the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
29 Jul 2021
29 Jul 2021
Evaluation of different APC operating strategies considering turbine loading and power dynamics for grid support
- Department of Engineering I, Control Engineering, Univ. of Applied Sciences HTW Berlin, Germany
- Department of Engineering I, Control Engineering, Univ. of Applied Sciences HTW Berlin, Germany
Abstract. This work focuses on the design, implementation, and implications of different operational strategies for wind turbines when providing active power control (APC). APC is a necessary functionality for contributing to the stabilization of the electrical grid. Specifically, two different operational strategies are used as the foundation for a model-based control design that allows the turbine to follow a given power demand. The first relies on keeping a constant rotational speed while varying the generator torque to match the power demand. The second approach varies both, the generator torque and rotational speed of the turbine to yield the desired power output. In the power reduction mode, both operational strategies employ the pitch to maintain the desired rotational speed of the turbine and therefore desired power output. The attainable power dynamics of the two closed-loop systems to varying power demands are analyzed and compared. Reduced-order models formulated as transfer functions and suitable for the integration into an upper-level control design are proposed. It is found that the first strategy involving only the generator torque while keeping a constant rotational speed provides significantly faster power control authority. Further, the resulting fatigue loading in turbulent wind conditions is briefly discussed for the two operational strategies, where constant operational storage is emulated to enable a bidirectional variation of the power output. Without any additional load reducing control loops, the results also suggest that this operational strategy is more favorable with regard to the resulting loading of the turbine structure. The simulation studies are conducted for the 5 MW reference turbine using FAST.
Florian Pöschke and Horst Schulte
Status: final response (author comments only)
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RC1: 'Comment on wes-2021-80', Anonymous Referee #1, 13 Aug 2021
The paper presents two derating/active power control strategies and their performance in terms of loads and power tracking. Moreover, the authors modelled the turbine power response by using transfer functions which can be easily used for wind farm control design. The topic is very interesting and highly relevant for the readers who are interested in wind turbine and farm control. However, I found the structure of the paper is not well-organised and the contribution of the paper is not clearly highlighted. As mentioned by the authors, similar derating strategies were also proposed by (Aho et al. 2016) with studies of the loads. Can the authors highlight the key novelty of this paper?
In addition, the authors only presented two equations to describe their control design. It is unclear to me if these two strategies will work during the transition between normal controller and derating. In addition, from my understanding, the chosen simulation cases were assumed that the wind speed was always sufficient to generate the demanded power. In reality, the wind speed sometimes might not be high enough for producing the required power, then how would the controller behave? Does it need to switch back to the normal controller and how would it affect the loads?
Please find my further comments as follows.
- Title. Do not use unnecessary acronyms in the title. Not everyone knows what APC means.
- Introduction. I found there is a lot of relevant studies missing, including some of the earliest works on this topic. For example, [1], [2], [3], [4], [5] and [6].
- Introduction. '... the paper aims to feed the discussion on the integration of dynamical turbine models for control design and simulation study of large-scale power systems. I don't think this is the only contribution by the paper. Please clarify the novelty and contribution of this paper.
- Section 2. Equation (2). Do the authors assume derating always happens in the above-rated wind region? The paper claimed that the generator torque is a function of the wind speed, which is not typical for a normal controller. Did the authors use some sort of a wind speed-based look-up table to implement these derating strategies?
- Page 3, Line 75. Typo. (Fig. 1 and (2)) —> (Fig. 1 and 2)
- Section 3. The authors claimed that the linear model dynamics were obtained somehow. I didn't understand how the linear model was obtained. Is it via the linearization tool in FAST? Most importantly, I found Section 3 is redundant. Why would the readers need to know this 'control design' section? The derating strategies have already been presented in Section 2.
- Section 3. "... the observer estimates the current effective wind speed by a measurement of the rotational speed". Typically, the wind speed estimator also requires the knowledge of pitch angle and generator torque. Is there something missing or the authors are referring to some better designs?
- Section 4. The authors presented a blending of OS1 and OS2. I am wondering if there are any low-pass filters used for the torque signal and rotor speed set-point signal?
- Section 4.1 and Section 4 are similar. For example, both sections refer to the same figure. Perhaps it would be easier to read if both sections are combined. Also, the titles of Section 4.1 and 4.1.1 are similar.
- Figure 2. Caption. What are (a), (b) and (c)? They are not shown in the figure. In addition, the authors tend to put all plots into one figure, and in the text, different sections refer to the same figure. It is hard to read. For example, Figure 2 (a), (b), (c) are referred in Section 4.1 but Figure 2 (d) is mentioned in Section 4.1.1. I suggest the authors separate out Figure 2 (d) from Figure 2 as they are in a different time scale. Moreover, Figure 2(d) is more relevant to Fig 3 (a).
- Figure 3. Figure 3 (b) and (c) are not really linked Figure 3 (a). The authors should separate them.
- Section 4.1.1. '...fewer blade-tower interactions due to the reduced rotational speed,...'. Why was that? Isn't it that the opposite is true, that the reduced rotor speed makes the 3p frequency closer to the tower mode, thus, it will increase the coupling between 3p mode and tower mode?
- Section 4.2. Interesting power tracking studies. Did the authors consider the switching between the normal controller and OS1 or OS2? What do OS1 and OS2 behave when the wind speed is not sufficient to generate the required power?
- Section 5. '... it is discussed how different operational strategies can be designed for wind turbines using a model-based control design'. Did the authors use model-based design for developing the derating strategies in Section 2? Or did I miss something?
References:
[1] Deshpande, A. S., & Peters, R. R. (2012). Wind turbine controller design considerations for improved wind farm level curtailment tracking. IEEE Power and Energy Society General Meeting, 1–6. https://doi.org/10.1109/PESGM.2012.6343975
[2] Aho, J., Buckspan, A., Pao, L., & Fleming, P. (2013). An Active Power Control System for Wind Turbines Capable of Primary and Secondary Frequency Control for Supporting Grid Reliability. AIAA/ASME Wind Symposium, 1–13. https://doi.org/10.1.1.258.5692
[3] Jeong, Y., Johnson, K., & Fleming, P. (2014). Comparison and testing of power reserve control strategies for grid-connected wind turbines. Wind Energy, 17(3), 343–358. https://doi.org/10.1002/we.1578
[4] Mirzaei, M., Soltani, M., Poulsen, N. K., & Niemann, H. H. (2014). Model based active power control of a wind turbine. In 2014 American Control Conference (pp. 5037–5042). IEEE. https://doi.org/10.1109/ACC.2014.6859055
[5] Zhu, J., Ma, K., Soltani, M., Hajizadeh, A., & Chen, Z. (2017). Comparison of loads for wind turbine down-regulation strategies. 2017 11th Asian Control Conference (ASCC), 2784–2789. https://doi.org/10.1109/ASCC.2017.8287618
[6] Lio, W. H., Mirzaei, M., & Larsen, G. C. (2018). On wind turbine down-regulation control strategies and rotor speed set-point. Journal of Physics: Conference Series, 1037, 032040. https://doi.org/10.1088/1742-6596/1037/3/032040
- AC1: 'Reply on RC1', Florian Pöschke, 10 Sep 2021
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RC2: 'Comment on wes-2021-80', Anonymous Referee #2, 17 Dec 2021
The investigated problem is indeed very irelevant for wind turbine control. The article includes interesting results but their presentation and discussion should be improved. Please find my brief comments below and note that they are built on top of Reviewer #1 inputs:
- Introduction: There are many studies in this area as Reviewer #1 indicated. One addition to the previous suggestions is Christos Galinos et al. 2018 J. Phys.: Conf. Ser. 1104 012019 (https://backend.orbit.dtu.dk/ws/portalfiles/portal/160566480/Galinos_2018_J._Phys._3A_Conf._Ser._1104_012019.pdf) where the effect of turbine derating strategies are also investigated, mainly focusing on wake. Novelty with respect to all the listed previous studies should be clarified.
- Operating strategies: It is indeed not clear if down-regulation is activated below rated as well as "With ωopt (v) and Topt (v) being the optimal (or limited above rated power) rotational speed and generator torque depending on the current effective wind speed v..." reads as if the limits are introduced above rated only. Should be clarified.
- Control design: The need for linearization (and how does it capture dynamics really?) and its process should be clarified. Why not use the entire Cp surface if in the end a 'nonlinear decsription' is generated? The discretization induces additional uncertainties for your results and could potentially be avoided.
- Control design: How does "... the observer estimates the current effective wind speed by a measurement of the rotational speed"? Especially for OS1, where the rotational speed is kept at optimum, majority of the wind speed dynamics is reflected on the pitch acitivity. Another set of equantions is needed to explain the approach utilised for the wind speed observer.
- Results: Subsection 4.1.1 should be combined with 4.1 for a better flow in the article.
- Results (Section 4.1) Figure 3(a): What is Delta_Pd here? It seems like an isolated case within Figure 4, should be stated in the caption and the text for clarity.
- Results (Section 4.1) Figure 4: It is the most interesting result of the study in terms of load analysis and not discussed in the text at all. The behaviour should be analysed in detail and comparatively with respect to delta set-points.
- Conclusions: Suggested rewording "... where a de- or acceleration..." --> ""... where a deceleration or acceleration..."
- Conclusions: Potential implications of OS1 vs. OS2 to the wake of the turbine can be briefly discussed to open up the discussion for flow control studies.
Thanks for your efforts, looking forward to read the revised version!
- AC2: 'Reply on RC2', Florian Pöschke, 20 Dec 2021
Florian Pöschke and Horst Schulte
Florian Pöschke and Horst Schulte
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