Control of Wind Turbines

In pitch regulated wind turbines, the power output is regulated by adjusting the angle of the turbine blades to compensate for wind speed variations.  However, the active regulation of wind turbines presents a challenging control problem.

  1. The aerodynamic characteristics of the rotor are highly nonlinear.

  2. A primary control objective is to alleviate loads throughout the turbine in order to minimise fatigue damage.  This is nonlinear requirement which places little penalty on normal operating loads but a great penalty on occasional high loads.

  3. The actuator bandwidth is low with tight rate limits.

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Constant-speed wind turbines

Gain-scheduled control of constant-speed wind turbines

Owing to the nonlinearity of the aerodynamics, a single linear time-invariant controller is inadequate.  The standard approach is to employ a gain-scheduled controller.  Provided that an appropriate controller realisation is adopted, gain-scheduled control has been found to be extremely successful in practice. 

However, the conventional justification for the gain-scheduling approach is invalid since the wind speed varies rapidly across the entire operating envelope and prolonged gusts lead to sustained operation far from equilibrium.  The velocity-based framework (the derivation of which was originally motivated by this deficiency) provides a rigorous justification for the success of the gain-scheduling approach in the context of constant-speed wind turbines. 

Nonlinear control of constant-speed wind turbines

Conventional controllers employ gain-scheduling solely to compensate for the nonlinear aerodynamics.  However, the control objectives are also nonlinear.  By adopting a nonlinear control strategy, substantial improvements in performance are possible.   See on-line reports.

Accommodating actuator constraints

Owing to the large inertia of the rotor blades, wind turbine actuators are generally of low bandwidth and are subject to strict position and rate constraints.   The transients associated with encountering the position constraints can lead to a large performance degradation.  It has been found that, owing to the presence of both integral action and a low-frequency pole in the controller, conventional anit-wind-up methods are ineffective at reducing these transients and novel measures must be adopted.    The transients associated with the actuator rate constraints have little direct impact of performance.  However, these constraints can lead to a considerable reduction in the stability margins with a consequent requirement for controller de-tuning.  

Variable-speed wind turbines

Previous research on constant-speed wind turbines has shown that considerable performance improvements can be obtained by adopting appropriate controller realisations and nonlinear control strategies.  The application of similar approaches to variable-speed wind turbines is currently the subject of an extended investigation.

Previous renewable energy projects include:

Design and test of a controller for a variable speed wind turbine

The dependence of control systems performance on the wind turbine configuration

Strategies for the control of variable speed HAWT's

>An investigation of the benefits of nonlinear control for pitch regulated wind turbines.

An investigation of the design of the power-train for constant speeds HAWTS

Investigation of control characteristics of advanced wind turbines

Investigation of control strategies for VS45

Application of MBPC to wind turbines