Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 193

increase in pitch angle and a decrease in electrical output. Such an approach is
feasible with conventional pitch control. Above rated output the power reference
setting can be lowered - thus restricting the maximum power output (Figure 5.30c).
At lower turbine outputs the minimum pitch angle can be increased, again pushing
wind turbine operation away from the optimum (Holdsworth et al. , 2004).
For variable-speed wind turbines, power output regulation can also be
achieved by varying the rotor speed - a small decrease in rotational speed away
from the optimum tip-speed ratio will cause a reduction in electrical output. Since
speed control is ultimately achieved using power electronics (adjusting the injected
rotor quadrature voltage - Section 3.6.1) no moving parts are required. This clearly
contrasts with pitch regulation, and hence speed control is well suited for con-
tinuous, fine frequency regulation. Blade pitch control can provide fast-acting,
coarse control both for frequency regulation as well as emergency spinning reserve.
5.3.6.3 Implementation options
As conventional generation plant is displaced increasingly by wind generation, the
task of frequency regulation will naturally fall on the remaining generating units. If
wind variability is combined with existing demand variation, then at times the
flexibility of the conventional plant may be insufficient to regulate wind-induced
variations and maintain the frequency constant. It follows, therefore, that wind farms
will have to contribute to frequency control, either by maintaining a fixed load
profile, or contributing directly to system-wide frequency regulation. This is likely to
be an issue not only for small/isolated power systems, but for all power systems. A
simulation study, based on the E.ON German network (interconnected to the wider
European network), considered the addition of 3.5 GW of offshore wind, against an
existing background of 3.5 GW of onshore wind and a system capacity of approxi-
mately 33 GW (Koch et al. , 2003). It was proposed that the planned offshore gen-
eration provide frequency regulation capability, equivalent to an arbitrary 3 per cent
of the nominal wind capacity, enabling the regulating contribution from conven-
tional plant to be reduced from 28 to 11 per cent of the primary reserve requirement.
Wind turbine generators have good potential to provide frequency regulation
and spinning reserve, and, indeed, pitch-regulated turbines are likely to be much
more responsive than conventional plant. For example, in Denmark, under network
fault conditions, individual turbines are required to reduce the input power (by pitch
regulation) below 20 per cent of turbine rating within 2 seconds - an indicator of
turbine response. Provision of a conventional governor droop response implies that
an increase in power output should follow a fall in the system frequency, and hence
the wind turbines must operate below the potential output level for the current wind
conditions. Similarly, reserve can only be provided if the wind turbines have been
deloaded by the required amount. So, for example, the turbine pitch angle could be
adjusted for partial output, while maintaining a reserve margin of, say, 1-3 per cent
of rated output (delta control). SCADA systems enable turbine wind speed to be
measured, providing an estimate of potential power capture, with the turbine output
limited to a defined fraction of this value. The pitch angle can then be adjusted to
provide continuous frequency regulation or occasional spinning reserve.