SOPD Frequency Control 3 — Questions and Answers
Question 1: What is 'inertia' in the context of power system frequency control?
- The tendency of governors to resist changing generator output
- The kinetic energy stored in rotating generator masses that initially resists frequency change (Correct answer)
- The reactive power stored in transmission line capacitance
- The delay time before AGC can respond to a frequency event
Correct answer: The kinetic energy stored in rotating generator masses that initially resists frequency change
System inertia is the kinetic energy stored in spinning generator rotors that temporarily resists frequency change following a generation-load imbalance.
Question 2: Under-frequency load shedding (UFLS) programs are designed to activate at what general frequency threshold in North American systems?
- 59.95 Hz
- 59.7 Hz
- 58.5–59.5 Hz depending on scheme design (Correct answer)
- 57.0 Hz
Correct answer: 58.5–59.5 Hz depending on scheme design
UFLS schemes typically have multiple stages activating between approximately 58.5 Hz and 59.5 Hz, with exact thresholds varying by utility and regional criteria.
Question 3: A system operator is monitoring a declining frequency event. The Rate of Change of Frequency (ROCOF) is -1.2 Hz/s. What does this indicate?
- The system has low inertia and is experiencing a severe generation deficit (Correct answer)
- Frequency is stable and within normal operating range
- AGC has successfully arrested the frequency decline
- The event is minor and no corrective action is needed
Correct answer: The system has low inertia and is experiencing a severe generation deficit
A high ROCOF magnitude such as -1.2 Hz/s indicates low system inertia and a significant generation-load imbalance requiring immediate response.
Question 4: What is 'secondary frequency control' in power system operations?
- The activation of UFLS relays during severe under-frequency events
- AGC action that restores frequency to scheduled value and resets tie-line flows after primary response (Correct answer)
- The manual operator action of tripping generation to correct over-frequency
- Governor droop response that arrests initial frequency deviation
Correct answer: AGC action that restores frequency to scheduled value and resets tie-line flows after primary response
Secondary frequency control (AGC) acts over minutes to restore frequency to exactly 60 Hz and return tie-line flows to scheduled values after primary response arrests the deviation.
Question 5: Which factor does NOT directly reduce available primary frequency response in an interconnection?
- Increased penetration of non-synchronous renewable generation (solar, wind inverters)
- Generators operating at maximum output with no headroom
- Generators with governors in 'blocked' or manual mode
- High ambient temperature reducing transformer efficiency (Correct answer)
Correct answer: High ambient temperature reducing transformer efficiency
Transformer efficiency affected by ambient temperature does not directly reduce primary frequency response capability, which depends on generator headroom, governor status, and synchronous inertia.
Question 6: In the ACE formula, what does a negative ACE value indicate for a balancing authority?
- The control area is generating more than its scheduled net interchange
- The control area is generating less than its scheduled net interchange plus frequency obligation (Correct answer)
- Frequency is above 60 Hz and the area should reduce generation
- Tie-line meters are reversed and require recalibration
Correct answer: The control area is generating less than its scheduled net interchange plus frequency obligation
Negative ACE means the area is under-generating relative to its schedule and frequency obligation, requiring AGC to increase generation.
Question 7: What is the significance of the 'settling frequency' after a large generation trip?
- It is the frequency at which UFLS relays reset after load shedding
- It is the steady-state frequency after governor response but before AGC fully restores 60 Hz (Correct answer)
- It is the maximum frequency deviation allowed by NERC reliability standards
- It is the frequency at which automatic generation commitment begins
Correct answer: It is the steady-state frequency after governor response but before AGC fully restores 60 Hz
Settling frequency is the quasi-steady-state value after primary (governor) response arrests the decline but before secondary (AGC) response restores 60 Hz.
SOPD Frequency Control 3
What is 'inertia' in the context of power system frequency control?