A Transverse Flux High-Temperature Superconducting Generator ...

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Physics Procedia 36 (2012) 759 – 764

Superconductivity Centennial Conference

A Transverse Flux High-Temperature Superconducting Generator Topology for Large Direct Drive Wind Turbines Ozan Keysan, Markus A. Mueller Institute for Energy Systems, School of Engineering, University of Edinburgh, Edinburgh, EH93JL, UK

Abstract The cost and mass of an offshore wind turbine power-train can be reduced by using high-temperature superconducting generators, but for a successful commercial design the superconducting generator should be as reliable as its alternatives. In this paper, we present a novel transverse flux superconducting generator topology which is suitable for low-speed, high-torque applications. The generator is designed with a stationary superconducting field winding and a variable reluctance claw pole motor for simplified mechanical structure and maximum reliability. 3D FEA simulation results of a 70 kW prototype is presented.

© Publishedbyby Elsevier Selection peer-review under responsibility of the Editors. c 2012 2011 Published Elsevier Ltd.B.V. Selection and/orand/or peer-review under responsibility of Horst Rogalla andGuest Peter Kes. Open access under CC BY-NC-ND license.

Keywords: Keywords: Superconducting generator, offshore wind, transverse flux, direct drive, claw pole

1. Introduction The past decade has seen the rapid development of offshore wind turbines. Although, offshore wind turbines are usually supported by government subsidies in order to meet carbon emission targets for the next decade, the wind energy market becomes more and more competitive. The aim of the industry is to reduce the cost of offshore turbines from $3.7 million/MW down to $2.7 million/MW [1]. The onshore wind turbine technology is now quite mature and turbine sizes are usually limited by transportation issues and available wind resources. Offshore wind turbines, on the other hand, may be installed without worries on transport and can benefit from higher wind speeds. The installation and maintenance cost per MW and hence the cost of energy can be reduced by increasing the power rating of the wind turbine. According to the offshore wind turbine market projections of [2], the power rating of the wind turbines will significantly increase in the next decade. For instance, 85% of the wind turbine installations in 2020(5200 MW) will compose of turbines larger than 5 MW. Reliability is a very important issue for offshore wind turbines. Geared power take-off systems dominate the wind turbine market, but the gearbox is usually the most problematic part of a turbine. Although gearbox faults are quite rare, they are responsible for the highest down-time periods (average of 340 hours per failure, according to [3]) which results in lost generation income (almost 4% of the annual income) on top Email addresses: [email protected] (Ozan Keysan), [email protected] (Markus A. Mueller)

1875-3892 © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Guest Editors. Open access under CC BY-NC-ND license. doi:10.1016/j.phpro.2012.06.039

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of the repair cost. Note that, these data have been collected from onshore wind turbines. For offshore wind turbines, down-time related to any fault will be much longer. Thus, it is desirable to eliminate the gearbox and use a mechanically simplified power take-off system such as a direct-drive permanent magnet generator(DDPMG). There is a large volume of publications describing various DDPMG designs for wind turbines. DDPMGs draw attraction owing to their reliable structure and high efficiency. On the other hand, their cost has significantly increased due to steep increase in rare-earth permanent magnet prices because of recent export and mining regulations introduced by China. The price of rare-earth elements has increased more than tenfold since 2009 [4]. Neodymium prices increased by 65 percent in the last four months [5]. It is argued that the prices may go down after 2013 as new mines come on-stream, but the magnet prices will continue to be a big burden for DDPMGs. Another issue with DDPMGs is their enormous masses. The rotational speed of the wind turbines decrease as the power rating increases. For a wind turbine larger than 5 MW, the rotational speed is about 10 rpm. The only way to compensate low rotational speed is to increase the diameter which increases the mass of the generator exponentially. Any increase in the generator mass introduces extra cost on the tower design and installation. Thus, the demand for a high-torque-dense, reliable and cheap generator topology is clear. It is proposed in [1, 6, 7, 8, 9] that high-temperature superconducting generator(HTSG) may be a suitable candidate for offshore wind turbines. In [10], various direct-drive generators have been compared in terms of mass and torque capability. In [10], it is showed that, HTSGs are lighter than DDPMGs for applications with torque requirements larger than 3000 kNm. Abrahamsen et. al. stated in [8] that HTSGs have to prove that they are more reliable than alternative power-takeoff systems in order to penetrate into the renewable energy market. A homopolar HTS machine is proposed in [10]. Homopolar machines are reliable and have many advantages owing to stationary superconducting coil [11, 12]. On the other hand, the torque density of the homopolar machines are limited and they are quite heavy. In this study, a novel superconducting transversal flux generator will be presented. The generator utilizes a single stationary loop superconducting coil as the field winding and a simple variable reluctance rotor. The FEA transient simulations of a 70 kW generator prototype is presented. 1.1. Transverse Flux Superconducting Machine Transverse flux machines(TFM) differ from conventional machines with the direction of the magnetic flux which travels perpendicular to the direction of rotation. Transverse flux machines usually employ a rotor with permanent magnet poles. They usually have higher specific torque densities because of their high number of pole structure (especially evident at low speed range). Thus, principally automotive manufacturers have interest in TFMs for electric propulsion and starter motor applications. In [9], an air-cored superconducting TFM topology is presented. The machine has a very high power density owing to its air-cored structure but it has a rotating superconducting field winding. Conventional TFMs usually suffer from high leakage flux and low power factor, so it is important to have an optimized machine topology. One way of minimizing the leakage flux is to use a claw-pole arrangement. Claw pole machines are capable of delivering higher power densities than conventional machines [13]. A detailed analytical model of claw pole machines is presented in [14]. In claw pole machines magnetic flux travels in a three-dimensional way, so soft magnetic composites(SMC) are usually preferred instead of laminated steel. Eddy current losses in SMCs are much lower than electrical steels (because of high resistivity) [15]. On the other hand, the magnetic permeability of SMC materials are relatively low. Superconducting claw pole machines employ a superconducting field winding. Thus, the leakage flux can be compensated by the high magneto-motive force(MMF) generated by superconducting field winding. In [16], a superconducting claw pole machine is presented. The machine is optimized for a small diameter application and requires two separate superconducting field windings. In [17], the different claw pole types and dimensions are tested for the best performance.

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(a)

(b)

Fig. 1. Proposed transverse flux superconducting machine structure. a) Main magnetic flux direction through the claw-pole rotor(axial cross-section view). b) Main magnetic flux direction through the inner and outer stator(vertical cross-section view).

2. Proposed Generator Structure The proposed generator is a claw pole type transverse flux superconducting machine which is suitable for low-speed high-torque applications. The machine has a stationary superconducting field winding. The advantages of a stationary superconducting coil have been presented in [10]. Fig. 1 shows the axial and vertical cross section view of the proposed generator with the main magnetic flux vectors. The machine composes of two stationary sections(inner stator and outer stator) and a claw-pole variable reluctance rotor. There are two radial airgaps in the machine. The extra reluctance introduced by the second airgap can be compensated by the MMF generated by the superconducting field winding. The sections of the generator can be listed as: 2.1. Inner Stator From Fig. 1(a) and 1(b), it can be seen that the magnetic flux in the inner stator is three-dimensional (i.e. it travels both axially and radially). Thus, this section is manufactured using SMC material instead of laminated steel. Inner stator has a ring shape(see Fig. 2), but instead of manufacturing the inner stator as one big piece, it can be divided into small sections in radial direction. By this way, the manufacturing of SMC (i.e. the pressing of SMC powder) will be much easier. SMCs have superior magnetic properties than laminated steel for three-dimensional magnetic flux. On the other side, they are more expensive and mechanically weaker than laminated steel. In the simulations, Höganäs 3P Somaloy 1000 type SMC is preferred for its optimal mechanical and electromagnetic properties. 2.2. Superconducting Field Winding The field winding is a single stationary loop superconducting coil. It carries direct-current which ensures maximized critical current. Simple design of the field winding reduces the complexity of the cooling and the electrical excitation system (i.e. brushless exciters and cryocouplers are eliminated). Moreover, there are no transient torques and centrifugal forces acting on the winding which means simpler winding support structure and minimized damage risk to superconducting coil. In this study, a double field winding configuration is used (see Fig. 2(b)) to introduce some redundancy. By this way, even one of the coils fails, the other one can be used to generate power at rated load until the maintenance. Also, the cooling system may be simpler by stacking the cryogenic pipes between the superconducting coils. If a dry cryocooler system is to be used, a single coil configuration(with the equivalent MMF) may be preferred without sacrificing the performance.

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(a)

(b)

Fig. 2. a) FEA simulation model of the prototype(mechanical structure is not shown). b) CAD model of the prototype(with support and cooling structure).

2.3. Rotor From Fig. 1(a) and 1(b), it can be observed that, the claw poles only carry unidirectional magnetic flux (similar to a magnet), hence the eddy current loss in the rotor core is negligible. Although, the eddy current loss is negligible, it is preferred to use steel laminations in the rotor cores. Laminations force the magnetic flux to travel in the radial direction and minimizes the leakage flux between adjacent rotor poles by increasing the reluctance in the azimuthal direction. Adjacent rotor cores are fixed to each other by using a non-magnetic support structure (e.g. aluminium) and all cores are fixed to a disc structure(see Fig. 2(b)). Additionally, radial bearings along the circumference of the inner stator may be used to help the structure to keep the airgap clearance. The optimization of the claw pole shape is not covered in this study. An optimized rotor pole helps to reduce overall mass and decrease the leakage flux. 2.4. Outer Stator Outer stator is also made of laminated steel, but lamination direction differs from the rotor cores; the laminations are stacked in the axial direction. The purpose of the outer stator laminations is to reduce the eddy current loss. The stator tooth shape is not optimized and stator pole shoes are not used for easy replacement of armature coils when needed. Non-overlapped concentrated coils are preferred because of low end-winding resistance and easy maintenance(i.e. one coil can easily be replaced if a problem occurs). Concentrated coils and low number of stator slots may result in voltage harmonics and cogging torque. These can be reduced by skewing the stator teeth. On the other side, this will increase the mean turn length of the armature coils and hence the copper loss. Alternatively, a distributed winding schematic may be preferred. 3. Prototype Design In order to prove the concept we plan to manufacture a small prototype. In this study, 3D finite element analysis(FEA) is used to verify the prototype operation. Opera-Carmen software is used for transient FEA simulations. The full-model of the machine is presented in Fig. 2(a) with the mesh. The analysis is performed using a trimmed version of the full model to reduce computation time. The main specifications of the machine is given in Table 1. In the simulations, the data of SuperPower SCS12050 second generation HTS wire (based on BCO) is used. Although, the wire has a minimum critical current of 240 A, the current in the wire is taken as 216

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(a)

(b)

(c)

(d)

Fig. 3. a)Flux density vectors (front view). b) Flux density vectors (isometric view). c) Radial flux density in the stator teeth. d) Induced phase voltages at 100 rpm.

A. Total MMF generated in the field winding is 43,200 A.t. Fig. 3(a) and 3(b) show the direction of the magnetic flux in the generator. It can be observed that the claw poles are highly saturated. Fig. 3(c) shows the radial flux density in the stator teeth linking the armature coils. It can be seen from the figure that the radial flux density in the stator teeth is around ±1.4 T. In Fig. 3(d), the induced phase voltages are presented. During the first 125 msec, the generator accelerates from stand-still to 100 rpm. After steady state, the RMS value of the phase voltage is 350 V. When the generator is connected to a 4 Ω resistive load, the current density in the stator slots is 3.2 A/mm2 (which means 5.0 A/mm2 in the copper wire assuming a fill factor of 0.64). The current density can be increased easily with a better thermal design of the machine. From Fig. 3(d), it can be observed that the phase voltages have significant harmonic components which results in high cogging torque. This is due to non-optimum stator tooth shape design and rotor-stator pole number configuration. The voltage harmonics and cogging torque can be minimized by skewing the stator slots. The FEA simulations prove the operation of the proposed topology. 4. Conclusion This paper has presented a novel superconducting generator topology suitable for large offshore wind turbine direct-drive power-takeoff systems. The aim is to design a superconducting generator which is as reliable as possible. For that, a stationary superconducting field winding is chosen which eliminates cryocouplers and simplifies electrical excitation system. FEA simulation results for a prototype is presented. Although, the maximum flux density in the machine is limited by the saturation, the proposed generator uses considerably less superconducting wire than an air-cored superconducting topology.

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Ozan Keysan and Markus A. Mueller / Physics Procedia 36 (2012) 759 – 764 Table 1. Main specifications of the prototype

Rated Output Power Rated Speed Rated Torque Outer Diameter Axial Length Number of Poles Number of Coils Number of Turns(Armature) Induced Voltage SC Wire Current Mean SC Wire Diameter Number of Turns (SC) Total Field MMF Length of SC Wire

70 100 7,200 1.3 0.5 12 18 100 350 216 0.7 2x100 43,200 880

kW rpm N.m m m

per coil Vrms A m A.t m

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