Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 139

(and smoothing out the peaks) of electrical demand. However, the overall cycle effi-
ciency is likely to be in the range 60-80 per cent, depending on age and technology,
so large-scale energy storage is unlikely to be economic or even advantageous.
Ideally, electrical utilities would like the load demand to be invariant - the
cheapest and most efficient generation could then be selected to operate continuously
at the desired level. However the demand profile is shaped, sufficient generation plant
needs to be connected to the system, at any particular time, to meet the likely variation
in system demand. Since it may take several hours, or even days in the case of nuclear
generation plant, for an electrical generator to be started up and begin to generate, the
demand pattern must be predicted over a suitable period, and generating units
scheduled accordingly. Until recent times, when a single, vertically integrated utility
would have been responsible for generation, transmission and distribution of elec-
tricity, this unit commitment task would have been executed daily, with the objective
of minimising operating cost against a required level of system reliability. However,
as will be seen in Chapter 7 (Electricity Markets), many countries have now evolved
to a deregulated environment, where a market may exist for energy trading, along
with separate markets for a range of ancillary support services. The peculiarities of a
particular market structure will clearly influence the type of generating plant that are
deployed, the duties they are required to perform and any long-term investment
decisions. However, the underlying principles of operating the power system remain
the same. Thus, in order to focus on technical rather then economic issues, a tradi-
tional vertically integrated utility, personified as a system operator , will be assumed
in this chapter to have sole responsibility for system operation.
As the system demand varies throughout the day, and reaches a different
peak from one day to the next, the electric utility must decide in advance which
generators to start up and when to connect them to the network. It must also
determine the optimal sequence in which the operating units should be shut down
and for how long. A power system will consist of many generating units, utilising a
wide range of energy sources, employing different technologies and designs, and
providing a wide range of generating capacity. As a result, fuel, labour and main-
tenance costs can vary significantly between generating stations. Even within the
same power station the thermal efficiency of individual units will vary with power
output. Despite these complexities, a merit order can generally be defined whereby
all units are ranked in terms of operational costs. Consequently, the most economic
(or inflexible) units are committed to the system first, and so-called base load units
will typically be required to operate at 100 per cent of their rating for 24 hours per
day. At the other extreme, expensive peak load units may only be required for brief
periods during the day, or year, to meet the system demand maxima. In between
these extremes, the morning rise and evening fall in load, cf. Figure 5.1, is tracked
by flexible generation plant operating in a two-shift mode. Such plant, operational
for two shifts each day, and off for the other, are capable of responding fairly quickly
to changes in demand. Depending on indigenous resources and the government
policy of different countries, individual utilities may operate a significantly different
merit order. At present, modern coal-fired plant and combined cycle gas turbines
(CCGTs) tend to be the cheapest, while older coal-fired plant and oil-fired units tend