SOPD Voltage Control & System Reliability 5 ā Questions and Answers
Question 1: A power system is operating near its voltage stability limit. An operator increases generation at a remote plant to serve load. This action will most likely:
- Improve voltage stability by increasing active power supply
- Worsen voltage stability by increasing reactive losses on the transmission path (Correct answer)
- Have no effect on voltage stability since only reactive power matters
- Immediately cause a voltage collapse on the affected bus
Correct answer: Worsen voltage stability by increasing reactive losses on the transmission path
Increasing active power flow on a heavily loaded path also increases reactive losses (IĀ²X), consuming reactive power and pushing the system closer to voltage collapse.
Question 2: Which protection relay function is specifically designed to detect loss of excitation on a synchronous generator?
- 21 (Distance relay)
- 40 (Loss of field relay) (Correct answer)
- 51 (Time overcurrent relay)
- 27 (Undervoltage relay)
Correct answer: 40 (Loss of field relay)
ANSI function 40, the loss-of-field (LOF) relay, detects when a generator loses its excitation, preventing sustained under-excited operation that risks system voltage depression.
Question 3: Why is the Q-V curve used in voltage stability analysis in addition to the P-V curve?
- The Q-V curve shows the reactive power margin available at a specific bus under fixed active power loading (Correct answer)
- The Q-V curve eliminates the need for load flow studies
- The Q-V curve shows frequency deviations caused by reactive imbalance
- The Q-V curve is used only for distribution system analysis
Correct answer: The Q-V curve shows the reactive power margin available at a specific bus under fixed active power loading
The Q-V curve shows how much additional reactive power (MVAR margin) a specific bus can absorb before voltage collapse at a given active power level.
Question 4: An operator observes bus voltages dropping progressively across multiple substations despite reactive support being applied. This pattern most closely indicates:
- A frequency control problem caused by generation deficiency
- A progressive voltage collapse propagating through the system (Correct answer)
- A protection mis-operation causing unwanted line trips
- Normal diurnal load variation on a summer afternoon
Correct answer: A progressive voltage collapse propagating through the system
Progressive voltage decline across multiple buses despite reactive support is a hallmark of an evolving voltage collapse, requiring emergency intervention.
Question 5: Under NERC standard VAR-002, generators are required to:
- Operate only at unity power factor to minimize reactive costs
- Operate within the reactive capability limits specified by their plant operator and follow Transmission Operator voltage schedules (Correct answer)
- Supply reactive power only when directed by the Balancing Authority
- Report reactive power output annually to NERC for compliance
Correct answer: Operate within the reactive capability limits specified by their plant operator and follow Transmission Operator voltage schedules
VAR-002 requires generators to operate within their reactive capability and follow voltage schedules set by the Transmission Operator to support bulk electric system voltage.
Question 6: What is the primary reliability risk of operating a generator continuously at its under-excitation limit?
- Increased stator winding temperature
- Loss of synchronism due to insufficient synchronizing torque (Correct answer)
- Generator terminal overvoltage
- Excessive rotor heating from high field current
Correct answer: Loss of synchronism due to insufficient synchronizing torque
Operating at the under-excitation limit weakens the synchronizing torque, making the generator vulnerable to pulling out of synchronism with a small disturbance.
Question 7: A voltage schedule requires Bus A to be maintained at 1.02 pu. The operator observes Bus A at 0.98 pu. After exhausting local reactive resources, the NEXT appropriate action is to:
- Accept the deviation and log it without further action
- Coordinate with adjacent Transmission Operators for reactive support and notify reliability coordinator if limits are violated (Correct answer)
- Immediately shed 10% of load on Bus A
- Manually trip the lowest-loaded transmission line to force a reconfiguration
Correct answer: Coordinate with adjacent Transmission Operators for reactive support and notify reliability coordinator if limits are violated
When local reactive resources are exhausted, operators must coordinate with neighboring entities and notify the Reliability Coordinator if voltage limits cannot be maintained.
SOPD Voltage Control & System Reliability 5
A power system is operating near its voltage stability limit.
An operator increases generation at a remote plant to serve load.
This action will most likely: