Vibration and power regulation control of a floating wind turbine with hydrostatic transmission

We design a blade pitch controller employing linear parameter-varying (LPV) synthesis techniques for a floating hydrostatic wind turbine (HWT) with a barge platform, which is based on the LIDAR (Light Detection and Ranging) preview on the wind speed. The developed control system can simultaneously reduce barge pitch motions and regulate the power in Region 3. These two functions would normally disturb each other if designed separately. The state space model is not affinely dependent on the wind speed thus the LPV controller is obtained by satisfying multiple LMIs evaluated at a set of gridded points within the wind speed range in Region 3. An anti-windup compensation scheme is then used to improve the LPV controller’s performance when the pitch undergoes saturation around the rated wind speed. The simulations based on a high-fidelity barge HWT model show that our pitch controller significantly reduces barge pitch motions, loads on blade bearings & tower, and generator power fluctuations, compared with a gain-scheduled PI pitch controller

Control of Hydrostatic Transmission Wind Turbine

In this study, we proposed a control strategy for a wind turbine that employed a hydrostatic transmission system for transmitting power from the wind turbine rotor via a hydraulic transmission line to a ground level generator. Wind turbine power curve tracking was achieved by controlling the hydraulic pump displacement and, at the other end of the hydraulic line, the hydraulic motor displacement was controlled so that the overall transmission loss was minimized. Steady state response, dynamic response, and system stability were assessed. The maximum transmission efficiency obtained ranged from 79% to 84% at steady state when the proposed control strategy was implemented. The leakage and friction losses of the hydraulic components were the main factors that compromised the efficiency. The simulation results showed that the system was stable and had fast and well-damped transient response. Double wind turbine system sharing hydraulic pipes, a hydraulic motor, and a generator were also studied. The hydraulic pipe diameter used in the double-turbine system increased by 27% compared to the single-turbine system in order to make the transmission coefficient comparable between both systems. The simulation results suggested that the leakage losses were so significant that the efficiency of the system was worsened compared with the single-turbine system. Future studies of other behavioral aspects and practical issues such as fluid dynamics, structure strength, materials, and costs are needed

Quasi Self-Excited DFIG-Based Wind Energy Conversion System

This article introduces a new configuration of the doubly-fed induction generator (DFIG) based wind energy conversion system (WECS) employing only a reduced-size rotor side converter (RSC) in tandem with a supercapacitor. In the proposed structure, the grid side converter (GSC) utilized in conventional DFIG-based WECSs is successfully eliminated. This is accomplished by employing the hydraulic transmission system (HTS) as a continuously variable and shaft decoupling transmission unit. This transforms the conventional constant-ratio drives by providing an opportunity to control the power flow through the generator's rotor circuit regardless of the wind turbine's shaft speed. This feature of HTS can be utilized to control the RSC power and ultimately regulate the supercapacitor voltage without a need for GSC. The proposed system is investigated and simulated in MATLAB Simulink at various wind speeds to validate the results and demonstrate the dynamic performance of the system

Control of the offshore wind turbine and its grid integration

This thesis investigates the way to reduce the maintenance cost and increase the life cycle of the offshore wind turbines, as in the offshore case maintenance is highly difficult and expensive. Firstly, we study the possibility to replace the vulnerable and expensive DC link capacitor in wind power integration system by the virtual infinite capacitor (VIC), which is a power electronic circuit functioning as a large filtering capacitor. We propose a control algorithm for the VIC. Before applying it to the wind power system, we firstly test it in a simple power factor compensator (PFC) as the output filter capacitor. The simulation results show the effective filtering performance of VIC in low-frequency range. Then, we validate it experimentally by directly injecting the DC voltage together with a 50 Hz ripples to the VIC. The VIC successfully eliminates the ripple and extracts the DC voltage at the output terminals. Besides, the experiment performances are highly consistent with the corresponding simulations, which demonstrates the possibility to use VIC to replace the DC-link capacitor in wind power integration system and use it in other industrial systems. Since the VIC mainly filters the ripple in low frequency range while the DC-link voltage usually includes ripples in two distinct frequency ranges, we further develop it into the parallel virtual infinite capacitor (PVIC), aiming to suppress the voltage ripple in a wider frequency range. The PVIC is applied to replace the DC-link capacitor in wind turbine grid integration system. The simulations are conducted under different grid conditions with turbulent wind input. The results show that the PVIC provides much better voltage suppression performance than the equivalent DC-link capacitor, which facilitates the power generation control under normal operations and reduces the risks of converter failure under grid faults. In this way, the PVIC proves to be a great solution to substitute the vulnerable DC-link voltage in the offshore wind turbine power integration system. The wind power conversion system from the generator to the grid is composed of a DC-link capacitor and two back-to-back power converters. Though the application of PVIC removes the fragile DC-link capacitor in the power conversion system, the power converters are also fragile and expensive. In addition, the existence of power converters decouples the generator with the grid, which hinders the direct inertia support and frequency regulations from wind turbines. It would be desirable to completely remove the whole power conversion system. Hydrostatic wind turbine (HWT) may provide a suitable solution. The HWT is a wind turbine using hydrostatic transmission (HST) to replace the original heavy and fragile gearbox. The HST can provide the ‘continuously variable gearbox ratio’ , which allows HWT to be connected to a synchronous generator (SG) and then directly to the grid. We propose a coordinated control scheme for the HWT. The simulations are conducted with turbulent wind under variable system loads. The results indicate that with the proposed coordinated control system, the HWT (without power converters) provides efficient frequency support to the grid, which shows it is a promising solution for the future offshore wind power system. Finally, we consider to further reduce the maintenance cost and improve the performance of the HWT by using a new and novel control algorithm called model-free adaptive control (MFAC). It is applied to both torque control and pitch control of the HWT. Their control performances are compared to some of the existing algorithms. The simulation results demonstrate that the MFAC controller has much better tracking and disturbance rejection performances than the existing algorithms which can increase the fatigue life of the wind turbine and reduce the maintenance cost

Modelling and Optimization of Wave Energy Converters

Wave energy offers a promising renewable energy source. This guide presents numerical modelling and optimisation methods for the development of wave energy converter technologies, from principles to applications. It covers oscillating water column technologies, theoretical wave power absorption, heaving point absorbers in single and multi-mode degrees of freedom, and the relatively hitherto unexplored topic of wave energy harvesting farms. It can be used as a specialist student textbook as well as a reference book for the design of wave energy harvesting systems, across a broad range of disciplines, including renewable energy, marine engineering, infrastructure engineering, hydrodynamics, ocean science, and mechatronics engineering. The Open Access version of this book, available at https://www.routledge.com/ has been made available under a Creative Commons Attribution-Non Commercial-No Derivatives 4.0 license

Control of large offshore wind turbines.

Several control strategies are proposed to improve overall performances of conventional (geared equipped) and hydrostatic offshore wind turbines. Firstly, to maximise energy capture of a conventional turbine, an adaptive torque control technique is proposed through simplifying the conventional extremum seeking control algorithm. Simulations are conducted on the popular National Renewable Energy Laboratory (NREL) monopile 5-MW baseline turbine. The results demonstrate that the simplified ESC algorithms are quite effective in maximising power generation. Secondly, a TMD (tuned mass damper) system is configured to mitigate loads on a monopile turbine tower whose vibrations are typically dominated by its first mode. TMD parameters are obtained via H2 optimisation based on a spatially discretised tower-TMD model. The optimal TMDs are assessed through simulations using the NREL monopile 5-MW baseline model and achieve substantial tower load reductions. In some cases it is necessary to damp tower vibrations induced by multiple modes and it is well-known that a single TMD is lack of robustness. Thus a control strategy is developed to suppress wind turbine’s vibrations (due to multiple modes) using multiple groups of TMDs. The simulation studies demonstrate the superiority of the proposed methods over traditional ones. Thirdly, the NREL 5-MW baseline turbine model is transformed into a hydrostatic wind turbine (HWT). An H∞ loop-shaping torque controller and a light detection and ranging-based linear-parameter-varying anti-windup pitch controller are designed for the HWT. The tests on a monopile HWT model indicate good tracking behaviours of the torque controller and much improved performances of the linear-parameter-varying pitch controller over a gain-scheduled PI pitch controller. Finally, the hydraulic reservoir of a barge HWT is made into a bidirectional-tuned- liquid-column-damper (BTLCD) to suppress barge pitch and roll motions. The simulation results validate the effectiveness of the optimal BTLCD reservoir in reducing the tower loads and power fluctuations

Design, validation and application of wave-to-wire models for heaving point absorber wave energy converters

Ocean waves represent an untapped source of renewable energy which can significantly contribute to the energy transition towards a sustainable energy mix. Despite the significant potential of this energy source and the multiple solutions suggested for the extraction of energy from ocean waves, some of which have demonstrated to be technically viable, no commercial wave energy farm has yet been connected to the electricity grid. This means that none of the technologies suggested in the literature has achieved economic viability. In order to make wave energy converters economically viable, it is essential to accurately understand and evaluate the holistic behaviour and performance of wave energy converters, including all the different conversion stages from ocean waves to the electricity grid. This can be achieved through wave tank or open ocean testing campaigns, which are extremely expensive and, thus, can critically determine the financial sustainability of the developing organisation, due to the risk of such large investments. Therefore, precise mathematical models that consider all the important dynamics, losses and constraints of the different conversion stages (including wave-structure hydrodynamic interaction and power take-off system), known as wave-to-wire models, are crucial in the development of successful wave energy converters. Hence, a comprehensive literature review of the different mathematical approaches suggested for modelling the different conversion stages and existing wave-to-wire models is presented, defining the foundations of parsimonious wave-to-wire models and their potential applications. As opposed to other offshore applications, wave energy converters need to exaggerate their motion to maximise energy absorption from ocean waves, which breaks the assumption of small body motion upon which linear models are based. An extensive investigation on the suitability of linear models and the relevance of different nonlinear effects is carried out, where control conditions are shown to play an important role. Hence, a computationally efficient mathematical model that incorporates nonlinear Froude-Krylov forces and viscous effects is presented. In the case of the power take-off system, mathematical models for different hydraulic transmission system configurations and electric generator topologies are presented, where the main losses are included using specific loss models with parameters identified via manufacturers’ data. In order to gain confidence in the mathematical models, the models corresponding to the different conversion stages are validated separately against either high-fidelity well-established software or experimental results, showing very good agreement. The main objective of this thesis is the development of a comprehensive wave-to-wire model. This comprehensive wave-to-wire model is created by adequately combining the subsystems corresponding to the different components or conversion stages. However, time-step requirements vary significantly depending on the dynamics included in each subsystem. Hence, if the time-step required for capturing the fastest dynamics is used in all the subsystems, unnecessary computation is performed in the subsystems with slower dynamics. Therefore, a multi-rate time-integration scheme is implemented, meaning that each subsystem uses the sample period required to adequately capture the dynamics of the components included in that conversion stage, which significantly reduces the overall computational requirements. In addition, the relevance of using a high-fidelity comprehensive wave-to-wire model in accurately designing wave energy converters and assessing their capabilities is demonstrated. For example, energy maximising controllers based on excessively simplified mathematical models result in dramatic consequences, such as negative average generated power or situations where the device remains stuck at one of the end-stops of the power take-off system. Despite the reasonably high-fidelity of the results provided by this comprehensive wave-towire model, some applications require the highest possible fidelity level and have no limitation with respect to computational cost. Hence, the simulation platform HiFiWEC, which couples a numerical wave tank based on computational fluid dynamics to the high-fidelity power take-off model, is created. In contrast, low computational cost is the main requirement for other applications and, thus, a systematic complexity reduction approach is suggested in this thesis, significantly reducing the computational cost of the HiFiWEC platform, while retaining the adequate fidelity level for each application. Due to the relevance of the nonlinearity degree when evaluating the complexity of a mathematical model, two nonlinearity measures to quantify this nonlinearity degree are defined. Hence, wave-to-wire models specifically created for each application are generated via the systematic complexity reduction approach, which provide the adequate trade-off between computational cost and fidelity level required for each application

Ein nichtlineares Modell für das dynamische Verhalten von schwimmenden Offshore-Fundamenten

Among the renewable energies, the exploitation of offshore wind energy in deep waters is becoming more and more important, and it is expected to increase even more because of the intrinsic potential and the large availability of this resource. Deep-water wind turbines are usually installed over moored floating supports. Their dynamics depends on the complex interaction between the system and the environment making the use of numerical models almost inevitable for the design and optimization of such structures. In this context, a nonlinear model for the dynamics of moored floating platforms is developed. The dynamic problem of the platform is formulated in the framework of the dynamics of rigid bodies, referring to the mixed representation of the motion, which consists in the simultaneous use of two different bases. The formulation is developed for a wide range of loads (forces and torques) with particular attention to the transformations of the state variables. Both follower and non-follower loads are considered. The differential problem is solved with a recently developed time integration algorithm that considers the Lie group structure of the configuration space overcoming some critical aspects associated with the typical use of nautical angles and their time derivatives. The resulting formulation is very general and in principle can be exploited for the study of every system modelled as a rigid body, such as ships and hulls. The developed dynamic solver can be coupled with other models to study specific problems. In this work the assessment of the loads related to both the mooring system and the hydrodynamic action is addressed. In particular, mooring lines are modelled by means of a quasi-static formulation, whereas wave loads are evaluated with the linear hydrodynamic theory. In general, both the formulations guarantee a satisfying level of accuracy, even though in some specific cases the inertia and damping of the mooring lines as well as higher-order hydrodynamic terms should be included to improve the reliability of the model. The numerical model is tested against a number of dynamic problems for which the exact analytic solution is known, allowing a detailed assessment of the capabilities of the method. The dynamics of moored floating platforms is then investigated discussing the effect of different strategies of simulation on the system response and the role of the main parameters affecting the motion.Die Bedeutung von Offshore-Windenergie unter den erneuerbaren Energieträgern nimmt momentan stark zu und weitere Zuwächse werden aufgrund der weiten Verfügbarkeit dieser Energiequelle und ihres intrinsischen Potenzials erwartet. Windkraftanlagen in Tiefwasser werden normalerweise mittels schwimmenden Offshore-Fundamenten am Meeresboden verankert. Ihr dynamisches Verhalten hängt von der komplexen Interaktion zwischen dem System und der Umgebung ab, weshalb numerische Modelle unerlässlich für die Entwicklung und Optimierung solcher Strukturen sind. Daher wird ein nichtlineares Modell zur Beschreibung des dynamischen Verhaltens von schwimmenden Offshore-Fundamenten vorgestellt. Dafür wird ein analytischer Ansatz aus dem Bereich der Dynamik starrer Körper unter Verwendung einer gemischten Darstellung der Bewegung entwickelt. Dieser Ansatz berücksichtigt eine große Bandbreite an Belastungen (Kräfte und Momente) unter Beachtung der Transformation der Zustandsvariablen. Dabei werden sowohl Lasten mit raumfesten als auch mit veränderlichen Wirkrichtungen berücksichtigt. Die Lösung des Differentialgleichungssystems erfolgt mittels eines Zeitintegrationsverfahrens, das die Struktur der Lie-Gruppe des betrachteten Raumes berücksichtigt und eine Vereinfachung in Bezug auf die allgemein verwendeten Eulerschen Winkel darstellt. Der resultierende Ansatz ist allgemeingültig und kann prinzipiell auf jedes beliebige, als starrer Körper modellierte System angewendet werden. Der dynamische Gleichungslöser wird mit zusätzlichen Modellen zur Ermittlung der, von der Verankerung verursachten, Reaktionskräfte und -momente und der Beanspruchung durch hydrodynamische Lasten gekoppelt. Dabei werden die Ankertrossen mittels eines quasi-statischen Ansatzes modelliert und die Belastung durch Wellen wird nach der linearen hydrodynamischen Theorie bestimmt. Generell liefern beide Ansätze eine hohe Genauigkeit, in einigen Spezialfällen sollten die Massenträgheit und Dämpfung der Verankerung sowie hydrodynamische Terme höherer Ordnung zur Verbesserung des Modells berücksichtigt werden. Das numerische Modell wird anhand einer Reihe dynamischer Problemstellungen mit exakten, analytischen Lösungen validiert, wodurch die allgemeingültige Anwendbarkeit der Methode belegt wird. Abschließend wird das dynamische Verhalten von schwimmenden Offshore-Fundamenten hinsichtlich verschiedener Simulationsstrategien und des Einflusses der Hauptparameter auf die Bewegung untersucht