Direct-drive Wind Turbine Research Articles

Basic Characteristics and Design of a Novel Hybrid Magnetic Bearing for Wind Turbines

This paper proposes a five-degree-of-freedom (5-DOF) hybrid magnetic bearing (HMB) for direct-drive wind turbines, which can realize suspension in the 4-DOF radial and 1-DOF axial directions. Only two sets of radial control windings are employed in the proposed HMB because only one set of radial control windings can achieve the 2-DOF suspension in the radial direction. Unlike the traditional active thrust magnetic bearings, this paper uses a cylindrical rotor core without a large thrust disc in the novel HMB. The numbers of the controller, power amplifier and system volume can be reduced in the magnetic suspension system. This paper also presents the structure and basic characteristics of the proposed magnetic bearing. A precision equivalent magnetic circuit analysis of the permanent magnet ring and control magnetic field is conducted in this study, in consideration of the non-uniform distribution of magnetic density. Accordingly, the mathematical models, including the suspension force expression, are derived based on the accurate equivalent magnetic circuit. The basic principle of the structure parameter design is presented, based on the given key parameters. The accuracy of the analytical method is further validated by 3D finite element analysis.

Multi‐objective design and prototyping of a low cogging torque axial‐flux PM generator with segmented stator for small‐scale direct‐drive wind turbines

The multi-objective design of an axial-flux permanent magnet (PM) generator with the yokeless and segmented armature is done, for use in the small-scale direct-drive wind turbines. Using the multi-objective genetic algorithm technique, an optimisation process is performed for maximising the efficiency and minimising the cost, size, and mass of active materials, considering the practical constraints of the wind generator. The Pareto optimal fronts are used to find the best solutions. Also, a comparative study is performed to determine the most appropriate configuration among 12 configurations with the different combinations of poles and stator segments. Then, the selected optimal configuration is analysed using the three-dimensional (3D) finite element analysis (FEA). The inherent cogging torque of the PM wind generator affects the cut-in speed and start-up of the turbine. Thus, to improve the turbine performance, especially at low starting speeds, the generator cogging torque should be reduced to an acceptable level. In this study, using a new and practical approach, the peak-to-peak value of the cogging torque is reduced largely (81.4%), without notable reduction in the generator loadability. A prototype of the designed generator is fabricated and tested. The experimental results show good agreement with the 3D FEA simulation results.

On the Optimization of Generators for Offshore Direct Drive Wind Turbines

The objective of this paper is to optimize direct drive permanent magnet synchronous generators for offshore direct drive wind turbines in order to reduce the cost of energy. A 6 MW wind turbine design is assumed and parametric electromagnetic and structural generator models are introduced for a surface-mounted magnet generator topology (using magnets with high ${\rm{BH}}_{{\rm{max}}}$ ) and a flux-concentrating variant (using magnets with lower ${\rm{BH}}_{{\rm{max}}}$ ). These are optimized using a hybrid genetic algorithm and pattern search process and the results show that the surface-mounted permanent magnet generator produces the lower cost of energy. The choice of objective function is addressed and it is found that a simplified metric incorporating generator cost and losses proxy produces similar designs to a full cost of energy calculation. Further steps to improve the quality of the model include the effect of generator mass on the design and cost of the turbine tower and foundation, which can add €0.4 m to the turbine cost. Further optimizations are carried out to show the impacts of magnetic material costs (doubling this leads to a €1.1/MWh increase in cost of energy) and generator diameter limits (increasing the upper limit from 6 to 8 m leads to a 0.9% drop in cost of energy) have on the choice of optimum independent variables.

Backstepping flux observer for nonlinear control of the direct-drive permanent magnet synchronous generator wind turbines

This article described a backstepping flux observer and a backstepping control strategy applied to the wind energy conversion system based on synchronous generator. The proposed control technique is developed for both converters: the stator-side converter and the grid-side converter. The stator flux is recovered by the backstepping flux observer, and simultaneously, we introduce stator resistance variations to test the robustness of nonlinear backstepping control strategy. Then, MATLAB simulation works show high performances in terms of tracking, stability, and robustness against parameter variations.

Current-Aided Order Tracking of Vibration Signals for Bearing Fault Diagnosis of Direct-Drive Wind Turbines

Vibration monitoring is one of the most popular, effective, and reliable methods for bearing fault diagnosis. A key issue in the vibration monitoring for the bearings used in variable-speed wind turbines is the elimination of the effect of the turbine shaft speed fluctuation in the vibration signals measured under varying-rotating-speed conditions. This paper proposes a new current-aided vibration order tracking method for bearing fault diagnosis of variable-speed direct-drive (i.e., no gearbox) wind turbines. The method explores a new simple and effective approach to acquire the reference signal from a current signal measured from the stator of the generator for vibration order tracking. First, the instantaneous fundamental frequency of the current signal is estimated in the time-frequency domain to obtain the shaft rotating frequency. Then, the shaft phase-time relationship is established. With this information, the envelope of the synchronously recorded vibration signal is subsequently resampled at the equal-phase-increment time points. Finally, bearing fault diagnosis is performed by observing the peaks at bearing characteristic frequencies in the power spectrum of the resampled vibration envelope signal. The proposed method is validated by successful diagnosis of different bearing faults in a direct-drive wind turbine under varying-speed conditions.

A 5 MW direct-drive generator for floating spar-buoy wind turbine: Drive-train dynamics

This article proceeds with investigations on a 5 MW direct-drive floating wind turbine system (FWTDD) that was developed in a previous study. A fully integrated land-based direct-drive wind turbine system (WTDD) was created using SIMPACK, a multi-body simulation tool, to model the necessary response variables. The comparison of blade pitch control action and torque behaviour with a similar land-based direct-drive model in HAWC2 (an aero-elastic simulation tool) confirmed that the dynamic feedback effects can be ignored. The main shaft displacements, air-gap eccentricity, forces due to unbalanced magnetic pull (UMP) and the main bearing loads were identified as the main response variables. The investigations then proceed with a two-step de-coupled approach for the detailed drive-train analysis in WTDD and FWTDD systems. The global motion responses and drive-train loads were extracted from HAWC2 and fed to stand-alone direct-drive generator models in SIMPACK. The main response variables of WTDD and FWTDD system were compared. The FWTDD drive-train was observed to endure additional excitations at wave and platform pitch frequencies, thereby increasing the axial components of loads and displacements. If secondary deflections are not considered, the FWTDD system did not result in any exceptional increases to eccentricity and UMP with the generator design tolerances being fairly preserved. The bearing loading behaviour was comparable between both the systems, with the exception of axial loads and tilting moments attributed to additional excitations in the FWTDD system.

A New Fault Diagnosis Algorithm for PMSG Wind Turbine Power Converters under Variable Wind Speed Conditions

Although Permanent Magnet Synchronous Generator (PMSG) wind turbines (WTs) mitigate gearbox impacts, they requires high reliability of generators and converters. Statistical analysis shows that the failure rate of direct-drive PMSG wind turbines’ generators and inverters are high. Intelligent fault diagnosis algorithms to detect inverters faults is a premise for the condition monitoring system aimed at improving wind turbines’ reliability and availability. The influences of random wind speed and diversified control strategies lead to challenges for developing intelligent fault diagnosis algorithms for converters. This paper studies open-circuit fault features of wind turbine converters in variable wind speed situations through systematic simulation and experiment. A new fault diagnosis algorithm named Wind Speed Based Normalized Current Trajectory is proposed and used to accurately detect and locate faulted IGBT in the circuit arms. It is compared to direct current monitoring and current vector trajectory pattern approaches. The results show that the proposed method has advantages in the accuracy of fault diagnosis and has superior anti-noise capability in variable wind speed situations. The impact of the control strategy is also identified. Experimental results demonstrate its applicability on practical WT condition monitoring system which is used to improve wind turbine reliability and reduce their maintenance cost.

Design Optimization of Rotor Lock Disc for Horizontal Axis Wind Turbine Generator

This study presents design optimization of rotor lock disc in the generator rotor which was analyzed by applying the respective lateral force taken from machine load calculation for a 2.5 MW permanent magnet direct-drive wind turbine generator. The machine load calculation result was derived from GH Bladed software output. The maximum tensile stress, internal stresses such as von Misses, maximum displacement and corresponding safety factor were calculated in terms of theoretical stress concentration factor due to discontinuity or change in cross-section in the region where the rotor lock is present. The rotor lock disc configuration was optimized to satisfy a safety factor of unity using exhaustive search method and Cosmos works software. Calculation results indicated that the safety factor of rotor lock disc with 40 mm thickness failed when design lateral force taken from the extreme load of rotor locking torque was used. However, the safety factor of the optimized rotor lock disc with 60 mm thickness has been improved when the inner radius and the total arc subtended angle are properly identified.

Condition monitoring and fault diagnosis of a small permanent magnet generator

Small wind turbines are often used in remote locations, making them difficult and expensive to repair. This suggests the need for remote condition monitoring and fault diagnosis which has not been used extensively for small turbines. In order to investigate small direct drive wind turbine generators working at variable speed and develop new condition monitoring and fault diagnosis techniques, a test facility based on a 500 W permanent magnet generator was built. The investigation concentrated on mechanical faults which include ball bearing outer race defect and a rotor imbalance. Electrical load imbalance was also investigated. Imbalance detection was performed through wavelet power spectrum analysis of vibration signals. Three time–frequency analysis techniques were performed on vibration signals and performances are compared. These were short time Fourier transforms, continuous wavelet transform, and order analysis. Order analysis proved to be a simple, intuitive, and reliable technique for vibration analysis under variable speed.

The Study of Fuzzy Proportional Integral Controllers Based on Improved Particle Swarm Optimization for Permanent Magnet Direct Drive Wind Turbine Converters

In order to meet the requirements of high precision and fast response of permanent magnet direct drive (PMDD) wind turbines, this paper proposes a fuzzy proportional integral (PI) controller associated with a new control strategy for wind turbine converters. The purpose of the control strategy is to achieve the global optimization for the quantization factors, ke and kec, and scale factors, kup and kui, of the fuzzy PI controller by an improved particle swarm optimization (PSO) method. Thus the advantages of the rapidity of the improved PSO and the robustness of the fuzzy controller can be fully applied in the control process. By conducting simulations for 2 MW PMDD wind turbines with Matlab/Simulink, the performance of the fuzzy PI controller based on the improved PSO is demonstrated to be obviously better than that of the PI controller or the fuzzy PI controller without using the improved PSO under the situation when the wind speed changes suddenly.

가변속 풍력 발전용 영구자석형 동기발전기의 적응 슬라이딩 모드 제어기 설계

This paper proposes a simple adaptive sliding mode control algorithm for controlling a permanent magnet synchronous generator (PMSG) of a MW-class direct-driven wind turbine system. The proposed adaptive sliding mode controller does not require accurate knowledge of the PMSG parameter or turbine torque values. The proposed controller can accurately track the reference angular speed computed by the maximum power point tracking(MPPT) algorithm. Finally, this paper gives Matlab/Simulink simulation results to verify the practicality and effectiveness of the proposed adaptive sliding mode controller.

Quantitative Evaluation of Wind Turbine Faults Under Variable Operational Conditions

Wind turbines have been widely used for clean and renewable electricity generation. The maintenance costs of wind turbines constitute a significant portion of the total cost of the generated electricity. Thus, health management systems are increasingly needed to reduce the maintenance costs and improve the reliability of wind turbines. This paper proposes a novel framework for the quantitative evaluation of faults and health conditions of wind turbines using generator current signals. A synchronous resampling algorithm is designed to handle nonstationary current signals for fault feature extraction. The extracted fault features are used to reconstruct new signals, whose correlation dimensions are then calculated by using the Grassberger–Procaccia (G–P) algorithm for fault and health condition evaluation of the wind turbines. Experimental studies are carried out for a direct-drive wind turbine equipped with a permanent-magnet synchronous generator (PMSG) in the healthy condition and two faulty conditions. Results show that the proposed framework can not only detect the faults but also quantify different health conditions for the wind turbine.

A novel combined MPPT-pitch angle control for wide range variable speed wind turbine based on neural network

The objective of this paper is to develop a novel combined MPPT-pitch angle robust control system of a variable-speed wind turbine. The direct driven wind turbine using the permanent magnet synchronous generator (PMSG) is connected to the grid by means of fully controlled frequency converters, which consist of a pulse width-modulation PWM rectifier connected to an inverter via an intermediate DC bus. In order to maximize the exploited wind power and benefit from a wide range of the wind speed, a novel combined maximum power point tracking (MPPT)-Pitch angle control is developed using only one low cost circuit based on Neural Network (ANN), which allows the PMSG to operate at an optimal speed to extract maximum power when this last is lower than nominal power, and limit the extra power. To achieve feeding the grid with high-power and good quality of electrical energy, the inverter is controlled by (PWM) in a way to deliver only the active power into the grid, and thus to obtain a unit power factor. DC-link voltage is also controlled by the inverter. The dynamic and steady-state performances of the wind energy conversion system (WECS) are carried by using Matlab Simulink.

Ripple Field AC Losses in 10-MW Wind Turbine Generators With a MgB2Superconducting Field Winding

Superconducting (SC) synchronous generators are proposed as a promising candidate for 10-20-MW direct-drive wind turbines because they can have low weights and small sizes. A common way of designing an SC machine is to use SC wires with high current-carrying capability in the dc field winding and the ac armature winding is made with copper conductors. In such generators, the dc field winding is exposed to ac magnetic field ripples due to space harmonics from the armature. In generator design phases, the ac loss caused by these ripple fields needs to be evaluated to avoid local overheating and an excessive cooling budget. To determine the applicability of different design solutions in terms of ac losses, this paper estimates the ac loss level of 10-MW wind generator designs employing a MgB 2 SC field winding. The effects on ac losses are compared between nonmagnetic and ferromagnetic teeth with different numbers of slots per pole per phase. The necessity of an electromagnetic shield is then discussed based on the obtained loss levels. The results show that the total ac loss is so small that ferromagnetic teeth can be applied in the generator design without using an electromagnetic shield.

Design of a Superconducting Synchronous Generator With LTS Field Windings for 12 MW Offshore Direct-Drive Wind Turbines

Offshore wind generation is an important trend of global wind power generation. To reduce the cost of energy of offshore wind generation, high-power direct-drive wind turbines are being pursued around the world. With the development of superconductors and related technologies, superconducting (SC) generator offers a possible technological approach to produce large-scale direct-drive wind turbines with a modest cost. A design of a SC wind generator (SCWG) for offshore direct-drive wind turbines is presented in this paper. The proposed SCWG is an electrically-excited synchronous generator with stationary field windings made of low temperature superconductor wires. The electromagnetic design considerations on SCWG are studied. The description on design of copper rotor and SC stator are presented. Several key design technologies including rotor cooling and supporting structure design, SC field winding design, and cryogenic refrigeration system for SC magnets are analyzed. The weight and cost of the proposed SCWG are estimated and compared with that of a permanent magnet wind generator (PMWG) design and several existing similar SCWG designs. It is found that the proposed SCWG is 46% lighter than the PMWG with the same specifications. Moreover, a rough estimation of material cost of the SCWG is carried out, and comparison analysis of cost is also performed.

Effective supervisory controllers designed for optimal energy management in permanent magnet synchronous generator wind turbine with battery storage

Optimal control of large-scale wind turbine has become a critical issue for the development of renewable energy systems and their integration into the power grid to provide reliable, secure, and efficient electricity. In this article, a new supervisory control system for the optimal management and robust operation of a wind turbine generator and a battery energy storage system is presented. The proposed coordinated controller can mitigate both active and reactive power disturbances due to the intermittency of wind speed and load change. Moreover, the control strategy ensures the maximum power extraction capability of direct-drive wind turbine. This supervisory controller is made up of three subsystems. The first one is used to track the maximum power point for variable wind speeds; the control algorithm employs a fuzzy logic controller and a second-order sliding mode controller to effectively meet this target. The task of the second one is to maintain the required DC-link voltage level. This is achieved by controlling the bidirectional power flow in the storage batteries using DC–DC converter. Whereas in the last, a second-order sliding mode controller is investigated to achieve smooth regulation of grid active and reactive power quantities, which provides better results in terms of attenuation of the harmonics present in the grid courant compared with the conventional first-order sliding mode controller. Extensive simulation studies under different conditions of wind speed are carried out in MATLAB/Simulink, and the results confirm the effectiveness of the new supervisory control system.

Detection and Quantization of Bearing Fault in Direct Drive Wind Turbine via Comparative Analysis

Bearing fault is usually buried by intensive noise because of the low speed and heavy load in direct drive wind turbine (DDWT). Furthermore, varying wind speed and alternating loads make it difficult to quantize bearing fault feature that indicates the degree of deterioration. This paper presents the application of multiscale enveloping spectrogram (MuSEnS) and cepstrum to detect and quantize bearing fault in DDWT. MuSEnS can manifest fault modulation information adaptively based on the capacity of complex wavelet transform, which enables the weak bearing fault in DDWT to be detected. Cepstrum can calculate the average interval of periodic components in frequency domain and is suitable for quantizing bearing fault feature under varying operation conditions due to the logarithm weight on the power spectrum. Through comparing a faulty DDWT with a normal one, the bearing fault feature is evidenced and the quantization index is calculated, which show a good application prospect for condition monitoring and fault diagnosis in real DDWT.

Dysprosium electrodeposition from a hexaalkylguanidinium-based ionic liquid.

The rare-earth element dysprosium (Dy) is an important additive that increases the magnetocrystalline anisotropy of neodymium magnets and additionally prevents from demagnetizing at high temperatures. Therefore, it is one of the most important elements for high-tech industries and is mainly used in permanent magnetic applications, for example in electric vehicles, industrial motors and direct-drive wind turbines. In an effort to develop a more efficient electrochemical technique for depositing Dy on Nd-magnets in contrast to commonly used costly physical vapor deposition, we investigated the electrochemical behavior of dysprosium(iii) trifluoromethanesulfonate in a custom-made guanidinium-based room-temperature ionic liquid (RTIL). We first examined the electrodeposition of Dy on an Au(111) model electrode. The investigation was carried out by means of cyclic voltammetry (CV) and X-ray photoelectron spectroscopy (XPS). The initial stages of metal deposition were followed by in situ scanning tunneling microscopy (STM). CV measurements revealed a large cathodic reduction peak, which corresponds to the growth of monoatomic high islands, based on STM images taken during the initial stages of deposition. XPS identified these deposited islands as dysprosium. A similar reduction peak was also observed on an Nd-Fe-B substrate, and positively identified as deposited Dy using XPS. Finally, we varied the concentration of the Dy precursor, electrolyte flow and temperature during Dy deposition and demonstrated that each of these parameters could be used to increase the thickness of the Dy deposit, suggesting that these parameters could be tuned simultaneously in a temperature-controlled flow cell to enhance the thickness of the Dy layer.

Analysis of starting resistance moment of direct drive wind turbine

Permanent magnet motor in the process of running, the permanent magnets provided the magnetic field , the core will have a strong attraction, produce the cogging torque and torque ripple, vibration and noise and other disharmony factors, seriously affect the control precision of the motor system.Therefore, in the design of permanent magnet wind turbine generator, weaken the cogging torque is very important.

Small Wind Turbine Rotor Design

The paper discusses the development procedure of a small direct drive wind turbine. Especially attention to the main rotor and to the wind blade design procedure is dedicated. Decisional technological steps are described, which makes the wind turbine design effective as environmental friendly product. All the design procedures are well documented by the clearly figures and by adequate descriptions as well. The starting conditions at different wind conditions are estimated and the interesting discussion about the necessity of the starting motor is given.