Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 190

generation (Milligan and Porter, 2006). Quoted annual capacity factors for Denmark
and Germany are around 20 and 15 per cent, respectively. The lower values can be
explained largely by lower average wind speeds, and stronger requirements for
system reliability (DENA, 2005). An investigative study of mainland Europe,
assuming an installed wind capacity of 40 GW, suggested that the capacity credit for
wind could vary between 5 and 35 per cent, with an average value of approximately
19 per cent (Giebel, 2000). The variation in figures quoted was achieved by exam-
ining the effect of time shifting a time series wind profile with respect to the system
demand pattern. The low figure (5 per cent) is partially explained by the working
assumption that conventional plant must operate above 50 per cent of maximum
output and hence wind curtailment can be required.
5.3.6 Ancillary service provision
Conventional generation, in addition to providing energy, may also be required to
provide a number of additional ancillary services - spinning reserve, load following/
frequency regulation, voltage/reactive power support and black start capability.
Traditionally, wind turbine generators have not provided any of these services, and
indeed fixed-speed machines, in particular, may introduce high/low local voltage
distribution network problems (Dinic et al. , 2006; Romanowitz et al. , 2004).
As wind penetration levels increase, however, there are likely to be significant
operational difficulties if the provision of these services becomes depleted. The
majority of existing wind farms have been installed during the last 15-20 years.
Sited largely on shore, and scattered across distribution networks, they present an
evolution of many different types of technology and provide significant variation in
controllability. At one extreme is the passive stall, fixed-speed wind turbine, intro-
duced in the 1980s, in the 10-100 kW range and most widespread in Denmark.
From a grid perspective, the output of such machines is uncontrollable (apart from
stopping/starting) and they are generally unresponsive to system needs - as dis-
cussed later in Section 5.3.7 they do provide an inertial response. At the other
extreme are modern, pitch regulated, full- or partial-variable speed wind turbines,
providing real-time active and reactive power control, and potentially capable of
contributing to dynamic stability, voltage support and network control.
The wind turbine market has grown significantly in the last 5-10 years, so that
most installed wind farms are controllable to some degree. The system benefits of
retrofitting secure communications for centralised monitoring and control, in
addition to modifying the existing control systems, are likely to be minimal for
older wind turbine designs, considering the low capacity of the installed plant, and
balanced against the cost of implementation and the remaining equipment life.
Instead, newer installations, likely to be of much greater capacity (particularly if
sited offshore), can be specified to include communication networking and
advanced control systems at a small fraction of the total project cost. A modern
SCADA (supervisory control and data acquisition) permits comprehensive infor-
mation to be collected from individual turbines, including meteorological data,
which enables optimum setpoints to be defined, along with external system
operator commands.