Brushless Doubly-fed Induction Machines (BDFIMs) have been envisaged as a potential candidate for generators in offshore wind farms.@en

The brushless Doubly-Fed Induction Machine (DFIM) is a special machine type that is of interest as a generator in wind turbine drive-trains. The brushless DFIM has a significant space-harmonic content compared to conventional machine types, due to its construction. This results in additional harmonic distortions, such as torque-ripple and time-harmonics. This paper studies the causes and origins of harmonic distortions in the brushless DFIM. Using time- and space-harmonic analysis an analytical evaluation method is derived to predict torque ripple frequencies and time-harmonic frequencies in stator and rotor voltages and currents. The evaluation method is applied to a prototype brushless DFIM to demonstrate the severity of harmonic related distortions that can be present in brushless DFIMs. The evaluation is validated by additional FE analysis and measurements. Furthermore, measures are proposed to reduce harmonic distortions. The insight into harmonic distortions in the brushless DFIM and the ability to predict and prevent them must lead to improved brushless DFIM designs in the future.@en

Large offshore direct-drive wind turbines of 10-MW power levels are being extensively proposed and studied because of a reduced cost of energy. Conventional permanent magnet generators currently dominating the direct-drive wind turbine market are still under consideration for such large wind turbines. In the meantime, superconducting generators (SCSGs) have been of particular interest to become a significant competitor because of their compactness and light weight. This paper compares the performance indicators of these two direct drive generator types in the same 10-MW wind turbine under the same design and optimization method. Such comparisons will be interesting and insightful for commercialization of superconducting generators and for development of future wind energy industry, although SCSGs are still far from a high technology readiness level. The results show that the SCSGs may not be too expensive regarding capital cost of energy. If other major costs and reliability factors related to superconductivity are taken into consideration, however, the SCSGs may not be competitive yet at the moment.@en

The brushless doubly-fed induction machine (DFIM) has great potential as a variable-speed generator for wind turbine applications. This special machine has a richer space-harmonic spectrum due to its special nested-loop rotor construction compared with conventional induction machines. It may result in higher iron losses, higher torque ripple and more time-harmonics adding to the grid total harmonic distortion (THD). This paper applies the 2D finite element (FE) model to investigate several different nested loop rotor constructions. It shows the outer loop makes more contribution to the torque while the inner loop plays a small role in the torque production. The most outer loop determines the overall THD level while the inner one has little influence on it. The THD could be reduced by increasing the number of the outer loops. More machine performances could be studied to derive more guidelines for designing the
middle loops.@en

The rapid increase of wind power in the power grid results in high grid connection requirements for wind turbines. Moreover, the reliability of wind turbines becomes more and more important, especially in offshore applications. One potential solution for these demands is the wind turbine drive-train based on the brushless doubly-fed induction machine (DFIM). This machine type has no brushes or slip-rings on the rotor side which provides an attractive alternative to the DFIM which is commonly employed in the current market. However, the brushless DFIM has not yet been commercialized. Therefore, the primary objective of this thesis focuses on ‘modeling and design of brushless DFIM, to advance the development of this machine type for wind turbine applications’. A computationally efficient FE model is proposed to evaluate the performance of the brushless DFIM. An efficient, flexible and accurate optimization approach is then developed by combining the computationally efficient FE model with the NSGA-II multi-objective optimization algorithm. Compared with normal induction machines, the brushless DFIM is expected to have more severe noise, vibrations and lower power quality due to many undesired space-harmonics. The 2D multi-slice FE model is applied to investigate whether skewing rotor slots is useful to overcome these drawbacks of brushless DFIMs. Based on the study of the space- and time-harmonics in brushless DFIMs, a computationally efficient method is proposed to investigate the effects of skew at the initial design stage. Sixteen constructions are evaluated to gain more design guidelines for nested-loop rotors. The complicated space- and time-harmonics, the influence of the rotor skew and the influence of the nested-loop configurations are studied and validated by carrying out measurements on a small-scale prototype with four different rotors. The 3D magneto-static FE model is applied to investigate the axial flux due to the skewed slots which is neglected in the 2D multi-slice FE model. Finally, all the modeling methods and the design guidelines are brought together to optimize the design of a 3.2MW brushless DFIM. The results show that the design is improved from the active material cost and the efficiency of the machine points of view by increasing the magnetic loading of the brushless DFIMs. However, the brushless DFIM does not show advantages compared with normal DFIGs and PM generators from the efficiency and the shear stress points of view. However, considering the additional advantages of maintenance and reliability, the brushless DFIMs may provide a feasible application for wind turbine drive-trains.@en

Brushless doubly-fed induction machines (DFIMs) have great potential as variablespeed generators in wind turbines. Undesired space harmonics exist because the special rotor needs to couple to two stator windings with different pole-pair numbers and different frequencies. These undesired space harmonics could lead to noise, vibrations and low power quality. Applying skew benefits to overcome these drawbacks. Previously, a 2D multi-slice finite element (FE) method was applied to study the effects of skew which was time consuming. In the stage of initial design, it is not efficient to use such a model to predict how much the average torque and the torque ripple would be reduced by skewing slots. This paper makes use of normal 2D FE results and applies skew factors in post-processing to investigate the influence of rotor skew. It also proves that to apply skew factors appropriately, not only the space-harmonic order needs to be considered, but also the time-harmonic order. The proposed method can give an approximate prediction of skew effects with limited computing time. The results also indicate that skewing the rotor slot over one stator slot pitch could be a good choice to minimize the torque ripple in a brushless DFIM with a nested-loop rotor.@en

The brushless doubly-fed induction machine (DFIM) has great potential for wind turbine applications. However, it has not yet been commercialized due to its complicated operating principle. Previously, a computationally efficient FE model has been developed. Some design guidelines for the stator pole-pair combinations and the nested-loop rotors have been gained from the previous work. This paper brings the model and design guidelines together to optimize the design of a 3.2MW brushless DFIM. Both the active material cost and the efficiency are optimized. The results show that the magnetic loading of the brushless DFIM is increased for a better design by using the FE based optimization tool. The optimized designs increase the efficiency and the shear stress while reducing the torque ripple and the THD level of the stator voltages. However, the optimized designs result in a high electric loading which would be a challenge for cooling.

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Brushless doubly-fed induction machines (DFIMs) can be potential variable-speed generators for off-shore wind turbine applications. This special machine has two stator
windings with different pole-pairs. There is no direct magnetic coupling between two stator windings. A unique nested-loop rotor is designed to couple both stator windings. However, it results in lots of undesired space-harmonics in the airgap fields compared to conventional induction machines. There are few papers working on the design of nested-loop rotors. To design a better rotor, it is important to understand the complicated magnetic coupling in brushless DFIMs. This paper analyzes the space-harmonics and time-harmonics to provide a better understanding of the magnetic cross-coupling through the nested-loop rotor. The harmonic analysis is validated by FEM simulation and by taking measurements on a prototype in simple induction operation mode.@en

Brushless doubly-fed induction machines have great potential as variable-speed generators in wind turbines. Undesired space harmonics exist because the special rotor needs to couple both stator windings which with different
pole-pair numbers and different frequencies. These undesired space harmonics lead to a bigger torque ripple compared with conventional induction machines. Previously, a 2D multi-slice finite element (FE) method was applied to study the effects of rotor skew on torque responses in brushless DFIMs. It results in a significant computing time because several 2D FE slices are coupled and calculated simultaneously in one model. It is not efficient to use such a model to predict how much average torques and torque ripples would be reduced by
applying skewed slots at the beginning of design. This paper makes use of normal 2D FEM results and applies skew factors in post-processing to investigate the influence of rotor skew on the torque responses. The proposed method can give an approximate prediction of skew effects on torque responses with limited computing time.@en

The brushless Doubly-Fed Induction Machine (DFIM) has many advantages over the conventional DFIG commonly applied in wind turbines. However, due to the complex motion of the magnetic field in this machine type, the inclusion of nonlinear iron saturation in brushless DFIM models has been proven to be challenging. This paper combines a brushless DFIM Electric Equivalent Circuit (EEC) model with an analytical derived magnetic field model. The saturated magnetic field is iteratively obtained using the secant method. Saturation is included in the EEC model by introducing complex saturation factors derived from the magnetic field. This results in an EEC model that is able to accurately determine brushless DFIM operating characteristics. The model is validated by application to a case study machine and comparing the results with those derived from a finite element model.@en

The brushless doubly fed induction machine (DFIM) shows great potential as a generator in large-scale wind turbines. The motion of the magnetic field in this machine is not a simple rotation, which makes it not so straight forward to understand its operating principles. This paper develops an analytical magnetic field model for the brushless DFIM that includes the effects of rotor time harmonics and space harmonics due to the winding distribution and slotting. Using a case study machine, the developed analytical model is then validated by comparison with finite-element (FE) calculations. In addition, a 2-D spectral analysis is applied to the FE derived radial air-gap magnetic field as a function of time. This analysis verifies the space–time relations of the rotating magnetic field components in the airgap of the brushless DFIM. Finally, the developed analytical magnetic field model is used to analyze the brushless DFIM operating principles. The interaction of the stator magnetic field with the rotor nested loops is explained, as well as the development of electromagnetic torque.@en

Permanent-magnet machines with fractional slot concentrated windings are easy to manufacture. Their popularity therefore is steadily increasing. Without a proper design, however, the induced eddy-current losses in the solid rotor get rather high. The modeling and the prediction of eddy-current losses for these machines are thus very important during the design process. This paper focuses on the finite-element analysis and the experimental validation of eddy-current losses for this kind of machine with a small axial length. Two-dimensional and three-dimensional transient finite-element models are developed for computing the eddy-current losses. The rotor motion is taken into account using an Arbitrary Lagrangian-Eulerian formulation. The total iron losses are measured experimentally and a method to separate the rotor iron losses from the total iron losses is presented. The validation results show that the twodimensional finite-element model overestimates the losses due to the end-effects being neglected. The three-dimensional model agrees much better with the measurements in both no-load and on-load operations.@en

The air-gap magnetic fields of brushless doubly fed induction machines (DFIMs) are complicated because of the crosscoupling between two stator fields via a special nested-loop rotor. Comparedwith classical analyticalmodels, transient finite-element (FE) modeling is easier to evaluate the machine performance taking saturation into account. However, it is not efficient to evaluate lots of candidates in a large design space using transient FE models considering the time cost. In the transient simulation, the induced rotor currents are calculated by solving several time differential equations using the backward differentiation formula. This paper presents a computationally efficient FE analysis for brushless DFIMs. The induced rotor currents can be calculated using a single magnetostatic FE simulation. The average torque, losses, and efficiency can also be predicted using one magnetostatic solution. One candidate design can be evaluated within 1 or 2 min on a personal workstation. The efficient analysis is validated by the transient FE results. The presented model is applied to the optimization of a prototype. The influence of two construction variables, namely, polepair combinations and the number of loops per nest, is studied. One pole-pair combination is selected for manufacturing a prototype. @en

Superconducting generators are being proposed and investigated for large offshore wind turbines because of their compactness and light weight. Cost of energy is the key performance indicator to evaluate the feasibility of commercially applying superconducting generators to wind energy. This paper models, estimates and evaluates the cost of energy of a 10 MW direct-drive wind turbine for three superconducting generator designs with MgB2 field windings. These superconducting generator designs are compared regarding the cost of energy as well as other important performance indicators. The results show the fully iron-cored design has the lowest cost of energy and superior overall performance.@en