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With pitch-regulated turbines, it is possible to reduce the output at any moment by pitching the blades. This could also be done with stall-regulated turbines, by shutting down individual turbines within a wind farm. Although this only provides relatively crude control, the output from the power system operator’s point of view is effective and valuable.
All forms of active power control in a wind turbine require a reduction in output power, which means a reduction in revenue. This is less of an issue for conventional power stations, where the lost revenue will be compensated, to some extent, by a reduction in fuel cost. Therefore, system operators and energy regulators recognise that a reduction in wind farm output should be used as a last resort.
The simplest method is a cap, which means that the wind farm (or a group of wind farms) is instructed to keep its output below a certain level. A more complex version of the cap is to insist that output to be kept at a fixed level (delta), below the unconstrained output available from wind.
In parallel with a cap, the wind farm may also be instructed to control ramp rate, i.e. to limit the rate at which the output power can increase (due to increasing wind speed, or turbines returning to service after some outage). The ramp rate is defined over periods of one minute or 10 minutes, for example. This limits the network operator’s demands on other forms of generation to change output rapidly.
Clearly, it is not possible for wind generation to control automatically the negative ramp rate if the wind drops suddenly. However, with good wind forecasting tools, it is possible to predict a reduction in wind speed in advance; the output of the wind generation can then be gradually reduced in advance of the wind speed reduction, thereby keeping the negative ramp rate at an acceptable level.
On systems with relatively high wind penetration, there is often a requirement for frequency response or frequency control. This can take many forms, but the basic principle is that, when instructed, the wind farm reduces its output power by a few percent, and then adjusts it in response to the system frequency. By increasing power when frequency is low, or decreasing power when frequency is high, the wind farm can contribute to controlling the system frequency.
Figure 5.1 Examples of SCADA functions for active power control of wind power plants
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