Control strategy for ensuring tower clearance allowance whilst maximizing Annual Energy Production

Introduction

The current trend in the design of wind turbines is to increment the length of the blades in order to increase the annual power production, by means of a compromise between lightness and rigidity. One of the greatest problems related to this is a higher flexibility of the blade. This flexibility in combination with an increase in wind speed causes a greater deflection of the blades in the direction of the wind. This means that for a windward-facing wind turbine (normal configuration), the tips of the blades are deflected towards the tower with the consequent risk of collision between the blade tip and the tower.


Approach

There exist a number of solutions to prevent the deflected blades from striking the tower, as should this occur, irreparable damage would be caused to both components.


A common practice consists of designing the rotor shaft in such a way that it is not horizontal, but is set at an angle from the horizontal. Another possibility consists of the use of hubs with a greater coning angle. Besides, there exist control strategies like dynamic fine pitch in order to reduce the thrust on the turbine. The first two practices bring a reduction in the area swept by the rotor perpendicular to the wind whilst the last one applies higher collective pitch values. All these known possibilities imply a reduction in the annual energy production.


In an attempt to solve this energy loss, some strategies have been developed in the state of the art whereby the blade pitch angle is modified cyclically for each blade exclusively in a sector of rotation where the blade passes in front of the tower.


Main body of abstract

The control strategy enables an increase in the distance between the tip of the blade which passes by the generator tower and the tower itself, to prevent possible collisions. To this end, the control method includes the application of a blade pitch angle control term which is calculated as a function of at least one signal indicative of wind speed and as a function of the azimuthal angle of each blade.


Given that the minimum distance occurs when the tip of the blade passes in front of the tower, an additional blade pitch angle term is added, whose value depends on the azimuthal position of the blade and whose maximum value is the result of an optimization over a set of representative simulations which take into account the delay of the pitch actuator, the azimuth sector in which the strategy will be applied as well as the additional pitch signal amplitude. The objective of the optimization is to reduce the associated loss of power production while avoiding collision between blade and tower.

The described strategy uses the following as a signal indicative of wind: the generated power in the sub-nominal production regime, and the pitch angle in the nominal production regime. Therefore, the function defining the value of the amplitude of the cyclic function as a function of the signal indicative of wind is doubly dependent on the generated power and on the blade pitch angle, in such a way that, in the sub-nominal power regime, the function depends on the generated power; and above the nominal wind speed, the function depends on the blade pitch angle.


Conclusion

A control strategy to prevent the collision between the blade tip and the tower has been presented with the characteristic that it tries to minimize the associated energy loss by restricting its application to a certain regimes of operation where the possibility of collision exists. Its effectiveness has been validated with field blade loads data.



Learning objectives
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