Start-up and shutdown times and costs: what gets measured, and what goes wrong.

In steady-state operation a 900 MW combined-cycle plant is, broadly, a known quantity. The hours when the plant is starting up and shutting down are different. Fuel goes in but no electricity comes out for the first thirty to a hundred minutes. Hot-gas-path components see thermal cycles the OEM tracks individually.

Those hours are also where the dispatch economics turn on a few specific numbers — and where field test campaigns most often produce measurement errors. The system operator wants the start-up cost in monetary terms per start, the start-up time in minutes from command to grid-synchronous output, the shutdown cost per shutdown, and the equivalent figures for each starting state.

The regulator wants all three measured, because the dispatch decision changes based on starting state. A plant in a hot state can be brought online quickly and cheaply; a plant in cold state cannot. The system operator uses these numbers to decide whether dispatching a particular plant is economically sensible given current market conditions.

Field testing all three states means running three separate start-up sequences, each from the correct thermal baseline. This is calendar-expensive. Coordinating with the grid to take a working plant offline long enough to reach a cold state is rarely possible — most cold-start measurements are taken during scheduled outage returns.

A combined-cycle plant entering a wholesale market for the first time, or a plant being re-characterized for an updated dispatch profile, needs a start-up and shutdown campaign in addition to the steady-state heat-consumption campaign. The two campaigns share instrumentation and personnel but have different measurement protocols, different time scales, and different deliverables.

For owners reading this in advance of commissioning: budget for the start-up campaign as a distinct line item. It is not a footnote on the heat-rate test. It is a separate engagement with its own calendar, its own field events, and its own dossier section.

The hours when the plant is starting and stopping are the hours when the dispatch economics get decided for the asset's life. Treating them as measurement footnotes is among the most expensive habits in plant operations.

Start/shut economics belong to the plant performance testing family — captured at the same time as the capacity and heat-rate acceptance tests.