SOPD Grid Reliability 4 — Questions and Answers
Question 1: When a generator experiences a sudden loss of field excitation, what is the primary reliability concern?
- The unit will immediately overspeed and damage the turbine
- The unit will absorb reactive power from the system, potentially depressing voltage (Correct answer)
- Active power output will surge, overloading nearby transmission lines
- Frequency in the control area will immediately rise above 60.5 Hz
Correct answer: The unit will absorb reactive power from the system, potentially depressing voltage
Loss of field excitation causes a generator to operate as an induction generator, absorbing reactive power (Mvars) from the system instead of supplying it, which can depress system voltage.
Question 2: What is 'load shedding' and when is it used as a reliability tool?
- Transferring load from one feeder to another during maintenance
- Intentionally disconnecting customer load to prevent frequency collapse or cascading outages when generation is insufficient (Correct answer)
- Reducing a generator's output to prevent overloading its step-up transformer
- Scheduling industrial customers to reduce demand during off-peak hours
Correct answer: Intentionally disconnecting customer load to prevent frequency collapse or cascading outages when generation is insufficient
Load shedding is a controlled interruption of customer service used as a last resort to balance generation and load, preventing frequency collapse and wider system blackouts.
Question 3: In power system stability, what type of instability is characterized by growing oscillations in generator rotor angles?
- Voltage instability
- Transient (angle) instability
- Small-signal (oscillatory) instability (Correct answer)
- Frequency instability
Correct answer: Small-signal (oscillatory) instability
Small-signal instability involves undamped or negatively damped low-frequency oscillations in rotor angles that grow over time, often associated with weak inter-area power system connections.
Question 4: A transmission line is operating at its Surge Impedance Loading (SIL). What does this condition mean?
- The line is carrying load equal to its thermal emergency limit
- The line neither consumes nor produces reactive power — it is in reactive power balance (Correct answer)
- The line is at risk of dynamic instability and must be de-loaded
- The line's protection relays are set to trip at exactly this loading level
Correct answer: The line neither consumes nor produces reactive power — it is in reactive power balance
At SIL, a line's capacitive charging exactly offsets its inductive reactive power consumption, making it reactive-power neutral — a useful reference point for voltage stability analysis.
Question 5: Which entity has the highest authority during a grid emergency affecting multiple Balancing Authorities?
- The Transmission Operator with the largest load served
- The Reliability Coordinator (RC) for the affected area (Correct answer)
- NERC's headquarters operations center
- FERC's emergency response team
Correct answer: The Reliability Coordinator (RC) for the affected area
The Reliability Coordinator has the widest area view and highest level of authority during system emergencies, with the ability to direct TOPs and BAs to take specific actions.
Question 6: What is the purpose of NERC's Disturbance Control Standard (DCS) / BAL-002?
- To establish voltage limits for all transmission elements
- To require Balancing Authorities to recover Area Control Error to within limits following a disturbance within a specified time (Correct answer)
- To define mandatory underfrequency load shedding thresholds
- To set maximum allowable ramp rates for generating units
Correct answer: To require Balancing Authorities to recover Area Control Error to within limits following a disturbance within a specified time
BAL-002 requires Balancing Authorities to restore ACE to within defined limits within 15 minutes following a reportable disturbance, ensuring the area does not become a sustained burden on the interconnection.
Question 7: Which protection relay function is specifically designed to detect and isolate a generator operating out of synchronism with the power system?
- Distance relay (21)
- Out-of-step relay (78) (Correct answer)
- Underfrequency relay (81)
- Differential relay (87)
Correct answer: Out-of-step relay (78)
The out-of-step relay (ANSI function 78) detects when a generator's rotor has slipped poles and is no longer in synchronism, tripping the unit to protect it and the system from damaging currents.
SOPD Grid Reliability 4
When a generator experiences a sudden loss of field excitation, what is the primary reliability concern?