torque?
- DC series motor (Correct answer)
- DC shunt motor
- DC compound motor
- Permanent magnet DC motor
Correct answer: DC series motor
The DC series motor provides very high starting torque because its field winding is in series with the armature, so torque increases rapidly with current.
Question 4: What is the swing equation used for in power system stability analysis?
- Determining the rotor angle dynamics of a synchronous machine after a disturbance (Correct answer)
- Calculating the steady-state power flow in transmission lines
- Finding the short-circuit current during a fault
- Estimating transmission line losses under varying loads
Correct answer: Determining the rotor angle dynamics of a synchronous machine after a disturbance
The swing equation relates the accelerating power to the rotor angle of a synchronous machine, which is fundamental to transient stability studies.
Question 5: In a per-unit system, if a transformer has a per-unit impedance of 0.05 on its own base, what does this indicate about its voltage regulation?
- Approximately 5% voltage drop at full load under unity power factor (Correct answer)
- The transformer can handle 5% overload
- The transformer efficiency is 95%
- The short-circuit current is 5% of rated current
Correct answer: Approximately 5% voltage drop at full load under unity power factor
A per-unit impedance of 0.05 means the transformer drops about 5% of rated voltage at full load and unity power factor.
Question 6: What is the main advantage of using HVDC transmission over HVAC for long-distance bulk power transfer?
- Lower transmission losses and no reactive power compensation needed (Correct answer)
- Higher voltage levels can be achieved more easily
- Standard transformers can be used at both ends
- Protection systems are simpler and less expensive
Correct answer: Lower transmission losses and no reactive power compensation needed
HVDC eliminates reactive power losses and skin effect issues, resulting in lower overall losses for long-distance transmission compared to HVAC.
BEE FREE BEE Bachelor of Electrical Engineering Power Systems and Machines Questions and Answers 2
What is the primary purpose of a buck-boost transformer in a power distribution system?