Carlo Tibaldi

Effects of gain-scheduling methods in a classical wind turbine controller on wind turbine aero-servo-elastic modes and loads

aeroservoelastic modes and loads DTU Orbit (03/11/2019) Effects of gain-scheduling methods in a classical wind turbine controller on wind turbine aeroservoelastic modes and loads The eects of dierent gain-scheduling methods for a classical wind turbine controller, operating in full load region, on the wind turbine aeroservoelastic modes and loads are investigated in this work. The dierent techniques are derived looking at the physical problem to take into account the changes in the aerodynamic characteristics as a function of the wind speed. The modal analysis is performed with a high-order linear aeroservoelastic model computed with the frequency based stability tool HAWCStab2. The time series of the wind turbines loads are computed with the non-linear time domain tool HAWC2. Results show changes in the natural frequency and in the damping ratio of the speed regulator mode and of the tower longitudinal mode when using the dierent gain-scheduling schemes.

Aeroelastic Optimization of a 10 MW Wind Turbine Blade with Active Trailing Edge Flaps

This article presents the aeroelastic optimization of a 10MW wind turbine ’smart blade’ equipped with active trailing edge flaps. The multi-disciplinary wind turbine analysis and optimization tool HawtOpt2 is utilized, which is based on the open-source framework OpenMDAO. The tool interfaces to several state-of-the art simulation codes, allowing for a wide variety of problem formulations and combinations of models. A simultaneous aerodynamic and structural optimization of a 10 MW wind turbine rotor is carried out with respect to material layups and outer shape. Active trailing edge flaps are integrated in the design taking into account their achieved fatigue load reduction. The optimized ’smart blade’ design is compared to an aeroelastically optimized design with no flaps and the baseline design.

Aero-Elastic Optimization of a 10 MW Wind Turbine

This article presents the multi-disciplinary wind turbine analysis and optimization tool HawtOpt2 that is based on the open-source framework OpenMDAO, and interfaces to several state-of-the art simulation codes, which allows for a wide variety of problem formulations and combinations of models. In this article simultaneous aerodynamic and structural optimization of a 10 MW wind turbine rotor is carried out with respect to material distribution and outer shape. A set of optimal designs with respect to mass and AEP are presented, which shows that an AEP biased design can increase AEP with 1.5% while a mass biased design can achieve mass savings of up to 20% compared to the baseline DTU 10MW RWT. A newly developed frequency-domain based fatigue model is used to minimise fatigue damage, which achieves up to 8% reduction in the tower bottom fore-aft fatigue damage, with only limited reductions of the aerodynamic performance or increased mass.

Optimal tuning for a classical wind turbine controller

Fine tuning of controllers for pitch-torque regulated wind turbines is an opportunity to improve the wind turbine performances and reduce the cost of energy without applying any changes to the design. For this purpose, a method for automatically tune a classical controller based on numerical optimization is developed and tested. To have a better understanding of the problem a parametric analysis of the wind turbine performances due to changes in the controller parameters is first performed. Thereafter results obtained with the automatic tuning show that is possible to identify a finer controller tuning that improves the wind turbine performances. For the case study selected in this work, a 2% cost function reduction is achieved with seven iterations.

Design of an Aeroelastically Tailored 10 MW Wind Turbine Rotor

This work presents an integrated multidisciplinary wind turbine optimization framework utilizing state-of-the-art aeroelastic and strutural tools, capable of simultaneous design of the outer geometry and internal structure of the blade. The framework is utilized to design a 10 MW rotor constrained not to exceed the design loads of an existing reference wind turbine. The results show that through combined geometric tailoring of the internal structure and aerodynamic shape of the blade it is possible to achieve significant passive load alleviation that allows for a 9% longer blade with an increase in AEP of 8.7%, without increasing blade mass and without significant increases in ultimate and fatigue loads on the hub and tower.

Using Pretwist to Reduce Power Loss of Bend-Twist Coupled Blades

Bend-twist coupling of wind turbine blades is known as a means to reduce the structural loads of the turbine. While the load reduction is desirable, bend-twist coupling also leads to a decrease in the annual energy production of the turbine. The reduction is mainly related to a no longer optimal twist distribution along the blade due to the coupling induced twist. Some of the power loss can be compensated by pretwisting the blade. This paper presents a pretwisting procedure for large blade deflections and investigates the effect of pretwisting on blade geometry, annual energy production, and fatigue load for the DTU 10 MW Reference Wind Turbine. The analysis was carried out by calculating the nonlinear steady state rotor deflection in an uniform inflow over the operational range of the turbine. The steady state power curve together with a Rayleigh wind speed distribution has been used to estimate the annual energy production. The turbine model was then linearised around the steady state and the power spectral density of the blade response, which was computed from transfer functions and the wind speed variations in the frequency domain, was used to estimate the fatigue loads by a spectral method.

Reduced design load basis for ultimate blade loads estimation in multidisciplinary design optimization frameworks

The aim is to provide a fast and reliable approach to estimate ultimate blade loads for a multidisciplinary design optimization (MDO) framework. For blade design purposes, the standards require a large amount of computationally expensive simulations, which cannot be efficiently run each cost function evaluation of an MDO process. This work describes a method that allows integrating the calculation of the blade load envelopes inside an MDO loop. Ultimate blade load envelopes are calculated for a baseline design and a design obtained after an iteration of an MDO. These envelopes are computed for a full standard design load basis (DLB) and a deterministic reduced DLB. Ultimate loads extracted from the two DLBs with the two blade designs each are compared and analyzed. Although the reduced DLB supplies ultimate loads of different magnitude, the shape of the estimated envelopes are similar to the one computed using the full DLB. This observation is used to propose a scheme that is computationally cheap, and that can be integrated inside an MDO framework, providing a sufficiently reliable estimation of the blade ultimate loading. The latter aspect is of key importance when design variables implementing passive control methodologies are included in the formulation of the optimization problem. An MDO of a 10 MW wind turbine blade is presented as an applied case study to show the efficacy of the reduced DLB concept.

Study on Controller Tuning of Wind Turbines with Backward Swept Blades

The objective of this study is to investigate the effects of backward swept blades on the dynamics of the controller of a wind turbine. When sweeping blades backward a coupling between flapwise bending toward the tower and torsion towards feathering is achieved. This coupling mitigates loads on the structure due to a decrease in the angle of attack. Changing the blade geometry can affect the behavior of the wind turbine controller. Hence, a detailed investigation of the effects generated by the blade sweep on the controller tuning is needed. Using the Basic DTU Wind Energy Controller, the aero-servo-elastic behavior of the 10 MW DTU Reference Wind Turbine (RWT) with swept blades is studied and considerations on the tuning of the controller are made in order to preserve and not compromise the dynamics of the system. This work focuses on understanding the interaction between the employment of backward swept blades and the dynamic behavior of the controller above rated wind speed. The purpose is to isolate loads variations due to the blade sweep from the loading differences brought by the controller’s actions. This objective is achieved by re-tuning the controller of the wind turbine with swept blades according to the dynamics of the baseline regulator mode.

Aeroservoelastic analysis of storm-ride-through control strategies for wind turbines

An investigation of a control strategy to allow wind turbines to operate at high wind speeds by derating the rotor speed and generator torque set-points is presented. The investigation analyzes the wind turbine aeroservoelastic behavior in the above rated operational range by computing the aerodynamic gains and closed-loop eigenvalue solutions using a high-fidelity linear model. A simple strategy to reduce the reference rotor speed based on a pitch angle feedback is presented and analyzed. It is shown that high aerodynamic gains for operation at high wind speeds requires special handling in the scheduling of the controller gains. The computed closed-loop modal frequencies and damping ratios show how most turbine modes become less damped as the rotor speed is derated, and at very high winds the frequency and damping of the first drivetrain torsion mode are significantly reduced. Possible resonance problems can also be seen from the computed frequencies, and these problems may be worsened by the decreased damping during storm-ride-through. Finally it is shown that the dynamics of the pitch feedback to the derated generator speed is significantly affected by the operational wind speed, resulting in a slow response at high wind speeds.

PI controller design of a wind turbine: evaluation of the pole-placement method and tuning using constrained optimization

PI/PID controllers are the most common wind turbine controllers. Normally a first tuning is obtained using methods such as pole-placement or Ziegler-Nichols and then extensive aeroelastic simulations are used to obtain the best tuning in terms of regulation of the outputs and reduction of the loads. In the traditional tuning approaches, the properties of different open loop and closed loop transfer functions of the system are not normally considered. In this paper, an assessment of the pole-placement tuning method is presented based on robustness measures. Then a constrained optimization setup is suggested to automatically tune the wind turbine controller subject to robustness constraints. The properties of the system such as the maximum sensitivity and complementary sensitivity functions (Ms and Mt ), along with some of the responses of the system, are used to investigate the controller performance and formulate the optimization problem. The cost function is the integral absolute error (IAE) of the rotational speed from a disturbance modeled as a step in wind speed. Linearized model of the DTU 10-MW reference wind turbine is obtained using HAWCStab2. Thereafter, the model is reduced with model order reduction. The trade-off curves are given to assess the tunings of the poles- placement method and a constrained optimization problem is solved to find the best tuning.