04-01-2013, 09:59 AM
Unit Commitment
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Economic Dispatch: Problem Definition
Given load
Given set of units on-line
How much should each unit generate to meet this load at minimum cost?
Unit Commitment
Given load profile (e.g. values of the load for each hour of a day)
Given set of units available
When should each unit be started, stopped and how much should it generate to meet the load at minimum cost?
Observations on the example:
Far too few units committed: Can’t meet the demand
Not enough units committed: Some units operate above optimum
Too many units committed: Some units below optimum
Far too many units committed: Minimum generation exceeds demand
No-load cost affects choice of optimal combination
A more ambitious example
Optimal generation schedule for a load profile
Decompose the profile into a set of period
Assume load is constant over each period
For each time period, which units should be committed to generate at minimum cost during that period?
Issues
Must consider constraints
Unit constraints
System constraints
Some constraints create a link between periods
Start-up costs
Cost incurred when we start a generating unit
Different units have different start-up costs
Curse of dimensionality
Reserve Capacity Constraint
Unanticipated loss of a generating unit or an interconnection causes unacceptable frequency drop if not corrected rapidly
Need to increase production from other units to keep frequency drop within acceptable limits
Rapid increase in production only possible if committed units are not all operating at their maximum capacity
How much reserve?
Protect the system against “credible outages”
Deterministic criteria:
Capacity of largest unit or interconnection
Percentage of peak load
Probabilistic criteria:
Takes into account the number and size of the committed units as well as their outage rate
Types of Reserve
Spinning reserve
Primary
Quick response for a short time
Secondary
Slower response for a longer time
Tertiary reserve
Replace primary and secondary reserve to protect against another outage
Provided by units that can start quickly (e.g. open cycle gas turbines)
Also called scheduled or off-line reserve
Cost of Reserve
Reserve has a cost even when it is not called
More units scheduled than required
Units not operated at their maximum efficiency
Extra start up costs
Must build units capable of rapid response
Cost of reserve proportionally larger in small systems
Important driver for the creation of interconnections between systems
Environmental constraints
Scheduling of generating units may be affected by environmental constraints
Constraints on pollutants such SO2, NOx
Various forms:
Limit on each plant at each hour
Limit on plant over a year
Limit on a group of plants over a year
Constraints on hydro generation
Protection of wildlife
Navigation, recreation
Network Constraints
Transmission network may have an effect on the commitment of units
Some units must run to provide voltage support
The output of some units may be limited because their output would exceed the transmission capacity of the network
Summary
Some constraints link periods together
Minimizing the total cost (start-up + running) must be done over the whole period of study
Generation scheduling or unit commitment is a more general problem than economic dispatch
Economic dispatch is a sub-problem of generation scheduling