Environmental Engineering Reference
In-Depth Information
turbines provide some power between cut-in and cut-out wind speeds for 80% of the time.
The capacity factor does not determine back-up requirements, which must be assessed
statistically.
Statistical analysis confi rms that, for limited penetration, the capacity factor is a good guide
of the probability that the generator in question will be available to contribute towards
meeting demand [5]. On this basis variable sources are incapable of providing the same level
of reliable or
fi r m
power during demand peaks as conventional generators, but they are still
capable of providing a contribution to this. This fi rmness is known as the
capacity credit
and
is a measure of the amount of load that can be provided by variable plant with no change in
the LOLP. This can be illustrated by an example. A 1GW dispersed installation of
wind farms with a 30% load factor will provide the same yearly energy output as a 350 MW
gas fuelled plant of 85% load factor. Both plant statistically have a capacity credit of
300 MW.
The calculations above assume that the availability of generated power is completely
uncorrelated to the demand. Although this assumption is correct for the gas fuelled plant this
is certainly not so in the case of the wind plant. As will be shown in the next section, due to
aggregation of wind farms and the characteristics of the wind resource, there is a positive
correlation between the availability of wind power and the demand.
3.5 Impact of Renewable Generation on Frequency
Control and Reliability
3.5.1 Introduction
The introduction of variable RE generation into a network will have an impact and incur
associated costs in two main categories [4]. The fi rst can be labelled as the
balancing impact
and relates to the management of demand fl uctuations from seconds to hours. The second,
referred to as the
reliability impact
relates to the requirement that there is enough generation
to meet the peak demand. Both balancing and reliability involve statistical calculations. The
introduction of variable RE generation introduces additional uncertainties that can be quanti-
fi ed in terms of operational penalties that have to be taken into account when the value of
electricity from RE sources is calculated.
There is a widespread, but mistaken, belief that operation of an electricity system with
renewables causes serious problems. A common misconception is that signifi cant additional
plant must be held in readiness, to come on-line when the output from the wind plant ceases.
This would indeed be true in an island situation, with, for example, wind the principal source
of supply. Modest amounts of variable renewables within an integrated electricity system
pose, however, no threat whatsoever to system operation. The reason for this is that these
amounts do not add signifi cantly to the uncertainties in predicting the generation to ensure a
balance between supply and demand. Therefore the risk of changes in the output from vari-
able renewable sources has only a small infl uence on the needs for reserves.
In the following sections the discussion will be limited mainly to the integration issues of
wind power because this is currently the nonschedulable renewable energy source making
the largest impact, and is likely to remain so for the foreseeable future. In a later section
the impact from other renewables will also be briefl y considered. The discussion will review
