Environmental Engineering Reference
In-Depth Information
illustrates the demand profile in Great Britain during the evening of Sunday
9 July 2006.
During a typical evening there is a fall-off in demand as shops and businesses
close for the day, domestic (evening meal) cooking activity slows down and people
retire to bed. However, examination of the demand trace for 9 July reveals that,
superimposed on this behaviour, there are two rising trends in demand beginning at
approximately 7.45 pm and 8.50 pm. Italy were playing France in the final of the
2006 football World Cup on this day and so the peaks correspond to half-time and
full-time in the match - kettles are switched on for a cup of tea, visits to the
bathroom (water pumping), etc. The demand profile could thus be considered
predictable and generating plant could therefore be scheduled in advance to meet
the short-term peaks in demand. However, at the conclusion of normal play the
score was tied at 1-1, causing the match to extend into extra time (ending 1-1)
before concluding in a penalty shoot-out, which Italy won 5-3. Although the rapid
increase in demand after 9.45 pm is partially explained by lighting load, the utility
could not have known in advance that the match would not finish after 90 minutes.
5.2.1 Unit load-frequency control
For most electrical generation a fossil fuel (oil, coal, gas, etc.) is burned in a stream
of air producing heat of combustion and a supply of steam at high temperature and
pressure. The steam flows through a multi-stage steam turbine, and so drives the
rotor of an electrical generator. In a CCGT the exhaust combustion gases also drive
a gas turbine, which is coupled to the same, or a different, generator. Similarly, a
nuclear reactor may also be considered as a source of steam, which drives a multi-
stage turbine and generator. As illustrated in Figure 5.4, the steam flow, and hence
ultimately the power output of the thermal power station, can be controlled by
adjusting the governor valve feeding the steam turbine. The generator itself will
be a synchronous machine (commonly known as an alternator) which has the
characteristic that the frequency of the generated voltage is proportional to the
rotational speed of the machine.
If the system load increases the electrical generators connected to the system
will decelerate, with a consequent drop in rotor speed and system frequency.
By similar logic, if the system load decreases the generated frequency will increase.
+
w
ref
Δ
w
Controller
−
w
Superheated
steam
Three-phase
generat
or
Governor
valve
Steam
turbine
Figure 5.4
Steam turbine-governor control
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