BEE Power Systems Engineering 2

Question 1

In a power system, the per-unit impedance of a transformer is 0.05 pu on a 100 MVA base. If the base is changed to 200 MVA, what is the new per-unit impedance?

Accepted Answer

0.10 pu

Answer

Per-unit impedance scales linearly with the MVA base: Z_new = Z_old × (MVA_new / MVA_old) = 0.05 × (200/100) = 0.10 pu.

Question 2

Which relay type is most commonly used for primary protection of transmission lines because its operating time is inversely proportional to fault current magnitude?

Accepted Answer

Inverse time overcurrent relay

Answer

Inverse time overcurrent relays operate faster for larger fault currents, making them ideal for graded transmission line protection.

Question 3

A balanced three-phase system has line voltage of 415 V. What is the phase voltage?

Accepted Answer

239.6 V

Answer

Phase voltage = Line voltage / √3 = 415 / 1.732 ≈ 239.6 V in a balanced Y-connected system.

Question 4

What is the primary function of a shunt capacitor bank installed on a distribution feeder?

Accepted Answer

Improve power factor and reduce reactive power demand

Answer

Shunt capacitor banks supply reactive (VAR) power locally, improving power factor and reducing reactive current flow from the source.

Question 5

In the symmetrical component method, the positive-sequence, negative-sequence, and zero-sequence impedances of a generator are Z1, Z2, and Z0. For a single line-to-ground fault, the fault current is proportional to:

Accepted Answer

3 / (Z1 + Z2 + Z0)

Answer

For a single line-to-ground fault, Ia = 3Ea / (Z1 + Z2 + Z0), so fault current is 3/(Z1+Z2+Z0) times the prefault voltage.

BEE - Bachelor of Electrical Engineering Practice Test

BEE - Bachelor of Electrical Engineering Practice Test

BEE Power Systems Engineering 2