Environmental Engineering Reference
In-Depth Information
dependence of the load. All these issues will be discussed later in more detail. For now it is
suffi cient to recognize the interdependence between demand and frequency.
Complex power systems consisting of multiple interconnected generators supplying a large
number of consumers respond much as the single generator, but with all the generators and
loads contributing to the system response during demand changes. Just as for a single genera-
tor supplying a load, the frequency of a complex interconnected system is the same at all
parts of the network. For now, the idea of a unique system frequency should be accepted
without question, but it will be properly explained in Chapter 4. A power system is, of course,
never in equilibrium because the demand varies continuously as consumers switch on or off
their loads. It can be concluded that frequency shifts are an indicator of the imbalance between
supply and demand at a particular instant. Frequency drifts downwards when demand exceeds
supply and vice versa.
Conventionally, power systems are run so that their frequency remains within narrow
bounds because:
•
This ensures that electric motors operate at virtually constant speed. A fi xed speed is
required in many consumer applications where an AC electric motor is used to drive a
device at an approximately constant rate, e.g. a pump in a washing machine or a lathe in
an industrial workshop.
•
In electronic applications the mains frequency can be used as a basis for timing various
processes.
•
Transformers are sensitive to frequency variations and may be overloaded if the frequency
drifts substantially from the nominal.
•
Finally and most importantly, in traditional power stations the performance of the genera-
tors is dependent on the performance of all the auxiliary electric motor drives that deliver
fuel and air to the boiler, oil to bearings and cooling services to several systems. If these
auxiliaries underperform due to low speed caused by low frequency, power station output
can be reduced. As will be discussed later this phenomenon could lead to a runaway situ-
ation with cascaded shutdown of power stations and blackouts.
For a near-constant frequency to be maintained it is necessary that the supply of power
accurately tracks the variations in demand. How and why electricity demand changes is the
subject of the next section.
3.2 Electricity Demand
3.2.1 Demand Curves
Figure 3.1 shows the highly variable nature of the electricity demand over a day of a typical
individual house in the UK, with a minimum demand of a few watts, an average between
0.5-1 kW and the maximum in the range of 5-10 kW, i.e. 10 to 20 times the average load.
Using a dedicated electricity generator to supply this house alone would be hopelessly expen-
sive. The generator system would have to be large enough to meet the maximum demand,
but most of the time it would be running at a very small fraction of its rated capability. Any
fuel driven prime mover operated in this way would be woefully ineffi cient. Energy storage,
