EGSA Voltage Regulation and Excitation 2 — Questions and Answers
Question 1: What does a negative voltage regulation value indicate about a generator's performance?
- Terminal voltage rises from no-load to full-load (Correct answer)
- Terminal voltage drops from no-load to full-load
- The excitation system has failed
- The AVR setpoint is too low
Correct answer: Terminal voltage rises from no-load to full-load
Negative voltage regulation means the terminal voltage increases as load is applied, which is characteristic of generators with leading power factor loads.
Question 2: In a brushless excitation system, what component rectifies AC exciter output to supply DC to the main rotor field?
- Carbon brushes and slip rings
- A rotating diode assembly mounted on the rotor shaft (Correct answer)
- A stationary thyristor bridge
- The permanent magnet pilot exciter
Correct answer: A rotating diode assembly mounted on the rotor shaft
In brushless exciters, rotating rectifier diodes mounted on the rotor shaft convert AC output of the exciter armature to DC for the main generator field without brushes or slip rings.
Question 3: A generator is operating at rated voltage with no load. When full resistive load is applied, voltage drops 8% before the AVR corrects it. This initial dip is primarily caused by:
- AVR hunting
- Generator armature resistance and reactance voltage drops
- Exciter time constant delay (Correct answer)
- Incorrect droop setting
Correct answer: Exciter time constant delay
The initial voltage dip before AVR correction is largely due to the excitation system's time constant—it takes finite time for field current to change and terminal voltage to recover.
Question 4: What is the purpose of a derivative (rate) feedback signal in an AVR?
- To provide steady-state voltage accuracy
- To improve transient response and dampen voltage oscillations (Correct answer)
- To compensate for reactive droop
- To measure generator frequency
Correct answer: To improve transient response and dampen voltage oscillations
Derivative feedback responds to the rate of voltage change, helping to dampen oscillations and speed up transient response after sudden load changes.
Question 5: Reactive droop compensation in an AVR causes generator terminal voltage to:
- Rise proportionally as reactive load increases
- Drop proportionally as reactive load increases (Correct answer)
- Remain constant regardless of reactive load
- Fluctuate based on power factor only
Correct answer: Drop proportionally as reactive load increases
Reactive droop causes terminal voltage to decrease as reactive (lagging) load increases, enabling stable reactive load sharing between parallel generators.
Question 6: A static excitation system differs from a brushless system primarily because it:
- Uses a pilot exciter instead of an AVR
- Supplies field current via slip rings from a stationary thyristor rectifier (Correct answer)
- Cannot operate without external AC power
- Uses diodes instead of thyristors
Correct answer: Supplies field current via slip rings from a stationary thyristor rectifier
Static excitation systems use thyristor rectifiers on a stationary platform and deliver controlled DC field current to the rotor through slip rings and brushes.
Question 7: When two generators are operating in parallel and one has a higher voltage setpoint, what will happen to reactive power sharing?
- The generator with higher setpoint will absorb reactive power (motoring)
- The generator with higher setpoint will supply more reactive power to the bus (Correct answer)
- Both generators will share reactive power equally regardless
- The system frequency will drop
Correct answer: The generator with higher setpoint will supply more reactive power to the bus
A generator with a higher excitation voltage setpoint will push more reactive power into the bus, taking on a greater share of the total reactive load.
EGSA Voltage Regulation and Excitation 2
What does a negative voltage regulation value indicate about a generator's performance?