Hazim Namik

A Comparison of Multi-Blade Coordinate Transformation and Direct Periodic Techniques for Wind Turbine Control Design

The inherent periodic behavior of an operating wind turbine is not well accommodated by common time-invariant analysis and control techniques. A multi-blade coordinate transformation (MBC) helps to overcome this issue for rotors with three or more blades by mapping the dynamic state variables into a non-rotating reference frame. A number of researchers have applied MBC for modal analyses and individual blade pitch controller designs. They do so by assuming the transformed system model from MBC is time-invariant, which is not often the case. The paper explores the validity of the time-invariant assumption by comparison to direct periodic techniques, which retain all periodic system information. In a modal analysis study, eigenvalues of a system after MBC are compared to direct Floquet modes. In an individual blade pitch control design study, a linear quadratic regulation (LQR) design after MBC is compared to direct periodic LQR. A 5-MW three-bladed wind turbine model is used to quantify performance differences. Normal operating conditions are considered as well as conditions selected to increase the harmonics that are unfiltered by MBC. It is found that the direct periodic methods produce almost identical results to timeinvariant methods after MBC under all conditions studied. MBC is recommended for threebladed turbines, which can be followed by Floquet analysis or periodic control design methods if necessary.

State-Space Control of Tower Motion for Deepwater Floating Offshore Wind Turbines

The offshore wind energy potential is huge and to capture that energy, turbines have to be placed further offshore. Floating wind turbines offer a solution for deep waters. However, a floating wind turbine has extra motions that will affect the turbine in power production and structural loads. Therefore, the turbine control system has to be able to regulate power production and maintain safe operation of the turbine under incident wind and wave conditions. The work presented here discusses the application of state-space optimal controllers to regulate rotor speed and platform pitch above rated wind speed. A gain scheduled PI controller is used as a baseline to gauge the performance of the new controllers developed. A collective pitch linear quadratic regulator, designed to only regulate rotor speed and platform pitch, improves system performance but this improvement is thought to be due to better controller tuning as both controllers use the same mechanism to restore platform pitch and regulate speed. Individual blade pitch control using periodic control theory is applied because it uses a different mechanism to regulate platform pitch. Preliminary simulation results show that individual blade pitch control has platform pitch regulation over collective pitch controllers. However, unintended excitation of platform roll indicate that a more complicated controller may be required to ensure closed loop stability of the entire floating turbine.

Individual Blade Pitch Control of a Spar-Buoy Floating Wind Turbine

The spar-buoy floating wind turbine is one of the three main floating wind turbine concepts and one of the first to proceed to a full-scale prototype stage. Multiobjective linear state feedback controllers are implemented on the spar-buoy floating wind turbine with individual blade pitching (IBP). The spar-buoys deep draft results in a low platform pitch and roll natural frequencies. The low-frequency pitch and roll modes interact with other low-frequency modes of the system (i.e., surge and sway, respectively). Therefore, the linear state-space model used for control design must include the surge and sway degrees of freedom. Furthermore, a low platform pitch natural frequency limits the effectiveness of IBP at regulating the platform pitch around the first tower fore-aft (FA) resonant frequency. Simulations using a high-fidelity model are carried out according to design load case 1.2 of the IEC-61400-3 standard for fatigue load testing under normal operating conditions. Simulation results relative to a gain-scheduled proportional-integral controller show that a multiobjective state feedback controller is able to reduce tower FA and side-side bending fatigue loads by an average of 9%. This improvement is mainly due to IBP despite its limited effectiveness around the first tower FA resonant frequency.

Disturbance Accommodating Control of Floating Offshore Wind Turbines

Floating offshore wind turbines offer a feasible solution for water depths greater than 60m. However, floating wind turbines will experience additional motion induced by incident wind and waves due to the lack of rigid foundations. Therefore, the control system becomes an important component that can be used to reduce the induced motions or, if not carefully taken into consideration, could exacerbate the motions. In our previous work, we demonstrated that using state space controllers with multiple objectives resulted in improved performance with respect to a gain scheduled PI controller. Further improvements were obtained by implementing individual blade pitching through periodic control as it overcomes the limitations of collective blade pitching. However, the periodic controller destabilized un-modeled degrees of freedom (DOFs) and these DOFs had to be included in the design of the controller to ensure stability of the system. Disturbance accommodating controllers can further improve the performance by canceling or reducing the effects of persistent disturbances: incident wind and waves. Currently, only wind speed is modeled as a disturbance to reject. Simulation results show that only when the platform yaw degree of freedom is included in the controller design the performance is improved through better power and speed regulation.

Individual Blade Pitch Control of a Floating Offshore Wind Turbine on a Tension Leg Platform

A disturbance accommodating controller (DAC) to reject wind speed perturbations is applied on a 5MW wind turbine mounted on a tension leg platform (TLP). Multi-blade coordinate (MBC) transformation is used to address the periodicity of the floating wind turbine system. A method to apply DAC after applying MBC transformation is developed and several implementation options are presented. Simulations were carried out to assess the fatigue loads of the system in accordance with IEC61400-3 standard design load case 1.2; however simulations were restricted to region 3 analysis due to the lack of region transition logic currently implemented for the DAC. A gain scheduled PI (GSPI) controller is used as a baseline/reference controller to compare the performance of the DAC. Simulation results show that the DAC significantly improves power and speed regulation, reduces tower fatigue loads, and maintains the fatigue loads of the blades and the shaft to a comparable level to the baseline controller applied on the TLP. This improvement is due to the use of individual blade pitching and rejecting wind speed variations. Relative to an onshore wind turbine with a GSPI controller, the DAC reduces tower side-side fatigue load and maintains the blade and shaft loads to a comparable level. Tower fore aft fatigue loads remain higher by 27% (instead of 45% increase by the baseline controller on the TLP).

Active Structural Control with Actuator Dynamics on a Floating Wind Turbine

of passive and active structural controllers across the entire operational envelope of the wind turbine is investigated. Simulations of a 5 MW wind turbine mounted on a barge platform are carried out in accordance with the IEC 61400-3 standard design load case 1.2 { fatigue load testing. Simulation results show that both passive and active structural controllers consistently improve power regulation and reduce tower fore-aft fatigue loads and platform pitching motion. For example, the passive and active structural controllers reduce tower fore-aft fatigue damage equivalent load by an average of 13% and 31% relative to a baseline controller respectively. The maximum tower base bending moment can also be reduced by up to 20%. However, the unconstrained tuned mass damper stroke is large and may not be practical for real-world implementation in its current form.

A Study of Dynamic Coupling and Composite Load Control for Wind Turbines

*† ‡ Wind turbine components are sized according to their expected fatigue loads and any reduction in these would decrease costs and prolong working life. Wind turbine control is a multivariable problem with large interaction across multiple control channels (inputs to objectives). This article studies coupling and investigates a composite load controller for a wind turbine, consisting of a linear quadratic regulator for the loads and a baseline power controller for speed and torque regulation. The load objectives were to reduce tower foreaft, low speed shaft (LSS) tilt, and LSS yaw bending fatigue. Simulations in above rated wind conditions evaluated the composite design against the baseline controller alone, as well as a classical architecture with multiple single-input single-output (MSISO) loops. In this way, it is important to compare to a control design that is not aware of any coupling. Results showed significant mitigation of damage equivalent loads in the tower fore-aft, and LSS tilt and yaw directions across both designs when compared to baseline. The composite controller achieves better load reduction without the need for filtering out the effects of other channels, as would be required in the MSISO design.

A Review of Floating Wind Turbine Controllers

A review of the most important contributions made towards designing and testing controllers for floating wind turbines is presented in this chapter. These controllers range in complexity, description detail, simulation results and testing styles, and floating platform structure. Furthermore, they have not yet been compared against each other quantitatively using the same simulation conditions. However, the attributes of each control approach documented in the literature are listed in this chapter. Several approaches that deal with the reduction of platform pitch damping are discussed such as the use of individual blade pitching and tuned mass dampers.