EGSA Voltage Regulation and Excitation 4 — Questions and Answers
Question 1: A voltage sensing circuit in an AVR fails to read one phase. What effect will this typically have on excitation output?
- Excitation will drop to minimum to protect the generator
- The AVR will sense low voltage and over-excite, raising terminal voltage (Correct answer)
- The AVR will switch to manual control automatically
- Excitation will remain at the last known setpoint indefinitely
Correct answer: The AVR will sense low voltage and over-excite, raising terminal voltage
Loss of one sensing phase causes the AVR to perceive a lower-than-actual average voltage, driving excitation higher and potentially over-voltaging the generator.
Question 2: What is meant by the 'field forcing' capability of an excitation system?
- The ability to maintain rated voltage at rated load continuously
- The ability to momentarily drive field current well above rated value to improve transient voltage recovery (Correct answer)
- Forcing the field current to zero during de-excitation
- Setting the AVR to manual mode to force a fixed field current
Correct answer: The ability to momentarily drive field current well above rated value to improve transient voltage recovery
Field forcing is the short-term capability of the excitation system to apply field current several times rated value, improving voltage recovery after sudden large load application or system faults.
Question 3: The V/Hz limiter in an excitation system is designed to protect against:
- Overvoltage at rated frequency
- Magnetic core saturation caused by high volts-per-hertz ratio at low frequency (Correct answer)
- Reverse power flow to the exciter
- Stator insulation breakdown from high current
Correct answer: Magnetic core saturation caused by high volts-per-hertz ratio at low frequency
At low frequencies, magnetic flux increases for a given voltage, potentially saturating the core; the V/Hz limiter reduces excitation to keep the ratio within safe limits during startup or frequency dips.
Question 4: During parallel operation, if an AVR's reactive droop is set too high on one generator, the result will be:
- That generator will carry most of the reactive load, possibly exceeding reactive capability (Correct answer)
- Improved voltage regulation at the load bus
- Increased real power output from that generator
- System frequency will increase
Correct answer: That generator will carry most of the reactive load, possibly exceeding reactive capability
Excessive droop causes terminal voltage to fall steeply with reactive load, making that generator carry less reactive load than intended—if too low, it carries too much.
Question 5: What type of feedback signal does a current transformer (CT) provide to a compound excitation system?
- A voltage signal proportional to terminal voltage
- A current signal proportional to stator output current for compounding (Correct answer)
- A frequency signal for speed regulation
- A power factor signal for reactive load sharing
Correct answer: A current signal proportional to stator output current for compounding
In compound excitation, CTs sample stator output current to boost excitation proportionally to load current, helping maintain voltage as load increases without requiring an active AVR.
Question 6: A generator's open-circuit characteristic (OCC) becomes nonlinear at high field currents primarily due to:
- Increased copper losses in the field winding
- Magnetic saturation of the stator and rotor iron core (Correct answer)
- Frequency deviation from rated speed
- Brush contact resistance increasing with temperature
Correct answer: Magnetic saturation of the stator and rotor iron core
As field current increases, the magnetic iron saturates, requiring disproportionately more field current to produce additional terminal voltage—this creates the knee in the OCC curve.
Question 7: What is the purpose of an excitation system stabilizer (ESS) or PSS (Power System Stabilizer)?
- To maintain constant terminal voltage during steady-state operation
- To add damping to generator rotor oscillations by modulating excitation (Correct answer)
- To prevent overexcitation by limiting field current
- To synchronize the generator to the bus during paralleling
Correct answer: To add damping to generator rotor oscillations by modulating excitation
A PSS adds a supplementary signal to the AVR based on rotor speed or power oscillations to provide damping torque and prevent sustained inter-machine or inter-area oscillations.
EGSA Voltage Regulation and Excitation 4
A voltage sensing circuit in an AVR fails to read one phase.
What effect will this typically have on excitation output?