Environmental Engineering Reference
In-Depth Information
of any load or generator is dependent not only on its own characteristics but also on the
strength of the network to which it is connected.
Network operators are paying increasing attention to power quality. This is not specifi cally
in relation to distributed generation but mainly because of regulatory obligations set up to
protect increasingly discerning consumers from the generation of harmonics caused by the
plethora of electronic devices on the network.
6.4.5 Network Reinforcement
In some situations, the voltage effects discussed above could be reduced by reinforcing the
network and thereby increasing the fault level at the point of common coupling. For example,
it is sometimes possible to upgrade the conductors of an existing overhead line, which is
known as reconductoring. Alternatively, if permissions can be obtained, it is usually more
cost effective simply to build a new line.
When faced with an application to connect an embedded generator, it is common for the
local electricity company to claim that the nearest point for connection to the existing network
is too weak and to require that a new line be installed in order to reach a stronger part of the
network. The cost of installing such a line can be very signifi cant in comparison to the size
and cost of the generator itself and will normally fall to the owner of the generator, although
rules for such network reinforcement vary from country to country.
6.4.6 Network Losses
Enthusiastic proponents of renewable energy sometimes claim that, because distributed gen-
eration is nearer to the loads, it will reduce network losses. Unfortunately, this is not always
true.
High voltage transmission networks are generally very effi cient; thus reducing power fl ows
in transmission networks does not bring very large reductions in losses. In distribution net-
works, losses are proportionally far greater and so, for losses to be reduced to any signifi cant
extent, the distributed generation has to be located very close to matching loads. Furthermore,
its availability profi le in time should reasonably match that of the local demand.
6.4.7 Fault Level Increase
As noted previously, fault level can be used as a measure of grid strength and can be compared
with proposed generator capacity to give an indication of likely effects on line voltage.
However, as shown in Figure 6.6, the fault level can itself be increased by the connection of
embedded generators, particularly where synchronous machines are involved. This does not
invalidate the previous comments on voltage effects, but it does represent another factor to
be considered in the connection of a generator.
Switchgear is sized to suit the network fault level. Existing switchgear therefore represents
a limit to the amount of embedded generation that can be accommodated in a given area. It
is not just the switchgear on the site of the embedded generator that is important as a syn-
chronous generator will increase the fault level throughout the local network and so the rating
of all local switchgear must be considered.
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