BEE Power Systems Engineering 3 — Questions and Answers
Question 1: A transmission line has a sending-end voltage of 230 kV and a receiving-end voltage of 220 kV. What is the voltage regulation?
- 4.35%
- 4.55% (Correct answer)
- 10%
- 2.17%
Correct answer: 4.55%
Voltage regulation = (V_NL − V_FL) / V_FL × 100 = (230 − 220) / 220 × 100 ≈ 4.55%.
Question 2: In power systems, what is the purpose of an auto-recloser on a distribution feeder?
- Prevent voltage sag during motor starting
- Automatically restore service after a temporary fault by re-energizing the line (Correct answer)
- Limit inrush current to transformers
- Provide backup protection for the substation bus
Correct answer: Automatically restore service after a temporary fault by re-energizing the line
Auto-reclosers automatically open on a fault and reclose after a brief delay, restoring service if the fault was transient (e.g., lightning, tree branch).
Question 3: What is the significance of the 'critical clearing time' in transient stability analysis?
- The time beyond which a transmission line cannot carry rated current
- The maximum fault duration for which the system remains transiently stable (Correct answer)
- The minimum time for a relay to detect a fault
- The time constant of the governor response
Correct answer: The maximum fault duration for which the system remains transiently stable
Critical clearing time is the maximum time a fault can persist before the generator loses synchronism; faults cleared faster than this keep the system stable.
Question 4: In a power transformer, what causes hysteresis and eddy current losses?
- Load current flowing through the winding resistance
- Alternating magnetic flux in the core material (Correct answer)
- Capacitive charging current in the insulation
- Harmonic currents from nonlinear loads
Correct answer: Alternating magnetic flux in the core material
Both hysteresis and eddy current losses are core (no-load) losses caused by the cyclic magnetization and induced circulating currents in the iron core.
Question 5: A 3-phase, 60 Hz synchronous generator has 4 poles. What is its synchronous speed?
- 900 RPM
- 1200 RPM
- 1500 RPM
- 1800 RPM (Correct answer)
Correct answer: 1800 RPM
Synchronous speed Ns = 120f/P = 120×60/4 = 1800 RPM.
Question 6: What is 'reactive power' in an AC power system and what unit is used to measure it?
- Power consumed as heat, measured in watts (W)
- Power stored and returned by reactive elements, measured in volt-amperes reactive (VAR) (Correct answer)
- Apparent power supplied by the source, measured in volt-amperes (VA)
- Power dissipated in the resistance, measured in kilowatts (kW)
Correct answer: Power stored and returned by reactive elements, measured in volt-amperes reactive (VAR)
Reactive power (Q) represents energy oscillating between source and reactive elements (inductors/capacitors) and is measured in VAR or MVAR.
Question 7: In the equal-area criterion for transient stability, what do the accelerating area and decelerating area represent?
- Energy gained by the rotor during fault and energy lost during post-fault swing (Correct answer)
- Real and reactive power consumed during disturbance
- Positive and negative sequence network contributions
- Load increase and load shedding during a contingency
Correct answer: Energy gained by the rotor during fault and energy lost during post-fault swing
The accelerating area is kinetic energy gained by the rotor during the fault; the decelerating area is kinetic energy absorbed post-fault; stability requires the decelerating area to be at least as large.
BEE Power Systems Engineering 3
A transmission line has a sending-end voltage of 230 kV and a receiving-end voltage of 220 kV.
What is the voltage regulation?