Introduction - Wind Power Integration: Connection and System Operational Aspects - page 16

additional uncertainty that the introduction of wind energy imposes on a network.
Demand and generation levels cannot be predicted with total precision on any
network and typical demand prediction errors are around 1-2%. This means that
some spinning reserve must always be scheduled and it must also be able to cope
with the loss of the largest single unit on the system.
The precise way in which wind generation is integrated into an electricity
network - and the additional cost - depends on the characteristics of the other plant.
Systems with hydro or pumped storage (which can be used to respond to changes in
wind output) can absorb more variable renewables. Systems with high proportions
of nuclear or combined heat and power tend to be less flexible. What is clear is that
the vagaries of the wind or the sun, at modest levels of penetration, are not a
problem for most centralised systems. Despite the steadily increasing penetration of
wind generation on the Danish grid, energy from the plant continues to be absor-
bed. Greater threats to stability abound, particularly the loss of interconnectors or of
large thermal plant.
1.7.2 Capacity credits
Few topics generate more controversy than capacity credits for wind plant. The
capacity credit of any power plant may be defined as a measure of the ability of the
plant to contribute to the peak demands of a power system. Capacity credit is often
defined as the ratio (firm power capability)/(rated output). As thermal plant is not
100% reliable, values for all plant are less than unity. To a first order, 1,000 MW of
nuclear plant corresponds to about 850 MW of firm power and hence has a capacity
credit of 0.85; coal plant has a capacity credit of about 0.75. These figures are,
roughly, the statistical probability of the plant being available at times of peak
demand.
Almost every authoritative utility study of wind energy in large networks has
concluded that wind energy can provide firm capacity - roughly equal, in northern
Europe, to the capacity factor in the winter quarter (Milborrow, 1996). This implies
that if, say, 1,000 MW of wind plant was operating on the mainland UK network, it
might be expected to displace around 300 MW of thermal plant.
With smaller networks, which are not large enough to benefit from geo-
graphical dispersion, the capacity credit may be smaller, or non-existent. A study
of the Irish network, for example, assumed that wind had no capacity credit,
although it acknowledged that the evidence was conflicting and advocated further
work to clarify the position (Gardner et al. , 2003). However, the network operator
in Ireland has carried out an analysis of the impact of wind on the system
(ESBNG, 2004a) and suggested that the capacity credit, with small amounts of
wind, is about 30% of the rated capacity of the wind plant, which is in line with
most other European studies.
1.7.2.1 Power available at times of peak demand
As the risk of a generation deficiency is highest at times of peak demand, values of
capacity credit are strongly influenced by the availability of variable renewable
energy sources at times of peak demand. Several authors have examined this issue,