Permanent Magnet Wind Generator Research Articles

Measuring fault indicators in electric machines - learning experience

The authors read with keen interest an article in Instrumentation and Measurement Magazine titled, "Life after graduation: Africa- Where Metrology Began" [1]. Therein some research works from Cape to Cairo which employed measurement techniques were spotlighted. In this short article, more insight and the learning experience in one of the papers reviewed, namely: "Development of a Test Rig for Eccentricity Fault Studies on an Axial-Flux Permanent Magnet (AFPM) Wind Generator," [2] is presented. The testrig serves as a tool and platform for analyzing fault signatures associated with the AFPM machine. Its effectiveness which hinges on reliable instrumentation and measurement system is revealed in the experimental results in [3]-[5].

Electromagnetic Design of a Large-Scale Double-Stator Direct Driving HTS Wind Generator

High-temperature superconducting (HTS) direct driving wind generators have been extensively researched due to their inherent high torque density and small mass. In this paper, a double-stator HTS wind generator with rotating HTS field windings is proposed to further improve the torque density. The electromagnetic design of this generator is made and electromagnetic performances of different topologies with air-gap windings or iron teeth are compared and analyzed. In addition, two regular HTS wind generators and one double-stator permanent magnet wind generator with the same scale are put forward to make comparison with proposed double-stator HTS machines, and the feasibility and practical value of these five topologies are evaluated in terms of the weight, volume and cost of active materials. The results reveal double-stator HTS wind generator has great advantages in volume and mass reduction.

Series‐connected multi‐half‐bridge modules converter for integrating multi‐megawatt wind multi‐phase permanent magnet synchronous generator with dc grid

In this study, a new transformerless wind energy conversion system to integrate high-power multi-phase permanent magnet wind generators to medium-voltage local grids is presented. The proposed converter topology consists of multi-half-bridge modules with a high ac-direct current (dc) boosting capability. Each phase of an open winding multi-phase generator is connected to the ac side of a half-bridge module, whereas the dc sides of the half-bridge modules are connected in series forming the high-voltage dc-link output. The proposed architecture facilitates the employment of semiconductor switches with a relatively low-voltage rating, which equals the dc-link voltage divided by the number of generator phases. A detailed analysis of the proposed architecture along with the required closed-loop control is presented. The proposed architecture is simulated under different operating conditions using a typical 2 MW system, while the experimental validation is carried out using a low-scale prototype converter.

Interarea oscillation damping using H-infinity control for the permanent magnet wind generator

This paper investigates the use of the permanent magnet wind generator as a means for damping dynamic interarea oscillations in multi-area power systems. A power modulation controller is proposed based on the H-infinity design methodology. The controller uses as feedback the frequency deviation of the local power system area and applies effective control on the wind generator rotor speed, blade pitch angle and dc link voltage in order to provide sufficient power modulation at the output of the generator to damp interarea oscillations. The paper also investigates the potential use of the energy stored in the dc link capacitance as an auxiliary means to increase the power modulation capability of the generator. The method is applied to a three-area power system that exhibits undamped rotor oscillations. Results presented in the paper demonstrate the effectiveness of the wind generator in increasing considerably the system damping.

Influence of Rectifiers on Permanent Magnet Wind Generator Electromagnetic and Temperature Fields

Power rectifiers are very necessary in the wind power generation systems since they are the necessary channels that link the generator and power gird together. However, they have some effects on the permanent magnet wind generator due to their work on fast on-off transitions. Taking an 8kW 2000r/min wind-driven permanent magnet generator as an example, the system model and external circuit were established. Firstly, based on the field-circuit coupling calculation method, the voltage and current harmonics have been studied respectively when the generator was connected to rectifier loads and pure resistance loads, so did the total harmonic distortion. The mechanism of harmonic impacted by rectifiers was revealed. Secondly, combined the harmonic electromagnetic field theory, the stator core loss, armature winding copper loss and rotor eddy loss were analyzed when the generator connected different loads. Furthermore, according to the definition of nonlinear circuits PF, the numerical analysis method was adopted to calculate the power factor when the generator connected two loads respectively. The change mechanism of PF impacted by rectifiers has been revealed. In addition, the temperature field model has been established and the generator temperature was also analyzed. The temperature distributions were obtained when the wind generator was connected to different loads. Then, the relationship between losses and temperature was combined, the change rules of permanent magnet temperature by the eddy current loss were studied under different load. At last, it can prove that the rectifiers have influences on both electromagnetic field and temperature field through comparing the simulation results with experimental test data.

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.

Double PM-Rotor, Toothed, Toroidal-Winding Wind Generator: A Comparison With Conventional Winding Direct-Drive PM Wind Generators Over a Wide Power Range

The double rotor, toothed, toroidal-winding permanent-magnet (PM) machine is not a well known concept and has received very limited attention in literature. In this study, the concept is proposed for use as a direct-drive wind generator. Due to the varying design requirements of wind generators over the different power ranges, the PM generator is optimized over the entire installed wind power range. For each power level, the optimum design is compared with optimum nonoverlap-winding and conventional overlap-winding PM machine designs. This also gives a much broader indication on the scaling of different wind generator technologies. Although the electromagnetic design, by means of finite-element analysis, of the generator is the main focus of the paper, some of the implementation issues are also discussed. An existing 15-kW double rotor PM wind generator is modified to include a toroidal-winding, which is used as a case study. Both simulated results and practical measurements in the laboratory for the 15-kW case study toroidal-winding PM generator are presented in this paper.

Cogging torque reduction in axial-flux permanent magnet wind generators with yokeless and segmented armature by radially segmented and peripherally shifted magnet pieces

The inherent cogging torque component of the PM generators can cause problems at the start-up of wind turbines. To improve the operation of wind turbines, especially at low starting speeds, the wind generator cogging torque should be minimized. In this paper, an effective and practical approach is proposed for cogging torque reduction in the AFPM generators with yokeless and segmented armature (YASA). The proposed approach is based on the dividing magnets radially and shifting the magnet pieces peripherally with an appropriate shifting angle. Using a series of 3-D FEA simulations, the proposed approach is compared with the two conventional approaches, i.e. the magnet skewing approach and the selecting an appropriate magnet pole arc to pole pitch ratio, according to the cogging torque reduction and the negative impacts on the generator loadability. It is shown that using the proposed approach, the generator cogging torque can be greatly reduced (about 87%). Also, it is shown that compared with the other studied cogging torque reduction techniques, the proposed approach has less negative impact on the generator loadability. Some of the simulation results are verified using the experimental tests.

Development of Magnetic Losses During Accelerated Corrosion Tests for Nd-Fe-B Magnets Used in Permanent Magnet Generators

Sintered Nd-Fe-B magnets are critical components in permanent magnet wind generators. They are the strongest permanent magnets available and thus enable the construction of light and effective devi...

Numerical and experimental validation of multi-physics design models for axial flux permanent magnet wind generator

Numerical and experimental validation of multi-physics design models for axial flux permanent magnet wind generator

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.

Fault diagnosis and condition monitoring of axial-flux permanent magnet wind generators

This paper presents fault detection techniques in an attempt to discriminate between two common faults in axial-flux permanent magnet wind generators namely; static eccentricities and interturn short circuit faults. A combination of condition monitoring systems is proposed to achieve the discrimination; with detection techniques derived from current and vibration signatures for both faults using signal processing techniques. Parametric spectral estimation technique is applied as an alternative to the Fourier transform to process the fault signatures. This effectively overcame the problem of non-stationarity of signals due to variable wind speed as high frequency resolution was attained with less than three seconds of measurement data. The results will advance the development of standards and technical guidelines in the condition monitoring of permanent magnet synchronous generators in wind turbines.

Demagnetization Performance of a 7 MW Interior Permanent Magnet Wind Generator With Fractional-Slot Concentrated Windings

High reliability is one of the most important requirements for large-scale permanent magnet (PM) wind generators. However, the magnet suffers partially irreversible demagnetization due to the material property, especially under short-circuit conditions. To improve fault-tolerance capability and torque density, fractional-slot concentrated windings (FSCWs) have been adopted in wind generators. However, the rich harmonics of magnetomotive forces make the demagnetization characteristics different from that of generators with distributed windings. In this paper, the partial demagnetization performance of a 7 MW interior PM wind generator with FSCWs is investigated, under the symmetric short-circuit condition. In addition, influences of slot–pole combinations and rotor types are analyzed to improve the antidemagnetization capability. The transient magnetic behavior is evaluated by the finite-element analysis.

Design Optimization of Directly Grid-Connected PM Machines for Wind Energy Applications

The focus of the study in this paper is on the design of unconventional directly grid-connected PM wind generators without power electronic converters. As a case study, this analysis is based on a novel direct-drive wind generator system known as a slip-synchronous permanent-magnet (PM) wind generator. Attention is given to the definition of the design requirements of the directly grid-connected synchronous-generator unit from the relevant grid code specifications and to find the optimum design subject to these requirements. Due to the direct-grid connection, there are clear differences in the design requirements of this machine and those of conventional PM wind generators connected to the grid via a converter. Four different generator topologies are evaluated with regard to the ease of manufacturing, active mass, PM content, and suitability for direct-grid connection. Both simulated and measured results are given for an experimental direct-grid slip-synchronous PM generator.

The Structure and Finite Element Analysis of a New Type of Maglev Wind Generator

Abstract. In this paper, a new type of maglev wind generator is proposed. The working principle, structure characteristics and the finite element analysis of the new maglev wind turbine are introduced. The generator consists of a 2 degrees of freedom (DOF) maglev generator and a 3 DOF hybrid magnetic bearing. An eight poles active magnetic bearing with external rotor is added into traditional direct-drive permanent magnet wind generator. The rotor ring is hollowed, which can leads to the self-decoupling for magnetic fields between power generation system and suspension system. Compared with the conventional maglev wind generator, the proposed generator not only shows the same advantages of traditional maglev wind turbines, but also improves the axial length utilization and decrease the cost of motor control.

Optimal Design of Six-phase Permanent Magnet Synchronous Generator for directdrive Small-Scale Wind Turbines

Permanent Magnet (PM) generators are widely employed in directly coupled wind turbines. Therefore, optimal design of these generators is quite necessary to improve their overall performance. In this paper, single/multi-objective design optimization of six-phase permanent magnet synchronous generator (PMSG) for directly coupled wind turbines is carried out. The studied PM generator in this paper is the radial-flux surface mounted PM generator. The aim of optimal design is to reduce the cost of PMSG manufacturing and generator losses. For this purpose, the main equations of PM generator design are extracted. Therefore, the generator design steps are developed and then imperialist competitive algorithm is used to optimize the design variables of the generator. To find the best design for this generator, the cost and losses, separately, are optimized and are then optimized simultaneously. Finally, the optimum results are presented. Comparison between optimum results shows that simultaneous optimization of the cost and losses leads to suitable design for PM wind generator.

Modeling and Optimal Design of an Eddy Current Coupling for Slip-Synchronous Permanent Magnet Wind Generators

In this paper, the possibility of using an eddy current slip coupling to remove cogging torque and torque ripple in a slip-synchronous permanent magnet wind generator is investigated. Two different topologies are proposed. It is shown that two-dimensional (2-D) finite-element methods are inaccurate compared to three-dimensional (3-D) finite-element methods when solving eddy currents in eddy current couplings. The 3-D finite-element transient simulations are validated using a manufactured prototype 15-kW eddy current coupling. An analytical approximation is developed and used for the design optimization of two eddy current coupling topologies. Using the optimally designed eddy current coupling, it is shown that the analytical and 3-D finite-element solutions compare very well and that the proposed eddy current coupling topology has no torque ripple. The analytical approximation is well suited to the rapid design optimization of eddy current couplings that use a conductive material in the air gap.

Influence of Pole and Slot Combinations on Magnetic Forces and Vibration in Low-Speed PM Wind Generators

In this paper, radial forces and torque ripple characteristics are investigated in permanent magnet (PM) machines having different pole and slot combinations. Using the PM machines with concentrated windings could be beneficial in direct-drive wind generators since it is possible to reduce the size and weight of the generator. The PM machines with concentrated windings having a large number of poles are compared to investigate the effect of pole and slot combinations on force and vibration characteristics in low-speed generators. Cogging torque waveforms and torque ripple are investigated using time-stepping finite-element analysis. Analysis of radial forces is presented, including investigation on radial force density distribution, total forces on teeth, and time-dependent force waveforms on a tooth. Structural analysis and experimental modal analysis are performed on the prototype generator. The main mode of vibration in the prototype machine is observed experimentally and the results are in good agreement with the simulations.

An adaptive radial basis function neural network (RBFNN) control of energy storage system for output tracking of a permanent magnet wind generator

The converters of a permanent magnet synchronous generator have to be properly controlled to achieve maximum transfer of energy from wind. To achieve this goal, this article employs an energy storage device consisting of an energy capacitor interfaced through a voltage source converter which is operated through a smart adaptive radial basis function neural network (RBFNN) controller. The proposed adaptive strategy employs online neural network training as opposed to conventional procedure requiring offline training of a large data-set. The RBFNN controller was tested for various contingencies in the wind generator system. The adaptive online controller is observed to provide excellent damping profile following low grid voltage conditions as well as for other large disturbances. The controlled converter DC capacitor voltage helps maintain a smooth flow of real and reactive power in the system.

A Control Method of Permanent Magnet Wind Generators in Grid Connected Wind Farm to Damp Load Frequency Oscillation

A Control Method of Permanent Magnet Wind Generators in Grid Connected Wind Farm to Damp Load Frequency Oscillation