EGSA Voltage Regulation and Excitation 3 — Questions and Answers
Question 1: The per-unit voltage regulation formula is (V_NL - V_FL) / V_FL. If no-load voltage is 480V and full-load voltage is 460V, what is the voltage regulation?
- 4.35% (Correct answer)
- 4.17%
- 2.08%
- 8.70%
Correct answer: 4.35%
VR = (480 - 460) / 460 = 20/460 = 0.0435 = 4.35%.
Question 2: What is the function of a power factor controller (PFC) used with some AVRs?
- To maintain a constant terminal voltage regardless of load
- To maintain the generator at a fixed power factor setpoint (Correct answer)
- To prevent reverse power flow
- To balance three-phase voltages
Correct answer: To maintain the generator at a fixed power factor setpoint
A PFC mode AVR regulates excitation to hold a target power factor rather than a target voltage, commonly used when a generator operates in parallel with an infinite bus (utility).
Question 3: What does the term 'synchronous reactance' (Xs) represent in a generator?
- The resistance of the stator winding
- The AC reactance that limits armature current under steady-state fault conditions (Correct answer)
- The reactance of the rotor field winding
- The leakage reactance of the exciter transformer
Correct answer: The AC reactance that limits armature current under steady-state fault conditions
Synchronous reactance is the effective AC impedance of the stator that governs steady-state current flow and is a key parameter in calculating voltage regulation.
Question 4: A generator's AVR is set to maintain 480V at the generator terminals. A remote sensing option instead measures voltage at:
- The AVR input terminals
- A point further in the distribution system, such as main bus or load center (Correct answer)
- The exciter output terminals
- The neutral grounding resistor
Correct answer: A point further in the distribution system, such as main bus or load center
Remote sensing allows the AVR to regulate voltage at a point downstream from the generator terminals, compensating for feeder voltage drops under load.
Question 5: Which condition can cause an AVR to ceiling excitation (drive field current to maximum)?
- Generator operating at rated load and unity power factor
- A three-phase fault near the generator terminals causing voltage collapse (Correct answer)
- Lightly loaded generator at leading power factor
- Loss of one phase of the sensing voltage signal
Correct answer: A three-phase fault near the generator terminals causing voltage collapse
During a terminal fault, generator voltage collapses to near zero, causing the AVR to detect a large error and drive excitation to its ceiling to try to restore voltage.
Question 6: What is the primary purpose of an underexcitation limiter (UEL) in a generator excitation system?
- To prevent overheating of the rotor field winding
- To prevent the generator from absorbing excessive reactive power and losing synchronism (Correct answer)
- To limit stator current during overload
- To protect the exciter diodes from reverse voltage
Correct answer: To prevent the generator from absorbing excessive reactive power and losing synchronism
The UEL prevents the excitation from dropping so low that the generator absorbs reactive power beyond its capability curve limits, which could lead to loss of synchronism.
Question 7: In a compound excitation system, what provides the excitation source at zero speed (before the machine is self-exciting)?
- Residual magnetism in the rotor
- A separate battery or PMG-based starting supply (Correct answer)
- The AVR draws power from the generator output
- Capacitor bank self-excitation
Correct answer: A separate battery or PMG-based starting supply
Compound excitation relies on current transformers from the generator output, so a separate startup supply (battery, PMG, or auxiliary source) is needed at zero output.
EGSA Voltage Regulation and Excitation 3
The per-unit voltage regulation formula is (V_NL - V_FL) / V_FL.
If no-load voltage is 480V and full-load voltage is 460V, what is the voltage regulation?