Model-based design of a wave-feedforward control strategy in
ﬂoating wind turbines

Fontanella, Alessandro; Al, Mees; van Wingerden, Jan-Willem; Belloli, Marco

doi:https://doi.org/10.5194/wes-2021-9

Preprints

https://doi.org/10.5194/wes-2021-9

Preprints

01 Mar 2021

Review status: this preprint is currently under review for the journal WES.

Model-based design of a wave-feedforward control strategy in ﬂoating wind turbines

Alessandro Fontanella¹, Mees Al², Jan-Willem van Wingerden³, and Marco Belloli¹ Alessandro Fontanella et al.

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¹Mechanical Engineering Department, Politecnico di Milano, Milano, Via La Masa 1, 20156, Italy
²Sowento GmbH, Donizettistraat 1A, 70195 Stuttgart, Germany
³Delft Center for Systems and Control, Delft University of Technology, Delft, 2628 CD, The Netherlands

Received: 05 Feb 2021 – Accepted for review: 27 Feb 2021 – Discussion started: 01 Mar 2021

Abstract. Floating wind turbines rely on feedback-only control strategies to mitigate the effects of wave excitation. Improved power generation and lower fatigue loads can be achieved by including information about the incoming waves into the wind turbine controller. In this paper, a wave-feedforward control strategy is developed and implemented in a 10 MW floating wind turbine. A linear model of the floating wind turbine is established and utilized to show how wave excitation affects the wind turbine rotor speed output, and that collective-pitch is an effective control input to reject the wave disturbance. Based on the inversion of the same model, a feedforward controller is designed, and its performance is examined by means of linear analysis. A gain-scheduling algorithm is proposed to adapt the feedforward action as the wind speed changes. Non-linear time-domain simulations prove that the proposed feedforward control strategy is an effective way of reducing rotor speed oscillations and structural fatigue loads caused by waves.

Alessandro Fontanella et al.

Status: open (until 27 Apr 2021)

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RC1:
'Comment on wes-2021-9', Anonymous Referee #1, 15 Mar 2021 reply

The use of input-output analysis and wave-feedforward control strategy is believed to be novel in controller design for floating offshore wind turbines.

I have the following comments/questions:

Line 63: Does this tuning imply that the bandwidth of the blade pitch controller is reduced below the natural frequency of the floater pitch motion? (Arriving at 0.0186Hz ) If yes, could you elaborate on the effect of this tuning which seems to give a controller bandwidth that is reduced with a factor ~4-5 compared to a conventional controller for bottom fixed wind turbines. In particular with respect to aerodynamic loads within the bandwidth of a conventional controller but not within the bandwidth of the tuned controller, and with respect to the effectiveness of the proposed wave disturbance rejection.

Line 84: Something seems to be wrong with the reference.

Line 390: "As it has been shown, a large part of rotor speed oscillations is caused by wind

turbulence" Could some of these variations and associated DELs have been reduced if a conventional (bottom fixed) controller bandwidth had been applied?

Reply

Citation: https://doi.org/10.5194/wes-2021-9-RC1
- AC1: 'Reply on RC1', Alessandro Fontanella, 20 Mar 2021 reply
  
  Dear Referee,
  
  Thank you very much for your comments and feedback.
  Much of the work about floating wind turbine control carried out so far focus on the interaction between blade pitch controller and platform pitch motion, and how to avoid the negative-damping issue. The Authors are grateful to the Referee for introducing this topic in the discussion, and believe that clarifying how the pitch controller tuning relates with wave disturbance rejection makes the research more interesting for both the scientific community and industry.
  
  Concerning the first comment, about the effect of detuning on the effectiveness of the wind turbine controller against wind and waves:
  In case of original gains, wind loads are inside the CPC bandwidth: at the controller cut-off frequency the wind spectrum is around 3% of it's maximum value. Wave loads are just above the cut-off frequency (how much above depends on the sea state). The CPC with original gains rejects the wind disturbance, but is ineffective against wave. In case of detuned gains, the bandwidth is shorter: the wind spectrum is 18.6% of its maximum value at the controller cut-off frequency. Moreover, the disturbance sensitivity in the controller bandwidth is increased as the rotor-speed tracking performance is degraded. Hence the controller is less effective against the wind disturbance. The capability or rejecting wave loads is not influenced much by detuning. This is exemplified by the figure attached to the answer, where the typical PSD of wind and waves (rescaled) is compared to the sensitivity of the feedback pitch controller with original and detuned gains. In conclusion: the feedforward controller complements the feedback CPC, and targets wave loads. Therefore, its benefits are weekly related to the tuning of the feedback controller.
  
  Concerning the third comment, about the effect of detuning on rotor speed oscillations and fatigue DEL:
  The effectiveness of CPC with detuned gains is decreased, but detuning is needed to make the floating system stable without modifying the structure of the FB controller. The bandwidth of the FB controller, and hence its effectiveness against wind turbulence, could be increased by means of NMPZ-compensation [doi: 10.1049/iet-rpg.2012.0263], where pitch control is used in combination with dynamic generator-torque. Another possibility is to replace the FB controller with a more complex multivariable controller [doi:10.1088/1742-6596/753/9/092006]. Both these techniques can be used in synergy with feedforward control to further improve the floating wind turbine response to environmental loads.
  
  Finally, concerning the comment about the wrong reference, the Authors found that reduced-order modeling of an FOWT rotor is also discussed in "Multibody modeling for concept-level floating offshore wind turbine design" [Lemmer 2020, https://onlinelibrary.wiley.com/doi/abs/10.1002/we.2408], with a greater level of detail than in "Robust gain scheduling baseline controller for floating offshore turbines", currently cited in the manuscript.
  
  Reply
  
  Citation: https://doi.org/10.5194/wes-2021-9-AC1

Alessandro Fontanella et al.

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