Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 140

to be more expensive. Nuclear plant, while not necessarily the cheapest to run, is
comparatively inflexible, and may therefore operate as base load. Hence, countries
such as Sweden have a high penetration of nuclear plant, backed up by flexible
pumped storage and hydro-electric generation, respectively. In contrast, combined
heat and power (CHP) plant is foremost in Denmark, while CCGTs (and coal-fired
generation) are increasingly dominant in Great Britain and Ireland. Combining the
demand profile of Figure 5.1, and a likely merit order of generation fuel sources,
results in the unit scheduling of Figure 5.2.
A fixed cost can generally be assigned to shutting down a unit, but the start-up
cost will depend on how long the unit has been shut down. For a conventional
fossil-fuelled power station, the boiler metal temperature and hence the fuel
required to restore operating temperature will depend on the duration of cooling.
A hot plant, which has only recently been shut down, with metal temperatures close
to design conditions, may require 2-4 hours to be synchronised and brought to full
load. A cold plant, which has been offline for some time, may instead require
6-10 hours to complete the same process. The start-up and shutdown times will
vary significantly with generation technology, plant age and operator practice.
Since there is a cost (and time) associated with the start-up or shutdown of an
individual generating unit, then, from an economic viewpoint, the scheduling of
units at a particular time cannot be determined without considering the current
configuration of committed units, the standby (hot/cold) state of uncommitted
plant, and the expected variation in system demand. Consequently, the operating
cost at one particular time is related to the units selected for all other periods. Units
will be scheduled typically on a day-ahead basis. So, if the 24-hour period is split
into a number of stages (say half an hour long), a multi-stage cost minimisation
problem results, the objective being to select the combination of generating units at

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Figure 5.2 Combined merit order/system demand profile