Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 147

boiler output. The boiler is an energy store of limited capacity, and so over such a
short time scale, the boiler would not be able to increase, and sustain, such an
output. The steam pressure would inevitably fall, leading to droplets of water
forming in the steam, which could cause significant damage to the turbine blades.
The steam pressure therefore must be controlled, placing a limit on the reserve from
each unit, as shown in Figure 5.6. As the frequency falls, fuel and air supply to the
boiler will be increased. However, depending on fuel type and the configuration of
the plant, it may be tens of seconds before the increased fuel flow appears as a
sustained, increased power output.
Since the provision of spinning reserve incurs an operational cost, a balance
has to be struck between the cost of the reserve and the likelihood of it being called
upon. For small, or synchronously isolated, power systems it may prove too
expensive to provide 100 per cent cover following loss of the largest infeed.
A decision may be reached that only a certain fraction, say 80 per cent, may be
covered. The remaining shortfall of 20 per cent may then be obtained by dis-
connecting customers, either through selectively disconnecting part of the load, or
through the establishment of an interruptible tariff with large industrial consumers.
As load shedding is a somewhat drastic control measure, and disruptive to consumers,
it is usually implemented in stages, with each stage triggered at a different frequency
level, so that the minimum amount of load is lost. In practice, this measure may not
be necessary, as the system load tends to be frequency-sensitive: as the frequency
falls the demand also falls, hence reducing the initial generation-demand imbal-
ance. System load is also sensitive to voltage and hence, amongst other measures,
some utilities switch in reactors (using frequency-sensitive relays) at suitable net-
work points following loss of generation to depress the local voltage and hence the
locally connected demand. Also, if pumped storage is present and operating in
pumping mode, then it can be disengaged (reducing the system demand to its nat-
ural level), and then switched to peak output in generating mode. For many power
systems,

pumped storage may be the most responsive load/generation plant

available.

5.2.2.1 Inertial system response
Immediately following a significant generation-demand imbalance, the system
frequency will change rapidly. The initial rate of change of frequency (ROCOF)
will determine how quickly generation plant must respond before system difficul-
ties begin to worsen. If an individual generator is rotating at a nominal angular
speed w 0 , and has inertia I gen , arising from the inertia of the multi-stage turbine and
the alternator, then the rotational stored energy, E 0 , is given by

2 I gen w 0 2 . As the
system frequency varies, and hence the generator rotational speed changes, energy
will either be stored or released, tending to negate the original disturbance in the
system frequency. A convenient indicator of a generator's inertial response is
provided by its inertial constant, H gen , which is defined as the generator stored
energy divided by the rating of the machine. It can be interpreted as the time that
the generator can provide full output power from its own stored kinetic energy,
with typical values in the range 2-9 seconds (Grainger and Stevenson, 1994).

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