EGSA Paralleling and Switchgear Controls 5

Question 1

What is the primary reason phase sequence must be verified before paralleling a generator to a bus for the first time?

Accepted Answer

Incorrect phase sequence results in a massive short-circuit current when the breaker closes

Answer

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?

Accepted Answer

To detect normal source failure and initiate transfer to the alternate source

Answer

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?

Accepted Answer

Sustained oscillation of generator speed and load caused by excessive governor gain or droop set too low

Answer

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?

Accepted Answer

51

Answer

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?

Accepted Answer

All synchronizing conditions (voltage, frequency, phase angle) are simultaneously within their set limits

Answer

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.

(EGSA) Electrical Generating Systems Association Certified Practice Test

(EGSA) Electrical Generating Systems Association Certified Practice Test

EGSA Paralleling and Switchgear Controls 5