Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 150

(5 þ minutes), they are comparatively expensive to run, so that longer-term repla-
cement reserve plant will also be requested to start as soon as possible. Depending
on the temperature of such plant, the delay may range from 30 minutes (hot) to
4 þ hours (cold). Customer loads, previously shed, will also be progressively
reconnected as the stability of the system permits. The final objective is to restore
the system to its original state, whereby an economic combination of units is
operational, and sufficient spinning reserve is available to cope with plausible fault
scenarios. How long this task takes will depend on the severity of the original
incident, but will also be affected by the time of day at which the disturbance
occurs. Depending on whether the system load is decreasing (evening time) or
increasing (early morning), and hence whether generation plant has been scheduled
a priori to go offline/online, system restoration may be completed in a matter of
minutes or may instead take several hours.

5.3 System operation with wind power

5.3.1 Overview of system operational challenges of wind power
Power systems generally consist of a large number of fossil-fired power stations,
able to maintain their output indefinitely at specified levels, to follow defined
loading/unloading schedules, or to vary their output in sympathy with system
demand. The introduction of significant wind generation presents an energy source
not amenable to central control, and which may not be available to generate when
required due to lack of wind. It is important therefore to consider the variability of
wind generation over various time scales, and to consider the behaviour of dis-
tributed wind farms from a system perspective. The load-following implications for
the remaining generation and the response to generation loss incidents can then be
examined.
In the normal operation of thermal power systems, generating units are
scheduled to meet the predicted load demand profile and spinning reserve
requirements at minimum operating cost, given a specified reliability criterion.
Units committed to the system must be capable of changing their output to match
changes in system demand over the scheduling period. This should include the
ability to follow normal daily load changes (particularly the morning rise and
evening fall), participate in frequency regulation and supply replacement power
following a loss of generation event, or other major contingency. Those units
required to load follow are capable of meeting normal load fluctuations as well as
sudden, unexpected changes in demand.
Increasing concerns about global warming have, however, led to greater
interest in, and exploitation of, renewable resources for electrical generation. Indi-
vidual countries have approached this challenge in different ways and with varying
levels of priority. The European Union has been perhaps the most forward thinking,
particularly with regard to its available wind resource. The EU Renewables Elec-
tricity Directive requires that the amount of electricity supplied from renewables in
Europe should increase to 20 per cent by 2020. As part of the directive, member