Generating units
may be classified into three categories based on their mode of operation. These
are:
1. Base
Load;
2.
Intermediate;
3. Peaking.
Base load
units tend to be large units with low operating costs. They are generally
operated at full capacity during most of the hours that they are available.
They are
designed to operate for long periods of time at or near their maximum
dependable capability. Their low operating costs result from their use of
low-cost nuclear and coal fuels and/or lower heat rates (higher efficiencies) than
other units on the system.
For a
typical region, base load is on the order of 40% to 60% of the annual maximum
hourly load and, since this represents the amount of load that will be supplied
in the region at essentially all hours, it represents perhaps 60% to 70% of the
annual energy requirements of the region.
Base load
units are usually shut down for forced outages or maintenance only. Because of
their size and complexity, these units may require from 24 hours to several
days to be restarted from a “cold” condition.
Once the
decision has been made to shut down one of these units, periods of up to 24
hours may be required before another “start-up” may be attempted. When
operating a power system decisions on the time of restarting units play an
important role in hour-by-hour schedules for generation.
Intermediate
units are those generating units which are used to respond to the variations in
customer demand which occur during the day. They are designed to withstand
repeated heating and cooling cycles caused by changes in output levels.
Intermediate
units usually have lower capital costs, and somewhat higher heat rates (lower
efficiencies) than base load units. The intermediate load may be on the order
of 30–50% of the maximum hourly load for a typical system and represents
perhaps 20–30% of the annual energy requirements for the utility.
Peaking
units are those generating units that are called upon to supply customer demand
for electricity only during the peak load hours of a given period (day, month,
year). Combustion turbines, reciprocating engines and small hydroelectric units
comprise the majority of peaking units.
These are
ordinarily units with a low maximum capability (usually less than 150Mw), which
are capable of achieving full load operation from a cold condition within ten minutes.
Peaking units usually have the highest heat rate sand lowest capital costs of
the three categories of units.
In addition
to supplying system needs during peak load hours, they may be called upon to
replace the capability of other base load
or cycling units which have been suddenly removed from service due to forced
outages. They generally supply about 5% of the total energy requirements of a
system.
As
generating units age, unit efficiency and performance generally decrease. In
addition, newer, more efficient, lower operating cost units are continuously added
to a power system. These two occurrences tend to cause most generating units to
be operated fewer hours as they age.
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