SOPD Load Dispatching 4 — Questions and Answers
Question 1: What is the purpose of 'tie-line bias control' in interconnected system operations?
- Each control area adjusts generation proportional to both ACE and its frequency bias, preventing over-correction by any single area (Correct answer)
- Tie lines are physically biased toward higher voltage to reduce losses
- Generation units near tie lines are given priority dispatch to reduce congestion
- Bias control limits the maximum power flow allowed on inter-area connections
Correct answer: Each control area adjusts generation proportional to both ACE and its frequency bias, preventing over-correction by any single area
Tie-line bias control ensures each control area absorbs its share of a disturbance based on its frequency bias setting, preventing inadvertent over- or under-correction.
Question 2: A dispatcher notices that a heavily loaded 345 kV line is approaching its thermal rating. Which action directly relieves the thermal overload?
- Rescheduling generation to redirect power flow away from the overloaded line (Correct answer)
- Increasing exciter output on generators to raise system voltage
- Activating underfrequency load shedding relays in the affected region
- Requesting neighboring utilities to increase their spinning reserve
Correct answer: Rescheduling generation to redirect power flow away from the overloaded line
Re-dispatching generation changes the direction of power flow in the network, which is the primary tool for relieving thermal overloads on specific transmission elements.
Question 3: Which metric best describes a control area's ability to follow NERC's CPS1 standard?
- A statistical measure of how well ACE is controlled relative to a target bound over a rolling 12-month period (Correct answer)
- The percentage of time that tie-line flows are within ±1% of scheduled values
- The number of minutes per day that frequency deviates beyond 59.95 Hz or 60.05 Hz
- The average MW difference between scheduled and actual generator output
Correct answer: A statistical measure of how well ACE is controlled relative to a target bound over a rolling 12-month period
CPS1 (Control Performance Standard 1) measures the statistical correlation between a control area's ACE and system frequency error over a 12-month period.
Question 4: What is the difference between 'regulation' and 'load following' as ancillary services?
- Regulation handles fast, second-by-second frequency deviations via AGC; load following manages slower, predictable load trends over minutes (Correct answer)
- Regulation is provided only by hydroelectric units; load following uses thermal units
- Regulation applies to voltage control; load following applies to frequency control
- Regulation is an emergency service; load following is an economic service
Correct answer: Regulation handles fast, second-by-second frequency deviations via AGC; load following manages slower, predictable load trends over minutes
Regulation (via AGC) corrects short-term random frequency deviations, while load following accommodates the predictable, slower trend of load changing over 15-minute intervals.
Question 5: A generating unit operating at its maximum economic output is asked to 'back down' despite high system demand. What is the most likely reason?
- A transmission constraint requires reducing generation in that area to prevent a line overload (Correct answer)
- The unit's bid price is now above the system marginal cost
- The governor droop setting requires the unit to reduce output as frequency rises
- The unit's minimum run-time requirement has been met and shutdown is scheduled
Correct answer: A transmission constraint requires reducing generation in that area to prevent a line overload
Transmission congestion often forces dispatchers to curtail generation in specific areas, even at high demand, to keep power flows within equipment thermal ratings.
Question 6: What does 'N-1 security criterion' mean for a dispatcher planning load dispatch?
- The system must be able to withstand the loss of any single element without violating reliability standards (Correct answer)
- Each generator must carry at least 1 MW above its minimum load at all times
- Frequency must not deviate more than 1 Hz from nominal under any condition
- One backup dispatcher must always be available to take over control room duties
Correct answer: The system must be able to withstand the loss of any single element without violating reliability standards
The N-1 criterion requires that the system remain stable and within operating limits following the sudden loss of any single element such as a generator or transmission line.
Question 7: When a dispatcher 'curtails' an energy transaction, what action is being taken?
- Reducing or canceling a scheduled power transfer between parties due to reliability or congestion constraints (Correct answer)
- Shutting down a generating unit below its minimum operating point
- Removing a load block from service during a demand response event
- Lowering the voltage at a bulk power substation to reduce load
Correct answer: Reducing or canceling a scheduled power transfer between parties due to reliability or congestion constraints
Curtailment of an energy transaction means reducing or eliminating a scheduled interchange to relieve a transmission constraint or address a reliability issue.
SOPD Load Dispatching 4
What is the purpose of 'tie-line bias control' in interconnected system operations?