Abstract
The paper presents a novel approach for the synthesis of the open-loop pitch profile during emergency shutdowns. The problem is of interest in the design of wind turbines, as such maneuvers often generate design driving loads on some of the machine components.The pitch profile synthesis is formulated as a constrained optimal control problem, solved numerically using a direct single shooting approach. A cost function expressing a compromise between load reduction and rotor overspeed is minimized with respect to the unknown blade pitch profile. Constraints may include a load reduction not-to-exceed the next dominating loads, a not-to-be-exceeded maximum rotor speed, and a maximum achievable blade pitch rate.Cost function and constraints are computed over a possibly large number of operating conditions, defined so as to cover as well as possible the operating situations encountered in the lifetime of the machine. All such conditions are simulated by using a high-fidelity aeroservoelastic model of the wind turbine, ensuring the accuracy of the evaluation of all relevant parameters.The paper demonstrates the capabilities of the novel proposed formulation, by optimizing the pitch profile of a multi-MW wind turbine. Results show that the procedure can reliably identify optimal pitch profiles that reduce design-driving loads, in a fully automated way.
Highlights
Emergency shutdowns [1] often generate design driving loads on some components of a wind turbine
The picture is specific of a given machine and restricted to tower base and blade root, this situation is quite typical of contemporary wind turbines, for which the sizing of at least some of their main components are often driven by shutdown loads
The proposed approach for the synthesis of pitch profiles during shutdowns was tested with reference to a 2.5 MW three-bladed upwind wind turbine
Summary
Emergency shutdowns [1] often generate design driving loads on some components of a wind turbine. We show that, for the tower base, the dominating loads are generated during shutdowns These are followed in the ranking by idling loads; as the machine is not controlled in idling conditions, the reduction of loads in such cases is a design problem that can be dealt with by changing, for example, the rotor solidity and diameter, and/or the blade shape (with evident consequent tradeoffs with performance and loading on other components). At the instant of time when grid connection is lost, indicated in the plots by an asterisk and corresponding to the time of maximum wind speed gradient, the electrical torque drops suddenly and the rotor initially speeds up (moving to higher TSRs) before starting to slow down In this phase of the maneuver, the machine is operated by an open-loop pitch profile, which rapidly reduces the aerodynamic torque towards large negative values. A comprehensive set of design DLCs are assumed, to investigate the robustness of the computed solution
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