A crucial part of wave energy converters (WECs) is the power take-off (PTO) mechanism, and PTO sizing has been shown to have a considerable impact on the levelized cost of energy (LCOE). However, as a dominating type of PTO system in WECs, previous research pertinent to PTO sizing did not take modeling and optimization of the linear permanent magnet (PM) generator into consideration. To fill this gap, this paper provides an insight into how PTO sizing affects the performance of linear permanent magnet (PM) generators, and further the techno-economic performance of WECs. To thoroughly reveal the power production of the WEC, both hydrodynamic modeling and generator modeling are incorporated. In addition, three different methods for sizing the linear generator are applied and compared. The effect of the selection of the sizing method on the techno-economic performance of the WEC is identified. Furthermore, to realistically reflect the relevance of PTO sizing, wave resources from three European sea sites are considered in the techno-economic analysis. The dependence of PTO sizing on wave resources is demonstrated. @en

Superconducting permanent magnet generators (SCPMGs) are a potential candidate for 10 MW direct-drive wind turbine applications. This paper presents two 10 MW SCPMG designs using MgB<sub>2</sub> cables for the armature winding and investigates the short-circuit characteristics of the designed SCPMGs. The first part of the results shows that the SCPMGs can double the shear stress of a conventional low-speed permanent magnet (PM) generator (from 65 kPa to 130 kPa) whilst avoiding demagnetization of the PMs in rated-load operation. However, the power factor has to drop to a range of 0.7-0.8. The second part of the results shows that during a sudden three-phase short circuit, the superconducting armature winding is prone to quench and the PMs are likely to be demagnetized in both proposed designs.

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Doubly fed induction generator (DFIG) based wind turbines are most employed for onshore applications because of their cost-effectiveness. The drivetrain improvement is barely studied due to the maturity of the DFIG based systems. This paper investigates two methods for improving the annual energy production (AEP) of the DFIG based wind turbines. They are referred to as short-circuited and -Y-connected DFIGs. The origins of the AEP improvement are elaborated from the drivetrain perspective. The improvement is quantified by the aerodynamic model of the turbine and the steady-state model of the DFIG. The two methods are then compared when applied to six regions with different wind speed distributions. The AEP improvements at six regions are evaluated and compared to derive the feasibility of the methods for different locations.

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Downsizing the Power Take-off (PTO) rating has been proven to be beneficial for decreasing the Levelized Cost of Energy (LCOE) of wave energy converters (WECs). However, the linear permanent magnet (PM) generator has not yet been modelled and optimized in detail in previous feasibility studies. This paper extends the study of the PTO downsizing to further investigate the influence of the linear PM generator sizing on a WEC's techno-economic performance. The generator is sized for providing different maximum forces, and the effect of sizing on the generator performance is presented. The efficiency map of the selected linear generator design is applied to evaluate the annual energy production (AEP) and finally identify its influence on the techno-economic performance of a WEC.

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Ship hybridization has received some interests recently in order to achieve the emission target by 2050. However, designing and optimizing a hybrid propulsion system is a complicated problem. Sizing components and optimizing energy management control are coupled with each other. This paper applies a nested double-layer optimization architecture to optimize the sizing and energy management of a hybrid offshore support vessel. Three different power sources, namely diesel engines, batteries and fuel cells, are considered which increases the complexity of the optimization problem. The optimal sizing of the components and their corresponding energy management strategies are illustrated. The effects of the operational profiles and the emission reduction targets on the hybridization design are studied for this particular type of vessel. The results prove that a small emission reduction target of about 10% can be achieved by improving the diesel engine efficiency using the batteries only while the achievement of a larger emission reduction target mainly depends on the amount of the hydrogen and/or on-shore charging electricity consumed. Some design guidelines for hybridization are derived for this particular ship which could be also valid for other vessels with similar operational profiles.

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In recent years, permanent magnet superconducting (PMSC) generators have become a candidate for applying superconducting (SC) generators in large direct-drive wind turbines. This configuration keeps the SC armature winding and its cooling system stationary and eliminates rotational cooling couplings. However, the low excitation by permanent magnets may lead to poor power factors if the armature current is high. Furthermore, the permanent magnets are prone to demagnetization when the armature reaction is strong. This paper investigates the design challenges regarding the power factor, demagnetization and short circuit characteristics by analyzing two PMSC generator designs. The results show that the power factor cannot be as high as 0.9 and a low power factor such as 0.6 can take advantage of the high current carrying capability of the SC armature winding. However, this low power factor will cause demagnetization. The armature current may cause quenching of the SC wires during a three-phase short circuit. Demagnetization of the permanent magnets during the short circuit is strong and could be an intrinsic weakness of a PMSC generator.

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Compared with partially superconducting generators, fully superconducting generators (F-SCGs) can further increase the torque density in large direct-drive wind turbine applications. Design trends of F-SCGs intend to increase the electrical loading by applying superconducting wires and boost the current density in the armature winding to meet the critical current density with a safety margin. High currents may cause a low power factor and require the power electronic converter to have a much larger capacity. In an F-SCG, furthermore, torques could be too high, and field and armature currents may exceed the critical currents during a generator short circuit. This paper studies the design of a 20 MW F-SCG with consideration of the control strategy and the power factor, and then evaluates the short circuit characteristic of the F-SCG. The results analysis shows that a capacitive load control should be adopted to avoid a significant drop in the power factor and to make full use of the current-carrying capability of superconductors. An I_{d} = 0 control can also be used with a medium current level. During the short circuit, the negative side is that the phase currents exceed the critical currents and cause quenches. The positive side is that the field currents stay below the critical currents and the torques do not exceed the mechanical limitation of three times the rated torque.

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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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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

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 (BDFIMs) have been envisaged as a potential candidate for generators in offshore wind farms.@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

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

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

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 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

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

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