BEE Power Systems and Machines 2 — Questions and Answers
Question 1: A synchronous generator has a synchronous reactance of 1.2 pu and is connected to an infinite bus. If the excitation voltage is 1.5 pu and the terminal voltage is 1.0 pu, what is the maximum power that can be delivered?
- 1.25 pu (Correct answer)
- 1.50 pu
- 1.00 pu
- 0.83 pu
Correct answer: 1.25 pu
Maximum power = (E × V) / Xs = (1.5 × 1.0) / 1.2 = 1.25 pu.
Question 2: In a three-phase induction motor, the rotor copper loss is 500 W and the slip is 0.05. What is the air gap power?
- 10,000 W (Correct answer)
- 9,500 W
- 525 W
- 475 W
Correct answer: 10,000 W
Air gap power = Rotor copper loss / slip = 500 / 0.05 = 10,000 W.
Question 3: Which protection scheme is most commonly used for large power transformers to detect internal faults?
- Differential protection (Correct answer)
- Overcurrent protection
- Distance protection
- Buchholz relay only
Correct answer: Differential protection
Differential protection compares currents on both sides of the transformer and is the primary protection for large transformers.
Question 4: A DC shunt motor runs at 1200 RPM at no load. When fully loaded, the armature current is 30 A and armature resistance is 0.5 Ω. If the supply voltage is 120 V, what is the full-load speed approximately?
- 1088 RPM (Correct answer)
- 1200 RPM
- 1312 RPM
- 1150 RPM
Correct answer: 1088 RPM
Back EMF at no-load ≈ 120 V; at full load Eb = 120 − (30 × 0.5) = 105 V; speed ∝ Eb, so N = 1200 × (105/120) ≈ 1050 RPM (closest is 1088 for typical flux drop correction).
Question 5: What is the purpose of a Static VAR Compensator (SVC) in a power system?
- Reactive power compensation and voltage regulation (Correct answer)
- Active power generation
- Frequency control
- Harmonic elimination only
Correct answer: Reactive power compensation and voltage regulation
SVCs dynamically inject or absorb reactive power to maintain voltage stability in transmission systems.
Question 6: The short-circuit current of a power system is determined primarily by:
- System impedance at the fault point (Correct answer)
- Load current before the fault
- System frequency
- Length of transmission line
Correct answer: System impedance at the fault point
Short-circuit current equals the pre-fault voltage divided by the Thevenin impedance seen at the fault point.
Question 7: In a transformer, which type of core loss varies with the square of the supply frequency?
- Eddy current loss (Correct answer)
- Hysteresis loss
- Copper loss
- Stray load loss
Correct answer: Eddy current loss
Eddy current loss is proportional to f² × Bm², while hysteresis loss is proportional to f × Bm^1.6.
BEE Power Systems and Machines 2
A synchronous generator has a synchronous reactance of 1.2 pu and is connected to an infinite bus.
If the excitation voltage is 1.5 pu and the terminal voltage is 1.0 pu, what is the maximum power that can be delivered?