Environmental Engineering Reference
In-Depth Information
is not always clear-cut. Sometimes confusion arises when the word grid is used to refer to a
relatively small standalone electrical network. This is not necessarily wrong (though it may
indicate delusions of grandeur!) but it should always be clear as to the extent of the grid being
referred to.
The point on the network to which a renewable energy generator is connected is referred
to, for reasons to be explained later, as the
point of common coupling
(PCC).
1.4.3 Distributed Generation
Power systems have developed over the years to supply a varying demand from centralized
generation sourced from fossil and nuclear fuels. Unless nuclear fusion proves successful,
which will not be known for over 50 years, there is universal agreement that by the end of
this century the majority of our electrical energy will be supplied from RE sources.
Generators powered from renewable energy sources (except large scale hydro and large
offshore and onshore wind farms) are typically much smaller than the fossil fuelled and
nuclear powered generators that dominate today's large power systems. Small generators
cannot be connected to the transmission system because of the cost of high voltage transform-
ers and switchgear. Also, the transmission system is often a long way away as the geographi-
cal location of the generator is constrained by the geographical availability of the resource.
Small generators must therefore be connected to the distribution network. Such generation is
known as
distributed
or
dispersed generation
. It is also known as
embedded generation
as it
is embedded in the distribution network.
In traditional power systems power invariably fl ows from the large centralized power sta-
tions connected to the EHV network down through the HV and LV systems to be distributed
to consumers. In power systems with distributed generation power may travel from point to
point within the distribution system. This unusual fl ow pattern has some serious implications
in the effective operation and protection of the distribution network. Distributed generation
will be discussed in Chapter 6.
It may be concluded that present power systems will gradually have to evolve and adapt
so that, in the far future, a managed demand will be supplied from distributed, mostly vari-
able, RE generation. This transformation will be aided by the liberal use of power electronic
interfaces capable of maximizing the effectiveness of RE sources, controlling power fl ows
and ensuring reliability of supply. Some of these issues are discussed in the last chapter.
1.4.4 RE Penetration
The proportion of electrical energy or power being supplied from renewable sources is
generally referred to as the
penetration
. It is usually expressed as a percentage. When fuel
or CO
2
-emission savings are being considered, it is useful to consider the
average
penetration
:
(
)
Energy from renewable energy powered g
enerators kW h
Total energy delivered to loads kW h
Average penetration
=
(
)
In this case, the energy (kW h) is measured over a long period of time, perhaps a year. At
fi rst sight, it might seem more natural to express the denominator (total energy delivered to
loads) as: total energy from all the generators (including fossil fuelled generators). However,
