EGSA Paralleling and Switchgear Controls 5 — Questions and Answers
Question 1: What is the primary reason phase sequence must be verified before paralleling a generator to a bus for the first time?
- Incorrect phase sequence causes the governor to over-speed the prime mover
- Incorrect phase sequence results in a massive short-circuit current when the breaker closes (Correct answer)
- Incorrect phase sequence causes the AVR to lose voltage control
- Incorrect phase sequence prevents the synchroscope from rotating
Correct answer: Incorrect phase sequence results in a massive short-circuit current when the breaker closes
If phase rotation of the incoming generator differs from the bus, closing the breaker connects mismatched phases, creating a near-bolted fault condition with destructive fault currents.
Question 2: In an automatic bus transfer scheme, what is the purpose of an 'under-voltage' supervisory relay on the normal source?
- To initiate load shedding when normal source voltage sags
- To detect normal source failure and initiate transfer to the alternate source (Correct answer)
- To prevent the generator from connecting to the bus during a sag
- To regulate bus voltage during paralleled operation
Correct answer: To detect normal source failure and initiate transfer to the alternate source
The normal source under-voltage relay monitors for voltage collapse or fault on the utility feed and initiates the automatic transfer sequence to the standby generator or alternate source.
Question 3: What is 'hunting' in the context of paralleled generator governors, and what typically causes it?
- Sustained oscillation of generator speed and load caused by excessive governor gain or droop set too low (Correct answer)
- A condition where one generator takes all the load while others unload
- The periodic cycling of AVR output due to reactive load imbalance
- Rapid switching of the bus tie breaker due to protection relay miscoordination
Correct answer: Sustained oscillation of generator speed and load caused by excessive governor gain or droop set too low
Governor hunting is sustained speed oscillation caused by over-sensitive (high gain) or overly aggressive governor response, and can lead to load swings between paralleled units.
Question 4: Which ANSI relay number is associated with a time-overcurrent (inverse time) relay?
- 50
- 27
- 51 (Correct answer)
- 87
Correct answer: 51
ANSI 51 is the time-overcurrent relay with an inverse time characteristic—trip time decreases as current magnitude increases above the pickup setting.
Question 5: What condition must be true for the 'synchronizing window' timer to start in an automatic synchronizer?
- All synchronizing conditions (voltage, frequency, phase angle) are simultaneously within their set limits (Correct answer)
- Only the frequency condition is within limits
- The bus voltage has been above 90% for at least 5 seconds
- The generator has been running for a minimum warm-up period
Correct answer: All synchronizing conditions (voltage, frequency, phase angle) are simultaneously within their set limits
The synchronizing window timer begins only when voltage magnitude, frequency difference, and phase angle difference are all simultaneously within their respective limits, confirming a valid close opportunity.
Question 6: In a switchgear control scheme, what is the purpose of a 'breaker failure' (BF or ANSI 50BF) protection function?
- To prevent breaker close when phase sequence is incorrect
- To trip adjacent upstream breakers if the primary breaker fails to open after a trip command (Correct answer)
- To detect internal arc flash inside the switchgear compartment
- To inhibit synchronizing when the breaker is already closed
Correct answer: To trip adjacent upstream breakers if the primary breaker fails to open after a trip command
The ANSI 50BF breaker failure relay starts a timer upon trip initiation; if current persists after the timer expires (indicating the breaker did not open), it issues a trip to backup breakers to clear the fault.
Question 7: When two generators operating in parallel both have isochronous governors (zero droop), what problem typically arises?
- Both generators run at different frequencies, causing a constant reactive power fight
- Load sharing becomes unstable because both governors fight for speed control at the same setpoint (Correct answer)
- The generators cannot produce enough kW to serve the load at unity power factor
- Phase angle between the two generators continually increases, causing separation
Correct answer: Load sharing becomes unstable because both governors fight for speed control at the same setpoint
Two isochronous governors on a common bus create an unstable loop—both try to maintain exactly 60 Hz, leading to load oscillations; one generator must use droop or a master/slave load sharing scheme.
EGSA Paralleling and Switchgear Controls 5
What is the primary reason phase sequence must be verified before paralleling a generator to a bus for the first time?