Environmental Engineering Reference
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achievable for longer time periods (several hours), so that the magnitude of more
extreme fluctuations, such as widespread turbine shutdown preceding an approach-
ing storm front, could be reduced.
5.3.4 System operational modes
Wind variability, unpredicted or even if predicted, occurring on the time scale of
tens of minutes to hours, will have a significant impact on economic and reliable
power system operation. The main concern is that a continuous balance must be
maintained between generation and demand, whilst ensuring an adequate reserve
capability. Extensive experience exists in predicting demand behaviour, and cred-
ible fluctuations in demand can be covered at minimal cost (see Section 5.2). The
introduction of large-scale, distributed, but variable power sources will clearly
impact on the scheduling of conventional generation and the operational procedures
implemented to ensure sufficient generation reserves. A number of challenging
scenarios can easily be imagined. For example, a large increase in load during the
morning rise, coupled with an unpredicted fall-off in wind generation, could
deplete the system's reserve margin, as well as strain the ramping capability of
conventional plant. Alternatively, at times of low demand and unexpected high
wind output, conventional generators could be wound down towards their
(less efficient) minimum output. Subsequently, should excess wind power be
curtailed, leading to an uneconomic and possibly high emission configuration? Or,
should some thermal units be de-committed, assuming that wind output will be
sustained? Solutions are required which make minimal use of fast-start but
expensive OCGT back-up generation, or the need for wind and/or load curtailment.
For simplicity, two extremes of system operation are proposed, namely fuel
saver mode and wind forecast mode. Fuel saver mode is essentially an extension of
current practice, with wind turbines treated as negative load devices, i.e. they
provide energy, and thus reduce the effective system load, but do not provide any
ancillary support services. The alternative to fuel saver mode is to include wind
forecasts directly within the unit commitment process. Assuming here for expe-
diency that wind curtailment will not be required, and that wind generation will not
provide any ancillary services, then unit commitment will be based on the net
demand forecast (equal to the demand forecast less the forecast wind generation).
The advantage is that a forecast of significant wind power could reduce the required
commitment of conventional generation, leading to a more cost-effective mode of
operation with lower emissions. Fuel saver operating mode
Assuming a vertically integrated utility structure, as in Section 5.2, the system
operator will complete unit commitment, typically for the following day, based
solely on the demand forecast, and ignore any potential contribution from wind
generation. The subsequent intention is that individual generating units will start up
and shut down in sympathy with the expected system demand pattern. In real time,
however, assuming that wind farms are operational and thus contributing to the
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