High-Temperature superconducting (HTS) generators are being considered as a competitive candidate in large direct-drive (DD) wind turbines because of their features of being lightweight and compact. Normally a large air gap is inevitable in partially HTS generators, sacrificing the torque producing capability. In this paper, multi-phase armature windings for HTS generators are investigated to reduce the air gap length in HTS generators while not compromising generators' performance. Therefore, the torque density of HTS generators can be improved without any added costs. Five different multi-phase armature winding schemes are studied in the paper. Their performance regarding torque production and rotor losses in a 10 MW DD HTS generator are examined. The findings show that employing multi-phase armature windings can reduce the mechanical air gap without generating extra eddy current losses in the rotor, and the torque production can be improved by up to 9.1%. In addition, the alternating magnetic field reaching the HTS field winding are also reduced by using multi-phase armature windings, resulting in lower AC losses and cooling costs.

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A method for comparing the levelized cost of energy (LCoE) of different superconducting drive trains is introduced. The properties of a 10-MW MgB₂ superconducting direct-drive generator and the cost break down of the nacelle components are presented and scaled up to a turbine with a rotor diameter of up to 280 m. The partial load efficiency of the generator is evaluated for a constant cooling power of 0, 50, and 100 kW, and the annual energy production is used to determine the impact on the LCoE.

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In recent years, superconducting synchronous generators (SCSGs) have been proposed as an alternative to permanent magnet synchronous generators (PMSGs). They are expected to reduce the top head mass and the nacelle size for such large wind turbines. In 2012, the INNWIND.EU project initiated this research to investigate SCSGs for 10-20MWdirect-drive offshore wind turbines. However, the feasibility of SCSGs was limited by a few critical issues, such as high costs, AC losses in the superconducting winding and excessive short circuit torque. Furthermore, SCSG designs proposed in the literature were various but all less competitive than PMSGs.@en

This paper aims at assessing the potential of partially superconducting generators for 10 MW direct-drive wind turbines by investigating their performance for a very wide range of excitation currents. Performance indicators such as shear stress and efficiency and other generator characteristics are compared for 12 different generator topologies. To be sufficiently attractive, superconducting generators must have significant advantages over permanent magnet direct-drive generators, which typically have shear stresses of the order of 53 kPa and efficiencies of 96%. Therefore, we investigate what excitation is required to obtain a doubled shear stress and an efficiency of 98%. To achieve this, the different topologies require a range of excitation from 200 to 550 kAt (ampere-turns) with a low armature current density of 2 A/mm2. The more iron that is used in the core of these topologies, the easier they achieve this performance. By examining the maximum magnetic flux density at the location of the superconducting field winding, feasible superconductors can be chosen according to their engineering current density capabilities. It is found that high- and low-temperature superconductors can meet the performance criteria for many of the topologies. MgB2 superconductors are feasible for the fully iron-cored topology with salient poles but need cooling down to 10 K.@en

This paper aims at finding feasible electromagnetic designs of superconducting synchronous generators (SCSGs) for a 10-MW direct-drive wind turbine. Since a lower levelized cost of energy (LCoE) increases the feasibility of SCSGs in this application, twelve generator topologies are compared regarding their LCoE in a simplified form of levelized equipment cost of energy (LCoEeq). MgB2 wires are employed in the field winding. Based on the current unit cost and critical current density capability of the MgB2 wire at 20 K, the topologies with more iron have a much lower LCoEeq than the topologies with more non-magnetic cores. The fully iron-cored topology with salient poles has the lowest LCoEeq. Then a scenario study shows that the difference of LCoEeq between the topologies will become much smaller when the unit cost of the MgB2 wire drops to a quarter and the current density capability of the MgB2 wire increases to 4 times. Then the topologies with more non-magnetic cores will become comparable to those with more iron. Aiming at a lower LCoEeq to increase the feasibility of SCSGs for large wind turbines, those topologies having the most iron in the core are the most promising for both now and the long term. If low weight is required, the topologies with more non-magnetic cores should be considered.@en

Innovative drive trains targeted at 10-20 MW offshore turbines are investigated in the INNWIND.EU project in order to determine the impact on the Levelized Cost of Energy (LCoE) resulting when installed in the ,North sea at 50 m of water [1]. The two main technologies studied are superconducting direct drive (SCDD)[2] and the magnetic pseudo direct drive (PDD) [3] generators, which are both capable to providing compact drive trains with low weight and a small number of moving parts compared to a gearbox based drive train (see figure 1a). Superconducting field coils are used to provide the torque in the direct drive generators, where the armature windings are based on conventional copper wire and magnetic steel laminates operated at ambient temperature. Magnetic pseudo direct drive generators consist of a magnetic gearbox made of an inner free rotor (rotating at a geared up speed to the blade input) and an intermediate drive rotor inserted into an outer static armature winding, where the electricity is harvested. @en

Superconducting synchronous generators (SCSGs) are drawing more attention in large direct-drive wind turbine applications. Despite low weight and compactness, the short circuit torque of an SCSG may be too high for wind turbine constructions due to a large magnetic air gap of an SCSG. This paper aims at assessing the effects of armature winding segmentation on reducing the short circuit torque of 10-MW SCSGs. A concept of armature winding segmentation with multiple power electronic converters is presented. Four SCSG designs using different topologies are examined. Results show that armature winding segmentation effectively reduce the short circuit torque in all the four SCSG designs when one segment is shorted at the terminal.@en

A direct-drive superconducting generator (DDSCG) is proposed for 10 MW wind turbines in the INNWIND.EU project. To fit the generator into the ``king-pin'' conceptual nacelle design, the generator structure with inner stationary superconducting (SC) field winding and outer rotating copper armature winding is investigated in the first research phase. Since the cost is an important performance indicator for this application, this paper presents a method to minimize the active material cost of the ``king-pin'' fitted DDSCG. In this method a relatively fast optimization program is developed with 2D non-linear finite element models. By implementing this method, three typical superconducting generator topologies are compared in terms of the active material cost and mass, the synchronous reactance and the phase resistance. The optimization method and the comparison results provide the DDSCG designers with a guideline for selecting a suitable machine topology.@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

Direct drive high temperature superconducting (HTS) wind turbine generators have been proposed to tackle challenges for ever increasing wind turbine ratings. Due to smaller reactances in HTS generators, higher fault currents and larger transient torques could occur if sudden short circuits happen at generator terminals. In this paper, a finite element model that couples magnetic fields and the generator’s equivalent circuits is developed to simulate short circuit faults. Afterwards, the model is used to study the transient performance of a 10 MW HTS wind turbine generator under four different short circuits, i.e., three-phase, phase-phase clear of earth, phase-phase-earth, and phase-earth. The stator current, fault torque, and field current under each short circuit scenario are examined. Also included are the forces experienced by the HTS field winding under short circuits. The results show that the short circuits pose great challenges to the generator, and careful consideration should be given to protect the generator. The findings presented in this paper would be beneficial to the design, operation and protection of an HTS wind turbine generator.@en

Direct drive high temperature superconducting (HTS) wind turbine generators have been proposed to tackle challenges for ever increasing wind turbine ratings. Due to smaller reactances in HTS generators, higher fault currents and larger transient torques could occur if sudden short circuits happen at generator terminals. In this paper, a finite element model that couples magnetic fields and the generator’s equivalent circuits is developed to simulate short circuit faults. Afterwards, the model is used to study the transient performance of a 10 MW HTS wind turbine generator under four different short circuits, i.e., three-phase, phase-phase clear of earth, phase-phase-earth, and phase-earth. The stator current, fault torque, and field current under each short circuit scenario are examined. Also included are the forces experienced by the field winding under short circuits. The results show that the short circuits pose great challenges to the generator, and careful consideration should be given to protect the generator. The results presented in this paper would be beneficial to the design, operation and protection of an HTS wind turbine generator.@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

Superconducting synchronous generators (SCSGs) are being proposed for 10-MW direct-drive wind turbines, because of their advantages of low weight and compactness. So far, however, there has not been a commonly accepted design philosophy of SCSGs and various possibilities with many tradeoffs remain for study. Partially SCSGs are considered a starting point since excessive AC losses in armature windings can be avoided. Many topologies can be applied to partially SCSGs and may significantly affect the performance indicators (PIs) of a wind turbine. Since cost of energy (CoE) is usually used as a key PI to evaluate the feasibility of an SCSG in wind turbine applications, this paper compares twelve topologies using MgB2 wires regarding the capital CoE as well as other resulting PIs. These topologies cover most possibilities for a radial-flux SCSG and four scenarios are investigated regarding the used MgB2 wire. The comparison results shows clear trends of these PIs over the twelve topologies and can be used as a reference for designing an SCSG for large direct-drive wind turbines.@en

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 MgB2 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.@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

Topologies of superconducting direct drive wind turbine generators are based on a combination of superconducting wires wound into field coils, copper armature windings, steel laminates to shape the magnetic flux density and finally structural materials as support. But what is the most optimal topology for superconducting wind turbine generators? This question is investigated by assuming some unit cost of the different materials and then minimizing the cost of the active materials of a 10 MW and 9.65 rpm direct drive wind turbine generator intended to be mounted in front of the INNWIND.EU King-Pin concept nacelle. A series of topologies are investigate by adding more iron components to the generator, such as rotor back iron, field winding pole, magnetic teeth and armature back iron. This method is used to investigate 6 topologies and to determine the optimal cost of the different topologies by using the current cost of 4 ∈/m for the MgB₂ wire from Columbus Superconductors and also a possible future cost of 1 ∈/m if a superconducting offshore wind power capacity of 10 GW has been introduced by 2030 as suggested in a roadmap. The obtained topologies are compared to what is expected from a permanent magnet direct drive generators and the further development directions are discussed. Finally an experimental INNWIND.EU demonstration showing that the current commercial MgB₂ wires can be wound into functional field coils for wind turbine generators is discussed.

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