SOPD Load Dispatching 2 — Questions and Answers
Question 1: What is the primary purpose of economic dispatch in power system operations?
- Minimize total generation cost while meeting load demand (Correct answer)
- Maximize power output from all generators equally
- Reduce transmission line losses to zero
- Maintain voltage within ±5% of nominal at all buses
Correct answer: Minimize total generation cost while meeting load demand
Economic dispatch allocates load among generators to minimize total fuel or operating cost while satisfying system demand and constraints.
Question 2: A dispatcher observes that the system frequency has dropped from 60 Hz to 59.8 Hz. Which immediate action is most appropriate?
- Shed non-critical load immediately
- Increase generation output from online units (Correct answer)
- Open transmission ties to neighboring systems
- Switch to manual control of all generators
Correct answer: Increase generation output from online units
A frequency drop indicates generation is insufficient to meet load, so the dispatcher must increase MW output from available online generators.
Question 3: What does 'area control error' (ACE) represent in an interconnected power system?
- The voltage deviation at the area's reference bus
- The difference between actual and scheduled net interchange plus frequency bias (Correct answer)
- The total reactive power deficiency in the control area
- The sum of all generation shortfalls across the region
Correct answer: The difference between actual and scheduled net interchange plus frequency bias
ACE is calculated as the difference between actual net interchange and scheduled net interchange, adjusted by the area's frequency bias setting.
Question 4: When a large industrial load trips unexpectedly, the system frequency begins to rise. What does this indicate?
- Generation is now in excess relative to remaining load (Correct answer)
- Transmission lines are overloaded and shedding load
- The automatic voltage regulators have failed
- Reactive power compensation is insufficient
Correct answer: Generation is now in excess relative to remaining load
A frequency rise after load loss means generators are producing more power than the remaining load requires, creating a surplus of generation.
Question 5: Which dispatch strategy is used when system reliability is threatened and cost is secondary?
- Economic dispatch
- Security-constrained dispatch (Correct answer)
- Merit order dispatch
- Marginal cost dispatch
Correct answer: Security-constrained dispatch
Security-constrained dispatch overrides purely economic solutions to ensure transmission limits and reliability criteria are maintained.
Question 6: A dispatcher is asked to evaluate 'spinning reserve.' What does this term specifically refer to?
- Generation capacity that is synchronized and can increase output within 10 minutes (Correct answer)
- Backup generators that can start and deliver power within 30 minutes
- Extra transmission capacity held in reserve for emergency rerouting
- Demand response resources enrolled but not yet activated
Correct answer: Generation capacity that is synchronized and can increase output within 10 minutes
Spinning reserve is capacity from synchronized (online) generators that can be rapidly increased to cover sudden loss of generation or unexpected load increases.
Question 7: What is the significance of the 'lambda' (λ) value in classical economic dispatch?
- It represents the system marginal cost, at which all generators should operate for optimal dispatch (Correct answer)
- It is the minimum load each generator must carry to remain online
- It indicates the maximum transmission capacity of the weakest line
- It defines the allowable frequency deviation band for the control area
Correct answer: It represents the system marginal cost, at which all generators should operate for optimal dispatch
Lambda is the system incremental cost ($/MWh) at which the incremental cost of each participating generator is equalized for an optimal economic dispatch solution.
SOPD Load Dispatching 2
What is the primary purpose of economic dispatch in power system operations?