EGSA Generator and Alternator Principles 4

Question 1

What does the open-circuit characteristic (OCC) of a synchronous generator represent?

Accepted Answer

Terminal voltage vs. field current at rated speed with no load

Answer

The OCC (also called the magnetization curve) plots terminal voltage against field current at rated synchronous speed with the armature open-circuited.

Question 2

During a three-phase fault at generator terminals, which reactance determines the initial subtransient fault current?

Accepted Answer

Subtransient reactance (Xd")

Answer

Subtransient reactance (Xd") is the smallest and determines the highest initial fault current during the first few cycles immediately after fault inception.

Question 3

What is the purpose of the Automatic Voltage Regulator (AVR) sensing input in a generator excitation system?

Accepted Answer

Sense terminal voltage and adjust field current to maintain set-point voltage

Answer

The AVR continuously senses generator terminal voltage and adjusts field excitation current to maintain the voltage at the desired set point under varying load conditions.

Question 4

What is meant by 'generator motoring' and why is it dangerous?

Accepted Answer

The generator drawing power from the bus and operating as a motor, which can damage the prime mover

Answer

Motoring occurs when the prime mover loses power but the generator remains connected, drawing power from the bus to motor the shaft, which can severely damage turbines or engines.

Question 5

In a delta-connected generator stator, what is the relationship between line voltage and phase voltage?

Accepted Answer

Line voltage = Phase voltage (they are equal)

Answer

In a delta connection, each winding is directly connected between two lines, so line voltage and phase voltage are identical.

(EGSA) Electrical Generating Systems Association Certified Practice Test

(EGSA) Electrical Generating Systems Association Certified Practice Test

EGSA Generator and Alternator Principles 4