BEE Power Systems and Machines 5 — Questions and Answers
Question 1: In a power system, the 'swing equation' describes:
- The relationship between rotor acceleration, mechanical power input, and electrical power output (Correct answer)
- The oscillation of voltage at the load bus
- The frequency response of protective relays
- The variation of reactive power with voltage
Correct answer: The relationship between rotor acceleration, mechanical power input, and electrical power output
The swing equation M(d²δ/dt²) = Pm − Pe governs the dynamic behavior of the generator's rotor angle during disturbances.
Question 2: An overhead transmission line has a distributed capacitance to ground. Under steady-state AC operation, this capacitance causes the line to:
- Generate reactive power (leading) (Correct answer)
- Absorb reactive power (lagging)
- Have no effect on reactive power
- Reduce active power transfer
Correct answer: Generate reactive power (leading)
Shunt capacitance generates leading reactive power (VArs), which can raise voltage levels along the line.
Question 3: A three-phase, 415 V induction motor draws 20 kW at a power factor of 0.8 lagging. What is the reactive power consumed?
- 15 kVAR (Correct answer)
- 20 kVAR
- 25 kVAR
- 16 kVAR
Correct answer: 15 kVAR
Q = P × tan(θ) = 20 × tan(cos⁻¹0.8) = 20 × 0.75 = 15 kVAR.
Question 4: Which type of DC generator has the best voltage regulation under varying load?
- Compound generator (cumulative) (Correct answer)
- Series generator
- Separately excited generator
- Shunt generator
Correct answer: Compound generator (cumulative)
A cumulative compound generator combines series and shunt field effects to maintain nearly constant terminal voltage from no-load to full-load.
Question 5: In symmetrical component analysis, a single line-to-ground (SLG) fault is characterized by:
- Positive, negative, and zero sequence currents all being equal (Correct answer)
- Only positive sequence current flowing
- Zero sequence current only
- Positive and negative sequence currents only
Correct answer: Positive, negative, and zero sequence currents all being equal
For an SLG fault, all three sequence currents (I1, I2, I0) are equal and the sequence networks are connected in series.
Question 6: What does the 'short circuit ratio' (SCR) of a synchronous generator indicate?
- The ratio of field current for rated open-circuit voltage to field current for rated short-circuit current (Correct answer)
- The maximum fault current the machine can supply
- The ratio of rated kVA to short-circuit kVA
- The generator's thermal withstand capability
Correct answer: The ratio of field current for rated open-circuit voltage to field current for rated short-circuit current
SCR = If,OC(rated V) / If,SC(rated I), and it is approximately equal to 1/Xd (pu), indicating the machine's sensitivity to excitation changes.
Question 7: The purpose of a tap changer in a power transformer is to:
- Adjust the turns ratio to regulate secondary voltage under varying load or supply conditions (Correct answer)
- Reduce transformer core losses
- Improve the transformer's power factor
- Provide short-circuit protection
Correct answer: Adjust the turns ratio to regulate secondary voltage under varying load or supply conditions
Tap changers vary the effective turns ratio, allowing operators to maintain the secondary voltage within acceptable limits despite changes in load or primary voltage.
BEE Power Systems and Machines 5
In a power system, the 'swing equation' describes: