Environmental Engineering Reference
In-Depth Information
Continuous Service
50.2
Time
10
mins
10
s 0
s
60
s
50.0
49.8
Primary
Secondary (to 30
mins)
Reserve
49.5
Occasional Service
49.2
Figure 3.10
Frequency response requirements. (Source: National Grid plc)
are selected on technical and economic merits and instructed by the system operator to operate
in frequency-sensitive mode (i.e. under active governor control) to provide this service. For
this to be achieved, some generators are held below maximum output.
An
occasional service
or
reserve
is available to contain signifi cant and abnormal frequency
excursions caused by sudden mismatches in the generation/demand balance (e.g. loss of
generation) [2]. Part-loaded large synchronized generators as well as deferrable loads fi tted
with frequency sensitive relays provide such services. The reserves are subdivided into
primary
and
secondary
categories. In Figure 3.10 the initial rate at which the frequency drops
after the incident at 8 seconds is controlled and limited instantly by the inertial energy release
from all the decelerating generators (and consumer drives) on the system. This provides a
breathing time before the fast governors of some generators begin to act. Massive steam
valves have to be opened hydraulically and increased steam fl ow has to be transported from
the boiler to the turbines. It therefore takes a fi nite time for the substantial stored energy in
the boiler to be exploited.
Primary reserves require the most rapid generator response. The key requirement for gen-
erators allocated this task is that they should be capable of increasing their active power
output within 10 seconds of predefi ned system frequency excursions and be capable of main-
taining this response for a further 20 seconds [2].
Secondary reserves require a slower initial response but maintained for longer periods of
time. This requires the capability of increasing the active power output within 30 seconds
and maintaining the response for a further 30 minutes [2]. A fast response capacity is also
provided by partly loaded hydro or pumped storage (when available), which are not bedev-
illed by the constraints imposed on thermal plant. Water driven plant can respond in a few
minutes and be started up automatically when the frequency falls below a critical value.
A fundamental feature of generators providing frequency response and reserves, collec-
tively known as the
operating margin
, is the requirement for generators to have
headroom
in order to increase output. The operating margin is the difference between available genera-
tion and actual demand. Generators providing such services will therefore be part-loaded.
