What is the difference between economic dispatch and unit commitment?

Short Answer:

Economic Dispatch (ED) and Unit Commitment (UC) are two important processes in power system operation, but they serve different purposes. Economic dispatch determines how much power each committed generator should produce at any moment to minimize cost, assuming those generators are already running.

Unit commitment, on the other hand, decides which generators should be turned ON or OFF over a time period (like 24 hours) to meet the demand at the lowest cost. So, unit commitment is about planning ahead, while economic dispatch is about real-time power sharing among running units.

Detailed Explanation:

Difference between economic dispatch and unit commitment

Power system operation involves careful planning and control to ensure that electricity is generated reliably and economically. Two important tools in this process are Economic Dispatch (ED) and Unit Commitment (UC). While both aim to minimize total cost of generation, they work at different stages and have different responsibilities in the power generation schedule.

Economic Dispatch (ED):

Economic Dispatch is a real-time or short-term operation process that determines the optimal output of already running (committed) generators. Its goal is to minimize the total fuel cost while ensuring that total power generation equals the demand and system losses. ED assumes that the generator units are already ON and available.

Key features of ED:

  • Decides how much power each online generator should produce.
  • Works under the constraint that total generation = demand + losses.
  • Uses fuel cost functions and penalty factors.
  • Does not include start-up, shut-down, or minimum up/down time constraints.
  • Solved at frequent intervals (like every 5 or 15 minutes).

Unit Commitment (UC):

Unit Commitment is a planning problem, usually for the next day or week. It determines which generators should be turned ON or OFF during different time intervals to meet the expected demand at minimum cost. UC takes into account several time-based constraints, such as start-up cost, shut-down cost, minimum up time, minimum down time, and reserve requirements.

Key features of UC:

  • Determines ON/OFF status of generators over time.
  • Solves a scheduling problem for 24 hours or more.
  • Considers operational limits like start-up time, ramp rate, and cost.
  • Helps prepare a schedule that ED follows during actual operation.
  • More complex due to binary decision variables (unit ON or OFF).

Main Differences at a Glance:

Aspect Unit Commitment (UC) Economic Dispatch (ED)
Purpose Which generators to run How much power each generator should produce
Time Horizon Long-term (e.g., 24 hours ahead) Short-term or real-time
Focus Generator scheduling Optimal load distribution
Decision Type ON/OFF status (binary) Continuous power output
Includes Start-Up Cost Yes No
Complexity High (combinatorial problem) Moderate (continuous optimization)
Conclusion:

Economic dispatch and unit commitment are both essential for efficient power system operation, but they work at different levels. Unit commitment answers which units should run, while economic dispatch answers how much each running unit should generate. Together, they ensure power is delivered reliably, efficiently, and economically by optimizing both the scheduling and real-time operation of power plants.