Non Cycling vs. Must Run

A Must Run unit must operate at its minimum capacity for all hours it is so designated regardless of economics. Dispatch any upper segment(s) based on economics. No unit commitment logic is required for this type of unit, because its outcome is already known/predetermined.

By contrast, a Non Cycling unit is simply one that cannot cycle on an hourly basis. Therefore, it is subject to unit commitment logic before proceeding to the dispatch.  If a unit is committed, then it must operate at its minimum capacity level, but only for its minimum up time.  

Again, during that min up period, any upper segment(s) are dispatchable based on economics (in this window it behaves similar to a Must Run unit). But after the min up time is fulfilled, the unit becomes eligible for decommitment, and the commitment logic looks ahead to determine if it is more economic to decommit the unit (and risk re-incurring start costs again a short time later) vs. staying committed (and risk losing money for a few hours).

For example, compare two units designated like the following:

Unit 5 is a non cycling or “commitment” unit, and unit 6 is designated as must run.

After running the zonal model for a week in July 2013, here are example results:

NOTE: While dispatch patterns are similar, unit 6 (Must Run) never backs down to 0. It “must run” all the time at its minimum capacity level or higher.

By contrast, unit 5 (commitment/non cycling) backs down to 0 output when not committed (percent committed = 0). However, when it is committed (percent committed = 100), it runs at least at its minimum capacity level, but often dispatches at a higher level based on economics.