EGSA Paralleling and Switchgear Controls 3 — Questions and Answers
Question 1: What parameter does a voltage matching relay check before permitting synchronization in an automatic paralleling system?
- Generator real power output vs. rated kW
- Difference in RMS voltage magnitude between the incoming generator and the bus (Correct answer)
- Phase sequence of the generator vs. the utility
- Generator stator temperature vs. thermal limit
Correct answer: Difference in RMS voltage magnitude between the incoming generator and the bus
The voltage matching relay ensures the incoming generator's terminal voltage is within a preset window (typically ±5%) of the bus voltage before enabling a close signal.
Question 2: A generator paralleling panel uses a 'check synchronizer' (25) relay. What does this device do?
- It continuously adjusts AVR output to maintain bus voltage
- It supervises the automatic synchronizer and blocks closing if synchronizing conditions are not met (Correct answer)
- It measures load sharing error between paralleled units
- It detects islanded operation and disconnects from utility
Correct answer: It supervises the automatic synchronizer and blocks closing if synchronizing conditions are not met
The ANSI 25 check synchronizer relay monitors voltage magnitude, frequency, and phase angle differences and blocks the breaker close coil unless all synchronizing windows are satisfied.
Question 3: In droop speed governing, increasing the droop percentage on a generator while paralleled will generally result in:
- The generator taking a larger share of load for a given frequency deviation
- The generator taking a smaller share of load for the same frequency deviation (Correct answer)
- The generator frequency becoming locked to the bus regardless of load change
- An increase in reactive power output from that generator
Correct answer: The generator taking a smaller share of load for the same frequency deviation
Higher droop means a steeper speed-vs-load characteristic, so the governor reduces fuel/speed less aggressively per unit of load increase, resulting in that generator picking up proportionally less load.
Question 4: What is the function of a bus tie breaker in a switchgear lineup with two main bus sections?
- It connects the two bus sections to allow power flow between them or to isolate a faulted section (Correct answer)
- It monitors bus voltage and trips the main breaker on undervoltage
- It synchronizes incoming utility power to the generator bus
- It provides ground fault protection for each bus section independently
Correct answer: It connects the two bus sections to allow power flow between them or to isolate a faulted section
A bus tie breaker interconnects two bus sections, enabling power transfer from one section to the other or allowing isolation of a faulted section to maintain continuity on the healthy section.
Question 5: Which ANSI device number is assigned to an instantaneous overcurrent relay?
- 51
- 50 (Correct answer)
- 27
- 59
Correct answer: 50
ANSI device 50 is the instantaneous overcurrent relay, which operates without intentional time delay when current exceeds its pickup threshold.
Question 6: In a generator protection scheme, what does a negative sequence overcurrent relay (46) detect?
- Unbalanced three-phase currents caused by asymmetric loads or faults (Correct answer)
- Excessive real power reversal into the prime mover
- Ground fault current on the generator neutral
- Overvoltage on one phase due to open delta transformer connection
Correct answer: Unbalanced three-phase currents caused by asymmetric loads or faults
The ANSI 46 relay measures negative sequence current components resulting from phase imbalance, which cause double-frequency eddy currents that can rapidly overheat rotor components.
Question 7: When manually paralleling two generators using a synchroscope, the operator should initiate the breaker close command when the synchroscope pointer is:
- At the 9 o'clock position and rotating counterclockwise
- At approximately 11 o'clock and rotating slowly clockwise (Correct answer)
- Stationary at the 12 o'clock position
- At the 3 o'clock position, indicating the machines are in phase
Correct answer: At approximately 11 o'clock and rotating slowly clockwise
The operator closes the breaker just before 12 o'clock with the pointer rotating slowly clockwise, accounting for breaker closing time so the machines are in phase at actual current flow.
EGSA Paralleling and Switchgear Controls 3
What parameter does a voltage matching relay check before permitting synchronization in an automatic paralleling system?