Voltage Regulation and Excitation Flashcards

A voltage sensing circuit in an AVR fails to read one phase. What effect will this typically have on excitation output?

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.

What is meant by the 'field forcing' capability of an excitation system?

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.

The V/Hz limiter in an excitation system is designed to protect against:

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.

During parallel operation, if an AVR's reactive droop is set too high on one generator, the result will be:

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.

What type of feedback signal does a current transformer (CT) provide to a compound excitation system?

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.

A generator's open-circuit characteristic (OCC) becomes nonlinear at high field currents primarily due to:

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.

What is the purpose of an excitation system stabilizer (ESS) or PSS (Power System Stabilizer)?

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.