BEE Power Systems and Machines 3

Question 1

A 50 MVA, 11 kV synchronous generator has a subtransient reactance of 0.15 pu. What is the subtransient fault current (in pu) for a three-phase bolted fault at its terminals?

Accepted Answer

6.67 pu

Answer

Subtransient fault current = 1.0 / X''d = 1.0 / 0.15 = 6.67 pu.

Question 2

Which of the following best describes the 'equal area criterion' in power system stability?

Accepted Answer

A graphical method to assess transient stability by comparing accelerating and decelerating energy areas on a P-δ curve

Answer

The equal area criterion states that a system is transiently stable if the decelerating area on the P-δ curve equals or exceeds the accelerating area.

Question 3

What is the slip of a three-phase induction motor running at 1440 RPM with a synchronous speed of 1500 RPM?

Accepted Answer

0.04

Answer

Slip s = (Ns − Nr) / Ns = (1500 − 1440) / 1500 = 60/1500 = 0.04.

Question 4

In per-unit system analysis, which quantity remains the same on both sides of a transformer?

Accepted Answer

Per-unit impedance (on the same MVA base)

Answer

When converting to a common MVA and kV base, per-unit impedances are the same on both sides, simplifying transformer analysis.

Question 5

A DC series motor is preferred for traction applications because it provides:

Accepted Answer

High starting torque at low speed

Answer

In a series motor, torque ∝ Ia², giving very high torque at startup when current is high, ideal for traction.

BEE - Bachelor of Electrical Engineering Practice Test

BEE - Bachelor of Electrical Engineering Practice Test

BEE Power Systems and Machines 3