FREE BEE Bachelor of Electrical Engineering Power Systems and Machines 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?

Accepted Answer

0.04

Answer

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?

Accepted Answer

It eliminates the need to refer impedances to the primary or secondary side of transformers.

Answer

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?

Accepted Answer

Ferranti Effect, caused by the line's capacitance and the resulting charging current.

Answer

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?

Accepted Answer

Core losses and copper losses

Answer

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?

Accepted Answer

The ability of the system to remain in synchronism following a large, sudden disturbance like a fault or loss of a generator.

Answer

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.

BEE - Bachelor of Electrical Engineering Practice Test

BEE - Bachelor of Electrical Engineering Practice Test

FREE BEE Bachelor of Electrical Engineering Power Systems and Machines Questions and Answers