Loading Reproduction And Power Smoothing For Wind Turbine Drive-train

The focus of this dissertation is put on the wind turbine drive trains due to their critical roles in wind power generation and regulation.The five-degree-of-freedom load reproduction and the power smoothing techniques are proposed to facilitate the performance evaluation and improvement of the turbine drive trains and hence to enhance the wind turbine reliability and durability.The remainder of this dissertation can be mainly organized as the following two sections.The five-degree-of-freedom turbine load reproduction technique for wind turbine drive trains.In chapter 2,the five-degree-of-freedom turbine loads are first defined in blade and hub reference frames and then a new loading system and the associated loading control strategies are presented for simulating the five-degree-of-freedom wind turbine loads.The five-degree-of-freedom turbine loads mainly include two orthogonal force components and three orthogonal moment components,and can be briefly calculated and defined in the blade and hub reference frames.For this loading system,the real wind rotor and blades are replaced by an equivalent rotating disc and driven by an electric motor.A set of equispaced electro-hydraulic loading actuators are symmetrically placed around this rotating disc and are controlled independently to fully reproduce the five degree-of-freedom turbine loads.The rotating disc is optimally designed to minimize the size and cost by using dynamic soft inertia compensation and an improved particle swarm optimization algorithm while considering the equivalent inertia and mass of the wind rotor and blades.Furthermore,two loading control strategies are presented to decompose such five-degree-of-freedom turbine loads into reference loading forces for each loading actuator by incorporating additional seven-degree-of-freedom dummy loads or by sorting and grouping the turbine loads in different directions.In chapter 3,the in-depth modelling and dynamic analysis of a typical loading actuator are presented and a back-stepping loading force controller is designed to track the decomposed reference loading force.This typical loading actuator is basically a critical-center valve controlled single-rod hydraulic cylinder combination and hence can be linearly modelled around equilibrium operating points.A proportional integral controller is then designed based on this linearized model to analyze the dynamics characteristics of this actuator in the time and frequency domains by varying the major hydraulic parameter values.Furthermore,a back-stepping loading force controller is designed to accurately track the reference loading force by considering model uncertainities in spite of the large external loading disturbances.In chapter 4,the single-chamber type loading actuators are proposed and their loading control characteristics are thoroughly investigated.The loading force generated from these actuators can be directly controlled to replicate the reference force by regulating the single-chamber pressure of the actuators.For these loading actuators,a loading actuator with the intrinsic pressure feedback loop and a bypass pressure valve controlled loading actuator are respectively designed to track the reference loading force.The dynamic model and the state space model of the loading actuators are established and thoroughly analyzed.A robust H? force controller and a robust H? filter are then designed to enhance the robustness of the bypass pressure valve controlled loading actuator against external disturbances and hence to improve the loading control accuracy by estimating the state variables in real time.Further,comparative simulations are conducted by using experimental data to evaluate the effectivenss of this type of loading actuators and the performances of the H? force controller in comparision with the conventional proportional integral controller.In chapter 5,a loading system is designed,built and tested to fully simulate the steady-state and dynamic turbine loads for a 100 kW wind turbine drive train.The detailed design procedure and manufacturing details are provided.This loading system mainly consists of a mechanical system,electro-hydraulic loading actuators and an industrial computer control system.The mechanical system is carefully designed and optimized to make the loading system work smoothly and quietly even under harsh loading conditions.The computer control system is also developed to facilitate the loading force regulation and condition monitoring for this loading system.Furthermore,typical loading conditions and experiments are defined and conducted to thoroughly analyze the operating characteristics of the loading system.This loading system has been tested under real loading conditions to thoroughly evaluate its dynamic characteristics and effectiveness in reproducing the five-degree-of-freedom turbine loads.The output power smoothing technique for wind turbine drive trains.In chapter 6,a servo-valve controlled pitch system and an electro-hydraulic digital pitch system are respectively proposed to smooth output power and drive-train torque fluctuations for large-scale wind turbines.This servo-valve controlled pitch system is driven by a valve controlled hydraulic motor and is precisely controlled to track the desired pitch angle trajectory.Planetary bevel gear pitch mechanism,nonlinear modeling and characteristics of this pitch system are then presented and analyzed.An adaptive nonlinear sliding mode pitch angle controller is also designed to effectively handle dynamic nonlinearities and external disturbances in this pitch system.The proposed digital pitch system can be actuated and controlled by a digital servo motor to regulate the blade pitch angle with relatively high control accuracy.This pitch system is characterized by an outer open control loop for enhancing the direct pitching motion and an intrinsic position control loop offering the benefit of sensor-less pitch control.Modelling,stability analysis and dynamic characteristics of this pitch system are presented.A pragmatic design procedure is provided and several key design parameters are determined or optimized.An ease to implement feed-forward compensation method is also designed and incorporated in this system to minimize the dynamic and steady-state tracking errors.Comparative simulation experiments are conducted to verify the effectiveness of the proposed pitch systems and controller in output power smoothing,control precision and robustness.In chapter 7,a megawatt-scale hydro-viscous transmission based continuously variable speed wind turbine is proposed to guarantee a smooth transition among different operating regions and hence to improve the power efficiency and quality.This type of turbine is achieved by highly integrating a hydro-viscous element into the turbine drive-train to mitigate the upstream wind loading fluctuations and output power fluctuations.This element allows the turbine speed to be directly regulated by continuously changing the oil film thickness.Torque characteristics and transmission efficiency of this type of wind turbine are analyzed.A particle swarm optimization algorithm based multi-objective optimization method is then employed to optimally design the hydro-viscous transmission.Major components of the wind power system are mathematically modeled and analyzed in detail.Furthermore,a hybrid output power control strategy is proposed and implemented to precisely control the generated power and torque for this system.The effectiveness of this type of wind power system is validated by a theoretical analysis and a comparative simulation study.