Environmental Engineering Reference
In-Depth Information
The topic of frequency control is of particular importance in systems with substantial
renewable energy penetration.
3.4.4 Generation Scheduling and Reserve
Demand forecasting provides a fairly accurate picture of the expected load over the following
24 hours so that enough generators are scheduled to provide the expected demand plus a
reserve
, a concept to be discussed in the following section. The process is complicated by
the disparate characteristics of plant on the system.
In Chapter 7, it will be shown how generators are loaded on economic grounds subject to
various constraints. In anticipation of an increase in demand a choice has to be made by a
utility on which uncommitted generating sets from a number of available generating units
(some sets may not be available as they are undergoing repairs or maintenance) have to be
prepared (heated, synchronized and loaded) and eventually shut down when the demand
declines. This is a complicated economic and technical choice. In a privatized energy market
one company, usually the one in charge of the transmission system, has the role of plant
selection, but here decisions are taken in the context of the contractual relations between the
different participants in the electricity market. Chapter 7 describes how regulatory tools,
tariffs and bidding systems are used in these circumstances to ensure that supply tracks
demand.
3.4.5 Frequency Control at Different Timescales
[2, 3]
The maintenance of frequency involves a
response
from generators over several timescales
ranging from seconds to days. This response is not only required to follow the demand's
fast variability from second to second and its slow variability over the day, but also sudden
substantial mismatches between generation and demand, for example during system
faults.
Figure 3.10 illustrates a typical system frequency trajectory plotted on a nonlinear time
scale. From the origin of the graph until about 8 seconds, the frequency exhibits the usual
noise associated with minor mismatch between the continuously varying demand and the
efforts of generation to match it through governor action. At 8 seconds something unpredicted
and serious takes place. It could be that a large power station trips because the overhead line
connecting it to the transmission system suffers a mechanical failure due to high winds and
accumulated snow. This
contingency
results in an instantaneous large shortfall of generation.
The trace describes a typical time history of the frequency and the measures taken to constrain
the frequency excursion within the statutory and operational limits. Such measures are taken
by all utilities but are given a variety of labels. Here the labels shown in Figure 3.10 are the
ones adopted in the UK.
A power system has at its disposal a number of generators with diverse characteristics.
These are arranged in a hierarchy of plant appropriate for operation at different timescales
as described below.
A
continuous
or
frequency response
is provided by generators equipped with appropriate
governing systems that control their outputs to counteract the frequency fl uctuations that arise
from relatively modest changes in demand or generation [2]. Large generators on the grid
