EGSA Generator and Alternator Principles 4 ā Questions and Answers
Question 1: What does the open-circuit characteristic (OCC) of a synchronous generator represent?
- Terminal voltage vs. load current at constant speed
- Terminal voltage vs. field current at rated speed with no load (Correct answer)
- Field current vs. armature current at rated load
- Power output vs. rotor angle at rated excitation
Correct answer: Terminal voltage vs. field current at rated speed with no load
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?
- Synchronous reactance (Xd)
- Transient reactance (Xd')
- Subtransient reactance (Xd") (Correct answer)
- Negative-sequence reactance (X2)
Correct answer: Subtransient reactance (Xd")
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?
- Sense rotor speed to control frequency
- Sense terminal voltage and adjust field current to maintain set-point voltage (Correct answer)
- Sense load current to limit short-circuit output
- Sense coolant temperature to adjust excitation
Correct answer: Sense terminal voltage and adjust field current to maintain set-point voltage
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?
- Running the generator above rated speed, causing overvoltage
- The generator drawing power from the bus and operating as a motor, which can damage the prime mover (Correct answer)
- Connecting two generators out of phase, causing high circulating currents
- Operating the generator at leading power factor, causing under-excitation
Correct answer: The generator drawing power from the bus and operating as a motor, which can damage the prime mover
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?
- Line voltage = Phase voltage Ć ā3
- Line voltage = Phase voltage / ā3
- Line voltage = Phase voltage Ć 2
- Line voltage = Phase voltage (they are equal) (Correct answer)
Correct answer: Line voltage = Phase voltage (they are equal)
In a delta connection, each winding is directly connected between two lines, so line voltage and phase voltage are identical.
Question 6: What is the primary cause of eddy current losses in the stator core of a generator?
- Hysteresis in the magnetic material
- Circulating currents induced in the core laminations by the changing magnetic flux (Correct answer)
- IĀ²R heating in the stator windings
- Windage friction of the rotating field
Correct answer: Circulating currents induced in the core laminations by the changing magnetic flux
Eddy current losses are caused by circulating currents induced in the conducting core material by the alternating magnetic flux; laminating the core reduces these losses.
Question 7: What is the 'power angle' (torque angle) Ī“ in a synchronous generator?
- Phase angle between voltage and current at the terminals
- Angle between the rotor magnetic field axis and the stator resultant field axis
- Angular displacement between rotor poles at no-load and full-load (Correct answer)
- Phase difference between two parallel generator outputs
Correct answer: Angular displacement between rotor poles at no-load and full-load
The power angle Ī“ is the angular displacement between the rotor pole axis and the stator resultant magnetic field, increasing with load; exceeding 90Ā° leads to loss of synchronism.
EGSA Generator and Alternator Principles 4
What does the open-circuit characteristic (OCC) of a synchronous generator represent?