Power System Unit Commitment Fundamentals

Unit Commitment in Power Systems

Unit commitment is the process of selecting the generating units that will supply the load of a system over a period of time at minimum cost, as well as provide a specified margin of operating reserve. The total load on the system will generally be higher during the day and lower during the night. It is not economical to run all the available units all the time. The cost of the system can be saved by turning off units when they are not needed.

Need for Unit Commitment

• Enough units will be committed to supply the system load.
• To reduce loss or fuel cost.
• By running the most economic unit, the load can be supplied by the unit operating closer to its best efficiency.

Constraints in Unit Commitment

Constraints that must be considered include:

• Spinning reserve
• Thermal unit constraints
• Hydro constraints

Spinning Reserve

Spinning reserve is the total amount of generation available from all units synchronized on the system, beyond the present load and losses being supplied. Spinning reserve is calculated as a percentage of forecasted peak demand and must be capable of making up the losses or the output of the most heavily loaded unit in a given time. This may be used when unusually high power demand occurs or when other generating units are offline for maintenance, repair, or refueling.

Thermal Unit Constraints

• Minimum Up Time

  Once a thermal unit is running, it should not be turned off immediately. There is a minimum time it must remain online.

• Minimum Down Time

  Once a thermal unit is decommitted, there is a minimum time before it can be recommitted.

• Crew Constraints

  If a plant consists of two or more units, they may not be able to be turned on at the same time if there are not enough crew members to attend both units while starting up. A certain amount of energy is expended to bring a unit online. This energy is not generated and is included in the unit commitment problem as startup cost.

Hydro Constraints

For a certain period of operation, if the storage of hydro reserve at the beginning and at the end of the time period is specified, and the water flow into the reservoir and inflow is known, then in hydro unit commitment, the problem is to determine the water discharges so as to minimize the cost of generation under the constraints of meeting the load demand and water availability.