Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 194

The Horns Rev offshore wind farm, located on a submerged sandbank off the
west coast of Denmark, provided the first true example of what is achievable.
Arranged in five clusters over a 20 km 2 area, the wind farm consists of (5 16)
2 MW, pitch-controlled OptiSpeed turbines, providing a total capacity of 160 MW.
Installed in 2002, Horns Rev was intended as a demonstration project of both
large-scale offshore technology and wind farm/individual turbine control func-
tionality. The control system is integrated with the network SCADA system
enabling bi-directional communication with the regional dispatch centre (Elsam)
and the TSO (Energinet.dk). This allows a number of control strategies to be
implemented (Christiansen, 2003): production may be constrained to a set refer-
ence (absolute production limiter; Figure 5.30c). Alternatively, the wind farm may
be required to participate in regional balance control (automatic generation control;
Figure 5.30d), such that power output is reduced at a defined ramp rate, and later
increased at a defined ramp rate (see Section 5.2.1). Ramping control can also be
applied to limit short-term variations in production, arising from rapid changes in
wind speed (Figure 5.30a). Output fluctuations of up to 100 MW have been seen
over 5 minutes. The above functions inherently imply wind curtailment, and hence
the possibility of regulating/spinning reserve provision. Alternatively, using delta
control, the power production is reduced by a defined setpoint, e.g. 25-50 MW
(Figure 5.30b). In combination with balance control, the Horns Rev wind farm can
thus provide system regulation, similar to conventional generation, but much fas-
ter. Each of the turbines also transmits an indicator of available production, such
that the extra reserve can be monitored by the TSO. Since all these control func-
tions may be active at the same time, integration of turbine/wind farm control
introduces communications and co-ordination challenges (Kristoffersen and
Christiansen, 2003). It should be noted that Horns Rev has had its problems. Faults,
originally with the transformers and later the generators, discovered in 2004,
required each of the nacelle assemblies to be brought ashore and retrofitted to
better handle sea conditions.
Given that the wind source is effectively free, energy is being wasted needlessly
by providing regulation - economic efficiency would suggest that this service could
be better provided by conventional generation. Wind farm developers would also
have concern about a loss of revenue from deloading their output. However, taking
Great Britain as an example, wind generators are able to specify the price at which
they are willing to be deloaded (Johnson and Tleis, 2005). The generator then
receives both holding and response energy payments. There may be short periods
during the year when such an approach is suitable - e.g. when a light-loaded con-
ventional unit is running mainly to provide reserve. Or, during periods of high wind
output and low system demand, when the operational alternative is to constrain off
entire wind farms, it may be more economic to request that wind turbines reduce
output and provide frequency regulation and reserve capability.
A general framework for the desired response from a wind turbine generator,
based upon the EirGrid (Ireland) grid code requirements, is illustrated in
Figure 5.31. Under normal conditions, the turbine operates within the deadband
region between points B and C, i.e. there is no contribution to load-frequency