SOPD Power Grid Operations & Management 3 — Questions and Answers
Question 1: Which phenomenon causes power to flow on unscheduled parallel paths through neighboring control areas?
- Loop flow (circulating power) (Correct answer)
- Frequency bias error
- Voltage flicker
- Ground fault current
Correct answer: Loop flow (circulating power)
Loop flow occurs because power follows Kirchhoff's laws and divides among all parallel paths, often flowing through unintended transmission corridors.
Question 2: What is the function of a phase angle regulator (PAR) on a transmission system?
- Regulate bus voltage by absorbing or supplying reactive power
- Control the magnitude and direction of real power flow by shifting the voltage angle (Correct answer)
- Protect the line from single-line-to-ground faults
- Provide reactive compensation during motor starting
Correct answer: Control the magnitude and direction of real power flow by shifting the voltage angle
A PAR introduces a phase angle shift between sending and receiving end voltages to control real power flow on a specific transmission path.
Question 3: During restoration after a blackout, why must operators carefully match voltage and frequency before closing ties to an energized island?
- To prevent meter calibration errors
- To avoid a damaging fault current surge that occurs when two unsynchronized sources are connected (Correct answer)
- To satisfy FERC tariff filing requirements
- To reset protective relay counters
Correct answer: To avoid a damaging fault current surge that occurs when two unsynchronized sources are connected
Connecting two asynchronous sources causes a large equalizing current that can damage equipment; synchronization checks ensure voltage magnitude, phase angle, and frequency are matched.
Question 4: What is the significance of the 'dead band' setting in a turbine governor?
- The speed range within which the governor does not respond, reducing wear from hunting (Correct answer)
- The minimum load below which a generator must be taken offline
- The thermal limit on high-pressure turbine blades
- The maximum rate of voltage change permitted by the AVR
Correct answer: The speed range within which the governor does not respond, reducing wear from hunting
A governor dead band prevents continuous hunting around the setpoint by ignoring small frequency deviations, balancing responsiveness with mechanical wear.
Question 5: A system operator needs to increase transmission capacity on an overloaded corridor. Which of the following is a valid operational tool?
- Raising line voltage above emergency limits indefinitely
- Adjusting generation dispatch to shift flows away from the overloaded line (redispatch) (Correct answer)
- Tripping the overloaded line to force load onto other paths
- Reducing reactive power production at all generators
Correct answer: Adjusting generation dispatch to shift flows away from the overloaded line (redispatch)
Redispatching generation re-routes power flows in accordance with physical network laws, relieving the overloaded corridor without removing equipment from service.
Question 6: Under NERC standards, what must a Balancing Authority do if its ACE cannot be corrected within the specified time limit?
- Immediately declare a system emergency and shed firm load
- Notify its Reliability Coordinator and implement corrective actions per the emergency operating plan (Correct answer)
- Unilaterally adjust tie-line schedules without coordination
- Shut down all interchange transactions
Correct answer: Notify its Reliability Coordinator and implement corrective actions per the emergency operating plan
NERC BAL standards require timely ACE correction; persistent ACE violations require notification of the Reliability Coordinator and execution of established emergency procedures.
Question 7: What is 'voltage stability margin' and why do operators monitor it?
- The difference between actual voltage and the maximum voltage a transformer can withstand
- The additional MW load that can be served before the system reaches the nose point of the PV curve and voltage collapses (Correct answer)
- The allowable range of frequency deviation from 60 Hz
- The margin between operating current and the line's ampacity rating
Correct answer: The additional MW load that can be served before the system reaches the nose point of the PV curve and voltage collapses
Voltage stability margin quantifies how much additional load can be absorbed before voltage collapse; monitoring it allows operators to take preventive action before instability occurs.
SOPD Power Grid Operations & Management 3
Which phenomenon causes power to flow on unscheduled parallel paths through neighboring control areas?