EGSA Paralleling and Switchgear Controls 2 — Questions and Answers
Question 1: What is the primary purpose of a dead bus close (DBC) relay in a paralleling switchgear system?
- To prevent closing onto an energized bus
- To allow closing onto a de-energized bus without synchronization checks (Correct answer)
- To trip a generator when bus voltage drops
- To measure reactive power imbalance
Correct answer: To allow closing onto a de-energized bus without synchronization checks
A dead bus close relay bypasses synchronization checks and allows a generator breaker to close onto a bus that has no voltage, enabling fast black-start restoration.
Question 2: In isochronous load sharing between two paralleled generators, what controls the power output of each unit?
- Voltage regulators proportionally split reactive load
- Speed droop governors adjusted by operators
- A load share controller that trims governor speed references (Correct answer)
- The generator with higher rated kW takes priority
Correct answer: A load share controller that trims governor speed references
An isochronous load share controller sends bias signals to each governor's speed reference to equalize kW output proportionally across paralleled units.
Question 3: Which condition must be verified before closing a generator breaker onto a live bus using a synchroscope?
- Generator voltage equals exactly 1.0 per unit
- Synchroscope pointer rotates slowly clockwise and approaches 12 o'clock (Correct answer)
- Generator frequency is 5 Hz higher than bus frequency
- Generator power factor equals unity
Correct answer: Synchroscope pointer rotates slowly clockwise and approaches 12 o'clock
The synchroscope pointer should rotate slowly in the fast (clockwise) direction and be closing on the 12 o'clock position, indicating near-synchronous conditions with the incoming machine slightly faster.
Question 4: What fault condition does a reverse power relay (32) protect against in a paralleled generator set?
- Over-frequency caused by load rejection
- Motorizing of the generator when it draws power from the bus (Correct answer)
- Reverse reactive power from a leading power factor load
- Negative sequence voltage on the bus
Correct answer: Motorizing of the generator when it draws power from the bus
A reverse power relay trips the generator breaker when the machine begins motoring—drawing real power from the bus rather than supplying it, which can damage the prime mover.
Question 5: In a transfer switch application, what defines a 'closed transition' transfer?
- The load is disconnected before the alternate source is connected
- Both sources are briefly connected simultaneously during the transfer (Correct answer)
- The transfer occurs only when the load is at zero current
- The normal source is locked out before the emergency source closes
Correct answer: Both sources are briefly connected simultaneously during the transfer
Closed transition transfer briefly parallels both sources so the load sees no interruption, requiring synchronization between the two sources before completing the transfer.
Question 6: What role does the phase angle matching requirement play during automatic synchronizing?
- It ensures generator terminal voltage magnitude matches bus voltage
- It prevents the breaker from closing when voltage phase difference exceeds a set limit (Correct answer)
- It controls the reactive current sharing between generators
- It measures the slip frequency between the two systems
Correct answer: It prevents the breaker from closing when voltage phase difference exceeds a set limit
The phase angle check window (typically ±5–10°) prevents the synchronizer from issuing a close command unless the instantaneous phase difference between generator and bus is within an acceptable range.
Question 7: When a generator experiences a loss-of-excitation (40) condition while paralleled to a bus, what is the expected behavior?
- The generator over-excites and exports excessive reactive power
- The generator under-excites and absorbs reactive power from the system, risking stator overheating (Correct answer)
- The generator immediately loses real power output and trips on reverse power
- The generator voltage rises above nominal and activates the overvoltage relay
Correct answer: The generator under-excites and absorbs reactive power from the system, risking stator overheating
Loss of field causes the generator to lose its excitation source, causing it to operate as an induction generator absorbing reactive (VAR) power from the bus, which can overheat the stator.
EGSA Paralleling and Switchgear Controls 2
What is the primary purpose of a dead bus close (DBC) relay in a paralleling switchgear system?