EGSA Voltage Regulation and Excitation 5 — Questions and Answers
Question 1: Which exciter type produces excitation voltage proportional to residual magnetism without any external power supply?
- Static exciter with thyristor bridge
- Self-excited shunt wound DC exciter using residual flux to bootstrap (Correct answer)
- Brushless AC exciter with PMG pilot exciter
- Battery-backed static excitation system
Correct answer: Self-excited shunt wound DC exciter using residual flux to bootstrap
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)?
- Increases real power output
- Increases reactive power output (lagging vars injected into grid) (Correct answer)
- Raises system frequency
- Reduces stator current automatically
Correct answer: Increases reactive power output (lagging vars injected into grid)
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:
- Ratio of full-load armature current to short-circuit current
- Ratio of field current for rated open-circuit voltage to field current for rated short-circuit current (Correct answer)
- Ratio of synchronous reactance to subtransient reactance
- Ratio of rated kVA to short-circuit kVA
Correct answer: Ratio of field current for rated open-circuit voltage to field current for rated short-circuit current
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:
- Field current falls below minimum for a set time
- Field current exceeds a set threshold for a defined time period (Correct answer)
- Terminal voltage exceeds rated voltage by more than 110%
- The AVR transfers to manual mode unexpectedly
Correct answer: Field current exceeds a set threshold for a defined time period
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?
- Terminal voltage drops smoothly to zero
- Terminal voltage develops ripple at twice the exciter frequency due to half-wave rectification (Correct answer)
- Terminal voltage remains normal because redundant diodes carry full load
- Terminal voltage increases due to reduced field losses
Correct answer: Terminal voltage develops ripple at twice the exciter frequency due to half-wave rectification
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.
Question 6: In AVR tuning, increasing the proportional gain (Kp) without adjusting other terms will typically cause:
- Slower voltage response with better steady-state accuracy
- Faster response but increased tendency for voltage oscillation or hunting (Correct answer)
- No change in transient response, only steady-state improvement
- Reduced exciter ceiling voltage
Correct answer: Faster response but increased tendency for voltage oscillation or hunting
Higher proportional gain increases the corrective action for a given error, speeding response but also amplifying noise and increasing the risk of voltage hunting or instability.
Question 7: What is the significance of the generator's 'ceiling voltage' specification in excitation system design?
- The maximum steady-state voltage the AVR will regulate to
- The maximum exciter output voltage available during transient forcing conditions (Correct answer)
- The maximum voltage the stator insulation is rated for
- The setpoint at which the V/Hz limiter activates
Correct answer: The maximum exciter output voltage available during transient forcing conditions
Ceiling voltage is the maximum instantaneous DC output the excitation system can produce during forcing; it determines how quickly field current can be driven up during voltage recovery events.
EGSA Voltage Regulation and Excitation 5
Which exciter type produces excitation voltage proportional to residual magnetism without any external power supply?