EGSA Voltage Regulation and Excitation 5

Question 1

Which exciter type produces excitation voltage proportional to residual magnetism without any external power supply?

Accepted Answer

Self-excited shunt wound DC exciter using residual flux to bootstrap

Answer

A self-excited shunt DC exciter uses the small voltage generated from residual magnetism to initiate field current, which amplifies magnetism until rated excitation voltage is reached.

Question 2

What effect does increasing generator excitation have on a generator operating in parallel with an infinite bus (grid)?

Accepted Answer

Increases reactive power output (lagging vars injected into grid)

Answer

When tied to an infinite bus, terminal voltage is fixed, so increased excitation causes more reactive (lagging) current to flow into the grid while real power remains governed by the prime mover.

Question 3

The short-circuit ratio (SCR) of a synchronous generator is defined as:

Accepted Answer

Ratio of field current for rated open-circuit voltage to field current for rated short-circuit current

Answer

SCR = If(OC rated V) / If(SC rated I), and it equals the reciprocal of per-unit synchronous reactance; a higher SCR indicates better voltage regulation capability.

Question 4

An overexcitation relay (OEL or 59F) on a generator excitation system will activate when:

Accepted Answer

Field current exceeds a set threshold for a defined time period

Answer

The overexcitation limiter monitors field current level and duration, reducing excitation or alarming/tripping when the rotor thermal limit is at risk from sustained overexcitation.

Question 5

What happens to a generator's terminal voltage waveform if one rotating rectifier diode in a brushless excitation system fails open?

Accepted Answer

Terminal voltage develops ripple at twice the exciter frequency due to half-wave rectification

Answer

A failed open diode converts three-phase rectification to an asymmetric configuration, introducing AC ripple on the DC field supply at twice the exciter frequency, which appears as terminal voltage modulation.

(EGSA) Electrical Generating Systems Association Certified Practice Test

(EGSA) Electrical Generating Systems Association Certified Practice Test

EGSA Voltage Regulation and Excitation 5