Environmental Engineering Reference
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49.8-50.2 Hz, while for the continental European grid, the equivalent range is
49.9-50.1 Hz. The synchronous zone spans 24 countries, reaching from Portugal
in the west to Romania in the east and from Italy in the south to western Denmark
in the north. The broader range adopted for Great Britain recognises that the
power system is much smaller (and possesses less inertia - see Section 5.2.2) than
that of mainland Europe. During significant system disturbances these limits can
be widened - perhaps from 49.5 to 50.5 Hz, or wider, depending on the size
(inertia) of the actual power system. If all generating units have the same droop
characteristic, then each will share the total system demand change in proportion
to its own rating (maximum output). Although this is a desirable feature, in terms
of sharing the regulation burden across all operational units, a significant change
in demand will probably result in an uneconomic dispatch of the load, since the
load is no longer distributed according to the merit order ranking of the units.
However, an optimal loading of the units can be restored by suitably adjusting
the reference frequencies of individual units. Given that the unit commitment
process typically defines the required unit trajectory for each scheduling period,
economic dispatch can be combined with the load following requirements of
individual units. Automatic generation control
A synchronously operated power system can extend over both regional and national
boundaries. Thus, power imbalances in one area will cause regulator action in all
other areas, minimising the effect of the original disturbance. However, since the
interconnection between subsystems and individual countries may be limited either
physically (transmission line rating) or by contractual agreement it is necessary that
interchanges between neighbouring areas are monitored and controlled. Germany,
for example, is split into four control areas, while France and Austria are operated
as one and three regions, respectively. For a particular region, the total error, D P tie ,
between the actual and scheduled tie-line interchanges with all neighbouring
regions can be determined by communicating information on the individual tie-line
power flows to the central area control. Similarly, by calculating D f , the error
between the reference system frequency and the actual frequency, the error in the
regional generation, D P area , can be determined if the system stiffness is known
(Wood and Wollenberg, 1996). The stiffness of an area quantifies the sensitivity of
the system frequency to imbalances between demand and generation, and will
depend on both the governor droops of operational units and the frequency
dependency of the load. The area control error, ACE , can then be defined as
ACE ¼ D P tie þ D P area MW
The area controller (regulator) must then determine the required change in P ref ,
the regional reference power, and commonly a proportional þ integral (PI) strategy
is applied,
ð ACE dt MW
D P ref ¼ b ACE þ
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