Free BEE Bachelor of Electrical Engineering Power Systems and Machines Questions and Answers — Questions and Answers
Question 1: A 4-pole, 50 Hz, three-phase induction motor is running at 1440 RPM. What is the slip of the motor?
- 0.08
- 0.04 (Correct answer)
- 0.96
- 1.04
Correct answer: 0.04
First, calculate the synchronous speed (Ns) using the formula Ns = 120f/P, where f is the frequency and P is the number of poles. Ns = (120 * 50) / 4 = 1500 RPM. Next, calculate the slip (s) using the formula s = (Ns - Nr) / Ns, where Nr is the rotor speed. s = (1500 - 1440) / 1500 = 60 / 1500 = 0.04.
Question 2: In power system analysis, which of the following is a primary advantage of using the per-unit system?
- It increases the precision of fault current calculations to more decimal places.
- It directly provides the physical values of voltage and current at any point in the system.
- It eliminates the need to refer impedances to the primary or secondary side of transformers. (Correct answer)
- It simplifies the calculation of transmission line capacitance and inductance.
Correct answer: It eliminates the need to refer impedances to the primary or secondary side of transformers.
A major advantage of the per-unit system is that the per-unit impedance of a transformer is the same whether computed from the primary or secondary side. This simplifies the analysis of networks containing multiple transformers with different voltage levels, as it eliminates the need for conversions across each transformer.
Question 3: A power systems engineer observes that the receiving-end voltage of a long, lightly loaded transmission line is higher than the sending-end voltage. What is this phenomenon called, and what is its primary cause?
- Skin Effect, caused by non-uniform current distribution in the conductor.
- Corona Effect, caused by ionization of air surrounding the conductor.
- Proximity Effect, caused by the magnetic fields of adjacent conductors.
- Ferranti Effect, caused by the line's capacitance and the resulting charging current. (Correct answer)
Correct answer: Ferranti Effect, caused by the line's capacitance and the resulting charging current.
The Ferranti effect is a phenomenon where the voltage at the receiving end of a long transmission line is higher than the sending end voltage. This occurs under no-load or very light load conditions. The primary cause is the charging current flowing through the distributed capacitance of the line, which leads the voltage and causes a voltage rise across the line's inductance.
Question 4: An open-circuit test and a short-circuit test are performed on a single-phase transformer. What parameters are primarily determined from these two tests, respectively?
- Copper losses and core losses
- Core losses and copper losses (Correct answer)
- Voltage regulation and efficiency
- Equivalent resistance and leakage reactance
Correct answer: Core losses and copper losses
The open-circuit (or no-load) test is performed at rated voltage and primarily determines the core (iron) losses and the shunt branch parameters (magnetizing reactance and core loss resistance). The short-circuit test is performed at a reduced voltage with rated current and primarily determines the full-load copper losses and the series impedance (equivalent resistance and leakage reactance).
Question 5: Which of the following best describes the 'transient stability' of a power system?
- The ability of the system to remain in synchronism during and after small, slow-changing load variations.
- The ability to withstand a temporary overvoltage without insulation breakdown.
- The ability of the system to remain in synchronism following a large, sudden disturbance like a fault or loss of a generator. (Correct answer)
- The overall efficiency of the power system under peak load conditions.
Correct answer: The ability of the system to remain in synchronism following a large, sudden disturbance like a fault or loss of a generator.
Transient stability refers to the ability of synchronous machines in a power system to remain synchronized after being subjected to a large, sudden disturbance. Such disturbances include short circuits, the sudden loss of a large load or generator, or a line switching event. This is distinct from steady-state stability, which deals with small, gradual changes.
Question 6: The capability curve of a synchronous generator defines the operational limits of the machine. Which of the following factors does NOT typically define a boundary on this curve?
- Stator armature heating limit
- Prime mover power limit
- Rotor field heating limit
- Generator's synchronous speed (Correct answer)
Correct answer: Generator's synchronous speed
The capability curve is a plot of active power (MW) vs. reactive power (MVAR) and is bounded by three main limits: 1) The MVA limit set by armature (stator) current and associated heating, 2) The MW limit set by the power rating of the prime mover (e.g., turbine), and 3) The maximum excitation limit set by the field (rotor) current and associated heating. The synchronous speed is a fundamental characteristic determined by frequency and poles, not an operational boundary on the P-Q plane.
Free BEE Bachelor of Electrical Engineering Power Systems and Machines Questions and Answers
A 4-pole, 50 Hz, three-phase induction motor is running at 1440 RPM.
What is the slip of the motor?