Direct-drive Wind Turbine Research Articles

Scale models formulation of switched reluctance generators for low speed energy converters

This study presents a design methodology based on scale models for low speed switched reluctance generators (SRG). This study is motivated by the application of SRG to direct-drive wind turbines and other low speed renewable energy systems. In direct drive energy converters considerable simplifications result from the elimination of the gear box which has generally been used to interface a slowly rotating shaft with the generator shaft. The comparison and evaluation of magnetic structures plays an important role in the SRG design. General design methodologies are usually oriented towards the evaluation of stator/rotor poles combinations for regular switched reluctance machines. Besides covering that feature, the formulation of scale laws proposed is also suitable to compare other SRG topologies distinguished by different characteristics of electric and magnetic circuits and their own relative position. In addition, this methodology can be extended to other physical phenomena such as thermal changes and magnetic saturation by introducing some constraints. The running example compares a modular short flux-path topology versus a low speed 20 kW prototype SRG designed for a direct drive wind turbine. The modular topology can optimise the efficiency and weight taking benefits from the significant gain of power per unit of volume and lower losses.

Simulation and Analysis of Direct-driven Wind Turbine

Direct-driven wind turbine has become one of the mainstream technologies in wind power generation. Studying the system structure and the control technology of direct-driven wind turbine is the basis for optimization operation of direct-driven wind turbine. The system structure of direct-driven wind turbines is analyzed and the main subsystem mathematical model of direct-driven wind turbine are established, including wind speed model, aerodynamic model, generator model, and grid side converter model. The overall model of direct-driven wind turbine is established by integration of each subsystem mathematical model. The overall model of the direct-driven wind turbine is modeled and analyzed, and the simulation results show that the direct-driven wind turbine model can accurately reflect the dynamic characteristics of the direct-driven wind turbine operation process.

Lightweight stator structure for a large diameter direct‐drive permanent magnet synchronous generator intended for wind turbines

A concept has been developed for a novel lightweight wheel structure intended for rotor and stator use in direct-drive wind turbine generators. It uses a slanted spoke and rim architecture to provide maximum static structural performance with minimum weight. A unique attribute of the structure is its use of layered sheet steel elements to form the spokes and rim. Friction between layers establishes structural integrity. The interaction between layers and the resulting increase in damping normal to the stack offers improved dynamic performance. A static structural analysis of a full-scale stator wheel structure for an 8 MW permanent magnet machine demonstrates the structural effectiveness of the architecture. To understand vibration characteristics of the lightweight wheel structure, a quarter-scale prototype was built and an experimental modal analysis was carried out to measure actual radial vibration responses. This data was used to verify a numerical model, and there was good agreement between measured and predicted behaviours. Finally, a modal analysis was carried out for the full-scale stator wheel structure. The dynamic performance of the wheel is acceptable for the stator of the 8 MW direct-drive permanent magnet synchronous generator embodiment.

Micro-Siting of Wind Turbine in Complex Terrain: Simplified Fatigue Life Prediction of Main Bearing in Direct Drive Wind Turbines

In Japan, 1,516 wind turbine accidents have been recorded between year 2004 and 2012, and 84% of them were for turbines in complex terrains. The longest downtime was associated with damage to main shafts or bearings with an average downtime of 5.7 months. Careful micro-siting in complex terrains can prevent these accidents from happening. The objective of the paper is to provide an intermediate step that allows consultants, developers, and wind farm owners to further evaluate micro-siting of wind turbines in complex terrains, prior to load simulations by manufacturers. The author developed a simplified method to predict fatigue life of a main rear bearing in direct drive wind turbines. The method uses hub-height 10 minutes wind data as an input. The validation with an actual accident showed practically good agreement of 12.0 years of the predicted life against 12.7 years of the actual life. The method was also applied to quantify the effect of a curtailment. The proposed curtailment increased the predicted life to 35.5 years with a relatively small range of wind speed at a direction of only 1 % frequency distribution. With the proposed method, it is possible to layout turbines where fatigue life of a main rear bearing is longer than its design life. The method can also be applied to existing turbines in order to spot turbines that require careful maintenance, and to consider an installation of condition monitoring system on a bearing.

Electromagnetomechanical Coupled Vibration Analysis of a Direct-Drive Off-Shore Wind Turbine Generator

In large direct-drive off-shore wind turbine generators one challenge is to engineer the system to function securely with an air gap length of about a thousandth of the outer rotor diameter. Compared to the large diameter of the generator rotor, the rolling element bearings can only be constructed with a relatively limited size. This makes it challenging to design appropriate constructions able to transmit the large applied magnetic forces encountered in the air gap of direct drive wind turbine generators. Currently, this challenge is met by designing stiff heavy rotors that are able to withstand the forces in the air gap. Incorporating flexibility into the design of the rotor structure can lead to a lighter less expensive rotor. In order to be able to do this the magnetomechanical coupling in the air gap and its effect on the structural dynamics need to be taken into account when predicting the intended flexibility. This paper introduces an approach for a multiphysical modal analysis that makes it possible to predict the dynamics of the strongly coupled magnetomechanical system. The new method is validated using measurements of a simple lab setup. It is then applied to a single-bearing design direct-drive wind turbine generator rotor to calculate the changes of the structural dynamics caused by the electromagnetomechanical coupling.

Controlling structural vibrations in wind turbines by constructing function V

For a variable speed large scale wind turbine, the vibration issues become a key problem that cannot be ignored in the turbine’s life cycle. Wind turbine tower vibration will cause superfluous mechanical loads. To resolve the vibration issue, a method for constructing the energy function V is proposed to meet the demands of safe operation. The Lyapunov theorem has been embedded in a wind turbine control algorithm, proving the theoretical feasibility of stability control based on function V. According to an analysis of this complex nonlinear model for the wind turbine, the general method of constructing an energy function suitable for a wind turbine is presented explicitly. The feasibility of applying an energy function to wind turbine vibration control is verified experimentally using a 3.0-MW direct drive wind turbine model. The experimental results indicate that the dynamic performance of the tested wind turbine model with energy function control is significantly better than that of the uncontrolled structure in terms of the reduction of nacelle acceleration, velocity, and displacement response.

Comparison Between Different Design Topologies for Multi-Megawatt Direct Drive Wind Generators Using Improved Second Generation High Temperature Superconductors

Various design topologies were investigated to study the weights and costs for a direct drive wind turbine generator using improved second generation (2G) YBCO high temperature superconductors (HTS). Five different design topologies using combinations of air core, iron core, salient and non-salient configurations for both stator and rotor were considered for a generator with the nominal rating of 10 MW, 8 RPM and 3300 V. The main objective of the investigation was to minimize the HTS quantities and the reduction of total cost to achieve the same rating performance. The electromagnetic design was performed using transient finite element modeling including the improved HTS conductor characteristic properties. The mechanical design includes the support structures for both full load operation and sudden three-phase terminal fault conditions which can cause severe stresses.

Comparison Study of Superconducting Wind Generators With HTS and LTS Field Windings

Large-scale direct-driven wind turbines are preferred for offshore wind generation from the viewpoint of cost of energy and system reliability. However, direct-driven generators of traditional technology are very difficult to be scaled up to a rated power of 10 MW or above. Superconducting generators offer a candidate to fill that gap. A feasible superconducting wind generator topology has rotor field windings made of superconductors and traditional copper stator windings carrying alterative currents. Nowadays, two kinds of superconductors are available in the market: the low-temperature superconductors (LTS) and the high-temperature superconductors (HTS). They have great differences in terms of material characteristics, product performance, price, and the relevant cryogenic and refrigeration system, which have a great influence on the design of superconducting generators. In this paper, a comparison study of superconducting wind generators with HTS and LTS rotor field windings is carried out, considering the performance and cost of superconductors and the cryogenic and refrigeration system. From the viewpoint of initial capital, technology maturity, and engineering feasibility, the LTS wind generator is recommended for subscale or full-sized prototype development in the near future.

Study of Variable Pitch Control for Direct-drive Permanent Magnet Wind Turbines Based on Fuzzy Logic Algorithm

Study of Variable Pitch Control for Direct-drive Permanent Magnet Wind Turbines Based on Fuzzy Logic Algorithm

Current-Based Mechanical Fault Detection for Direct-Drive Wind Turbines via Synchronous Sampling and Impulse Detection

Online fault detection is an effective means to improve wind turbine reliability and performance and reduce wind turbine downtime and operating and maintenance costs. Current-based wind turbine fault detection techniques have received more and more attention in academia and industry due to their nonintrusive character and economic advantages. This paper presents a novel computationally efficient high-resolution wideband synchronous sampling algorithm for the mechanical fault detection of variable-speed direct-drive wind turbines (i.e., no gearbox) only using nonstationary generator stator current measurements. The proposed algorithm synchronously resamples the current signals such that the varying characteristic frequencies of the excitations generated by wind turbine faults in the current signals become constant values. An impulse detection algorithm is then proposed to detect the faults by identifying the excitations from the frequency spectra of the synchronously sampled stator current signals. Experimental studies are carried out to demonstrate the effectiveness of the proposed algorithms for the detection of rotor eccentricity and bearing faults of a direct-drive wind turbine operating in variable-speed conditions.

Analysis, design and experimental verification of a field‐modulated permanent‐magnet machine for direct‐drive wind turbines

This paper presents the design and experimental verification of a field-modulated permanent-magnet (FMPM) machine using spoke-magnet outer rotor for direct-drive wind turbines. The fundamental operation and optimised design process of the proposed machine are elaborated and discussed in detail. The electromagnetic analytical calculation is used for the initial determination of the prototype design, mainly concerning the special constraints of wind power generation, and the two-dimensional finite element analysis is also applied to optimise and investigate the detailed characteristics. A 5 kW prototype is constructed, and the measured performances have verified the analysis and design, justifying that the proposed machine can offer significant advantages of high-torque capability and low-voltage regulation. Experience gained from this study has led to develop a general design guideline for the open-slot FMPM machine using spoke-magnet outer rotor for direct-drive applications.

English

This paper deals with permanent magnet synchronous generator (PMSG) based wind energy conversion system (WECS) integrated with grid with two back to back connected converters with a common DC link. The aim of this research is to model control of direct driven 1.5 MW wind turbine permanent magnetic synchronous generator (PMSG) which feeds alternating current (AC) power to the utility grid .The machine side converter is used to extract maximum power from the wind. In this paper a study of WECS is done by using a constant speed direct-driven wind turbine in Matlab. Moreover, by maintaining the dc link voltage at its reference value, the output ac voltage of the inverter can be kept constant irrespective of variations in the wind speed and load. An effective control techniques to extract maximum power from wind turbine is maximum power point tracking controller (MPPT), grid side controller also called voltage controller, pitch controller, phase lock loop controller (PLL) also used in this project, transformer used for isolation purpose, crow bar circuit used for protection the whole system. KEYWORDS-Permanent magnet synchronous generator(PMSG), wind energy conversion system(WECS), back to back PWM converter IGBT based, DC link capacitor, Direct-Driven, MPPT controller, PLL loop, generator side converter control, grid side converter control, pitch controller, crow bar protection and transformer.

Multimodel Modeling and Predictive Control for Direct-Drive Wind Turbine with Permanent Magnet Synchronous Generator

The safety and reliability of the wind turbines wholly depend on the completeness and reliability of the control system which is an important problem for the validity of the wind energy conversion systems (WECSs). A method based on multimodel modeling and predictive control is proposed for the optimal operation of direct-drive wind turbine with permanent magnet synchronous generator in this paper. In this strategy, wind turbine with direct-drive permanent magnet synchronous generator is modeled and a backpropagation artificial neural network is designed to estimate the wind speed loaded into the turbine model in real time through the estimated turbine shaft speed and mechanical power. The nonlinear wind turbine system is presented by multiple linear models. The desired trajectory of the nonlinear system is decomposed to be suitable for the reference trajectory of multiple models that are presented by the linear models of the nonlinear system, which simplifies the nonlinear optimization problems and decreases the calculation difficulty. Then a multivariable control strategy based on model predictive control techniques for the control of variable-speed variable-pitch wind turbines is proposed. Finally, simulation results are given to illustrate the effectiveness of the proposed strategy, and the conclusion that multiple model predictive controller (MMPC) has better control performance than the PI control method is obtained.

Recent Developments on Time-Delay Neural Networks

Over the past several decades, an amount of attention has been devoted to neural networks, which have wide applications in many areas, such as signal processing, pattern recognition, and combinatorial optimization. On the other hand, time delays occur in electronic implementation of analog neural networks because of the transmission of signal and the finite switching speed of amplifiers and can lead to the instability and poor performance of systems; thus neural networks with time delay have also been studied extensively because numerous interesting results have been obtained over the last decade, including stability analysis, state estimation, and passivity. In this special issue, we have accepted seven papers. The paper titled “New Pinning Synchronization of Complex Networks with Time-Varying Coupling Strength, NonDelayed and Delayed Coupling” by G. Wang et al. studies the pinning synchronization problem for a class of complex networks by a stochastic viewpoint, in which both time-varying coupling strength and nondelayed and delayed coupling are included. The paper titled “A Novel Model of Conforming Delaunay Triangulation for Sensor Network Configuration” by Y. Ma et al. proposes a new method for solvingDelaunayTriangulation problem,which is called endpoint triangle’s circumcircle model (ETCM).The paper titled “Multimodel Modeling and Predictive Control for DirectDrive Wind Turbine with Permanent Magnet Synchronous Generator” by L. Wang et al. establishes a method based on multimodel modeling and predictive control is proposed for the optimal operation of direct-drive wind turbine with permanent magnet synchronous generator. The paper titled “Independent Component Analysis Based on Information Bottleneck” by Q. Ke et al. provides the equivalence of two algorithms of independent component analysis (ICA) based on the information bottleneck (IB). In “Nonlinear Time-Delay Suspension Adaptive Neural Network Active Control” by Y. Zhu and S. Zhu, a quarter-vehicle magnetorheological active suspension nonlinear model with time delay is established, and an adaptive neural network structure for magnetorheological active suspension is presented. The paper titled “An Adaptive Regulator for Space Teleoperation System in Task Space” by C. Ge et al. studies the problem of the gravity information for bilateral teleoperation. The paper titled “Robust Stabilization of Linear Switching Systems with both Input and Communication Delays” by L. Jia and Z. Sheng considers the stabilization control of a class of linear switching systems with time delays in both the input and the communication channels.

Research on Compound Pitch Control Technology of Direct-drive Permanent Magnet Wind Turbine

The conventional PID pitch controller was usually used in direct-drive permanent magnet wind turbine in order to maintain the output power unchanged above the rated wind speed. However, due to the complex structure of wind turbine as well as high nonlinearity of wind speed, the effect of PID control method is often less than ideal. A new pitch control method combined with fuzzy adaptive PID and fuzzy feed forward controller is proposed in this paper. The fuzzy adaptive PID controller is able to ensure the unit having a better control result than PID controller at various wind speeds. The fuzzy feed forward controller has improved the responsiveness of pitch control system. With the simulation of a 1.5MW wind turbine, the responsiveness and stability of wind turbine can be effectively proved by the new pitch control method above the rated wind speed, the system’s overshoot is also reduced, which satisfies the demand of power control.

Dynamics and Control of Lateral Tower Vibrations in Offshore Wind Turbines by Means of Active Generator Torque

Lateral tower vibrations of offshore wind turbines are normally lightly damped, and large amplitude vibrations induced by wind and wave loads in this direction may significantly shorten the fatigue life of the tower. This paper proposes the modeling and control of lateral tower vibrations in offshore wind turbines using active generator torque. To implement the active control algorithm, both the mechanical and power electronic aspects have been taken into consideration. A 13-degrees-of-freedom aeroelastic wind turbine model with generator and pitch controllers is derived using the Euler–Lagrangian approach. The model displays important features of wind turbines, such as mixed moving frame and fixed frame-defined degrees-of-freedom, couplings of the tower-blade-drivetrain vibrations, as well as aerodynamic damping present in different modes of motions. The load transfer mechanisms from the drivetrain and the generator to the nacelle are derived, and the interaction between the generator torque and the lateral tower vibration are presented in a generalized manner. A three-dimensional rotational sampled turbulence field is generated and applied to the rotor, and the tower is excited by a first order wave load in the lateral direction. Next, a simple active control algorithm is proposed based on active generator torques with feedback from the measured lateral tower vibrations. A full-scale power converter configuration with a cascaded loop control structure is also introduced to produce the feedback control torque in real time. Numerical simulations have been carried out using data calibrated to the referential 5-MW NREL (National Renewable Energy Laboratory) offshore wind turbine. Cases of drivetrains with a gearbox and direct drive to the generator are considered using the same time series for the wave and turbulence loadings. Results show that by using active generator torque control, lateral tower vibrations can be significantly mitigated for both gear-driven and direct-driven wind turbines, with modest influence on the smoothness of the power output from the generator.

Mechanical calibration for the load measurement of a 750 kW direct-drive wind turbine generator system (KBP-750D)

A demonstration research through power performance and load measurements was performed for a 750 kW direct-drive wind turbine generator system (KBP-750D) at the Daegwanryeong Wind Turbine Demonstration Complex, Korea. A calibration test of mechanical loads by external force application was conducted as an operation of the load measurement based on the IEC TS 61400-13 standard. Strain gauge bridges were selected and attached to measure the fundamental loads of the wind turbine. A measurement system was then configured to measure total 40 channels of physical quantities, including the strain gauge bridges. Subsequently, a load measurement campaign using a developed data acquisition program lasted for several months. To calibrate the output signals, the zero-point offset of each strain gauge bridge was measured, and the calibration test by external force application was performed. This paper introduces the configuration of the measurement system installed in KBP-750D, reports the detailed processes of the zero-point offset measurement and load calibration tests, and presents the test results.

Wind Power Development in Power Grid and Study on Dispatching Strategy

Firstly, This paper summarizes the development situation of wind power in China, and analyzes the operation characteristics of all types of generating units one by one according to constant speed constant frequency asynchronous wind turbine, double-fed asynchronous wind turbine and direct-driven permanent magnet synchronous wind turbine. Then various effects of wind power integration on power grid are analyzed based on the actual situation of power grid. Finally, according to these effects, various measures of strengthening wind power integration management are proposed from management and technology levels. These measures are generally instructive and applicable to the same types of power grid with wind power.

Differential evolution method‐based output power optimisation of switched reluctance generator for wind turbine applications

The use of wind energy for electric power generation provides a clean and renewable source. Therefore there is an increasing interest in developing and exploiting natural energy generation system. Switched reluctance generators (SRGs) have the potential to be a robust and highly efficient electrical conversion system for variable-speed wind applications. This study presents a new approach for optimising performance of a SRG intended for variable-speed direct drive wind turbine applications. DC bus voltage level and phase voltage switching angles have been identified as control variables affecting power generation. Owing to highly non-linear characteristics of SRG, iterative simulation of the generator model on the range of control variables can be used for finding output power profile. Since it is a multidimensional search space, the number of iterations is very big. Differential evolution (DE) strategy has been introduced to find optimal firing angles and DC bus voltage level under multiple operating conditions. Optimisation of the control variables is performed using a machine model based on the measured characteristics. Selected operating points are experimentally tested using a 4 kW 1500 rpm SRG prototype. DE algorithm is a viable alternative for generating optimal control in multidimensional optimisation of SRG wind energy generation.

Monitor Method of Offshore Wind Turbine Based on Memory-Like and Neural Network Expert System

The number of offshore wind farms increases gradually because of the high capability of power generation. However, the costs of manufacturing, logistics, installation and maintenance of offshore wind turbine are higher than those of onshore wind turbine. Thus the introduction of fault diagnosis is considered as a suitable way to improve reliability of wind turbine and reduce costs of repairs and casualties. In this paper, 3 major failures of direct-driven wind turbine according to urgency and system responses are discussed. A "memory-like" model pretreatment method and a fault diagnosis method for the failures are investigated. The simulation results show that total amount of fault data to be processed and stored is reduced, and difficulties of knowledge gaining and fault reasoning are also decreased.