Other extensions to classical controllers

Other extensions to classical controllers have sometimes been used in order to further improve the performance in particular ways, for example the use of nonlinear gains, and variable or asymmetrical limits.

Non-linear gains are sometimes used to penalize large peaks or excursions in controlled variables. For example, the gain of a PI pitch controller can be increased as the power or speed error increases. A simple way to do this is to add to the input signal to the PI controller a term proportional to the square or cube of the error (remembering to adjust the sign if the square is used). This technique should be used with caution, however, as too much non-linearity will drive the system towards instability, in much the same way as if the linear gain is too high. This technique requires a trial-and-error approach since it is very difficult to analyse the closed-loop behaviour of non-linear systems using standard methods. Adding the non-linear term only when the power or speed is above the set-point will help to reduce peaks, but will also cause a reduction in the mean power or speed, similar to a reduction in set-point.

Asymmetrical pitch rate limits can also be used to reduce peaks. By allowing the blades to pitch faster towards feather than towards fine, power or speed peaks will be reduced. Once again the mean level will also be reduced by introducing this asymmetry. However, this technique is somewhat more 'comfortable' than the use of non-linear gains, in that it remains a linear system constrained by limits.

There is often a desire to reduce the set-point in high winds, to reduce the infrequent but highly-damaging loads experienced in those conditions at the expense of a small loss of output. It is straightforward to reduce the set-point as a function of wind speed (the pitch angle is usually used as a measure of the rotor-averaged wind speed, as for gain scheduling). However, the most damaging loads occur during high turbulence, and so it would be better to reduce the set-point in high winds only when the turbulence is also high. Rather than actually reducing the set-point, asymmetrical rate limits provide a simple but effective means of achieving this effect, since the rate limits will only 'bite' when the turbulence is high.

A further extension of this technique is to modify the rate limits dynamically, even to the extent of changing the sign of a rate limit in order to force the pitch in one direction during certain conditions such as large power or speed excursions. A useful application of this is in the control of variable slip systems, where it is important to keep the speed above the minimum slip point (point B in Figure 8.8). If the speed falls below this point, it then ceases to vary much as it is constrained by the minimum slip curve, and so the proportional term in the PI controller ceases to respond. Modifying the rate limits as a function of speed error as in Figure 8.13 is a useful technique to prevent this happening.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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