Powering Europe - European Wind Energy Association

fiGURE 10: PRinCiPlE of balanCinG in thE PowER systEM
50 Hz
Frequencyy
Power
Kinetic energy
0
Frequency dependent
load decrease
Consumption of electrical power varies per minute,
hour and day. Because the power balance must be
continuously maintained, generation is scheduled to
match these longer term variations. Such economic
dispatch decisions are made in response to anticipated
trends in demand (while primary and secondary
controls continue to respond to unexpected imbalances).
During the early morning period, for example, an
increase in load usually occurs from approximately 7
AM to midday or early afternoon. After the daily peak
is reached, the load typically falls over the next several
hours, finally reaching a daily minimum late at night.
Some generators require several hours to be started
and synchronised to the grid. That means that the generation
available during the midday peak must have
been started hours in advance, in anticipation of the
peak. In many cases, the shut-down process is also
lengthy, and units may require several hours of cooling
prior to restarting. The decision to utilise this type
of unit often involves a period of several days that the
unit must run prior to shutting down in order to be
economic. This time-scale is called unit commitment,
chApTEr 3 powersystemoperationswithlargeamountsofwindpower
Primary reserve
Load
Secondary
reserve
Long-term reserve
Time
Seconds Minutes Hours
and it can range from several hours to several days,
depending on specific generator characteristics and
operational practice.
During operations, the balancing task is usually taken
over from the individual power producers by the
system operator. This is cost effective, as the deviations
of individual producers and loads smooth out
when aggregated, and only the net imbalances in the
system area need to be balanced to control the frequency.
System operators have the information on
schedules for production, consumption and interconnector
usage. These schedules either are made by
themselves or are provided by electricity market or actors
involved (producers, balance responsible players
or programme responsible parties). They may also use
on-line data and forecasts of for example load and
wind power to assist in their operational duty. During
operations, they follow the power system and call producers
that have generators or loads as reserves to
activate them as and when they need, to balance the
power system.
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