EGSA Voltage Regulation and Excitation 4

Question 1

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

Accepted Answer

The AVR will sense low voltage and over-excite, raising terminal voltage

Answer

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?

Accepted Answer

The ability to momentarily drive field current well above rated value to improve transient voltage recovery

Answer

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:

Accepted Answer

Magnetic core saturation caused by high volts-per-hertz ratio at low frequency

Answer

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:

Accepted Answer

That generator will carry most of the reactive load, possibly exceeding reactive capability

Answer

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?

Accepted Answer

A current signal proportional to stator output current for compounding

Answer

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.

(EGSA) Electrical Generating Systems Association Certified Practice Test

(EGSA) Electrical Generating Systems Association Certified Practice Test

EGSA Voltage Regulation and Excitation 4