High Temperature Superconducting (HTS) synchronous generators can be designed with either an aircore type or ironcore type. The aircore type has higher efficiency under rated rotating speed and load than the ironcore type because of the iron losses which may produce much heat. However, the total length of HTS wire in the aircore type is longer than the ironcore type because the generated magnetic flux density of the aircore type is low. This paper deals with designs of 10 MW aircore and ironcore HTS wind power generators for wind turbines. Fully aircore, partially ironcore, and fully ironcore HTS generators are designed, and various stator winding methods in the three HTS generators are also considered, such as shortpitch concentrated winding, fullpitch concentrated winding, shortpitch distributed winding, and fullpitch distributed winding. These HTS generators are analyzed using a 3D finite elements method program. The analysis results of the HTS generators are discussed in detail, and the results will be effectively utilized for largescale wind power generation systems.
1. Introduction
Offshore wind power capacity contributed 470 MW to worldwide installations of wind power in 2011. The total installed capacity of offshore wind power from January of 2012 to October of 2012 was 1,314 MW. Offshore wind farms are currently under construction in Belgium, Denmark, Germany, the UK, China, Korea, and Japan with a total expected capacity upon completion of nearly 100 GW. The average annual growth rate for new installations is expected to be 81.8% until 2016
[1]
.
A directdrive type high temperature superconducting (HTS) synchronous generator has a smaller physical size and a higher power density per volume ratio than conventional generators. Because of these advantages, HTS generators are becoming an interesting solution to the need for large scale wind turbines
[2

4]
. The HTS generators can be designed with either an aircore type or ironcore type. The aircore type has higher efficiency under rated rotating speed and load than the ironcore type because of the iron losses that can produce a significant amount of heat. However, the total length of the HTS wire in the aircore type is longer than the ironcore type because the generated magnetic flux density of the aircore type is low
[5

7]
.
The main objective of this paper is to design 10 MW aircore and ironcore HTS wind power generators which are the fully aircore, partially ironcore and fully ironcore generators to find an optimal design for largescale wind power generation systems. The various stator winding methods of the HTS generators are also considered for more details of the optimal design, such as shortpitch concentrated winding (SCW), fullpitch concentrated winding (FCW), shortpitch distributed winding (SDW), and fullpitch distributed winding (FDW). The magnetic field distribution of the HTS generators is analyzed using a 3D finite elements method (FEM) program.
The most efficient design, the fully ironcore HTS generator with FDW, is recommended considering all items such as torque ripple, efficiency, volume, weight, and total length of HTS wire. The analyzed results will be effectively utilized for largescale wind power generation systems.
2. Basic Design of the 10 MW HTS Generators
 2.1 HTS wire and ironcore properties
The thickness, width, and minimum bend diameter of the HTS wire are 0.1 mm, 4 mm, and 11 mm, respectively. The detailed specifications of the engineering critical current density versus the perpendicular magnetic field of the HTS wire at the operating temperature are referred from SuperPower Inc.
[8]
. The iron cores in the three types of generators are exposed to a high magnetic field. The material of iron cores such as in the magnetic shield, rotor body, or stator body is M27.
Iron manufacturers generally provide the magnetic properties under 2 T. In the range of the significantly high magnetic field over the saturation magnetic field of ironcore, the magnetic properties of ironcore should be defined
[9
,
10]
.
Fig. 1
shows the nonlinear magnetization curve (BH curve) and relative permeability of M27 and the linear magnetization curve of aircore.
The magnetic curves of the ironcore and aircore
 2.2 Three types of the HTS generators
The three types of HTS generators consist of an HTS rotor coil, rotor body, vacuum vessel, stator copper coil, and stator body. Among the three types, the fully aircore type, which contains a rotor body and a stator body, uses nonmagnetic material such as stainless steel and fiber reinforced plastics. The magnetic shield in the fully aircore type uses magnetic material such as laminated silicon steel (M27). The partially ironcore type with a stator body and a rotor body consists of M27 and SUS304. In the case of the fully ironcore type, M27 is used in the rotor body and the stator body.
Fig. 2
shows the configurations of the fully aircore type HTS generator and ironcore type HTS generators. The partially ironcore type and the fully ironcore type have the same configuration.
The configurations of the HTS generators
The basic design specifications of HTS generators in wind power systems are shown in
Table 1
. The drivetrain of the HTS generators is the directdrive type, and the space factors of the HTS rotor coil and stator copper coil are 0.8 and 0.5, respectively. The HTS rotor coil is a racetrack type double pancake coil (DPC), and the current density of stator copper coil considering water cooling is 6 A/mm
^{2}
. The air gap length and effective length of DPC are 20 mm and 500 mm.
Basic design specifications of the 10 MW class HTS generators
Basic design specifications of the 10 MW class HTS generators
3. Detailed Designs of the HTS Generators Considering Stator Winding Methods
The stator winding methods are divided into a single layer and a double layer winding. The single layer winding has one coil side per slot, while the double layer winding has two coil sides per slot. The double layer winding can be classified as a fullpitch or a shortpitch winding.
When the two coil sides of the stator coil are 180 electrical space degrees apart, the winding is the fullpitch winding method, such as FCW or FDW. When the two coil sides of the stator coil are less than 180 electrical space degrees apart, the winding is the shortpitch winding method, such as SCW or SDW
In the case of SCW and FCW, the number of slots is 72 due to the concentrated winding i.e. one slot per pole per phase. In HTS generators with SDW and FDW, the number of slots is 144 due to the distributed winding, with two slots per pole per phase.
Fig. 3
shows the winding diagrams of the HTS generators. The magnetic field effects of the generators are different from each stator winding method. Hence, the control parameters of each generator should be controlled to generate the rated linetoline voltage.
Table 2
shows the control parameters of the HTS generators. The operating currents apply a 20% safety margin of critical currents in the superconducting field coils.
Winding diagrams of the HTS generators
Control parameters of the 10 MW HTS generators
Control parameters of the 10 MW HTS generators
4. Simulation and Comparative Analysis
 4.1 FEM analysis results of the HTS generators
The 1/24 models for the 10 MW HTS generators are designed in detail and simulated using 3D FEM tool.
Table 3
shows the total harmonic distortion (THD) values of the HTS generators.
THD values of the three types of 10 MW HTS generators
THD values of the three types of 10 MW HTS generators
The voltage THD while powering 100% resistive load of a synchronous machine must be less than 5%
[11]
. Therefore, all designs are appropriate to standard IEC 600341 conditions.
The torque ripple is an important property to consider when designing a rotating machine. A lower torque ripple is required to minimize the vibrations and noise of the machine, and to maximize the lifetime of the machine.
The torque ripple depends on the stator winding methods, slot shapes of stator, and so on. In this paper, the torque ripples of HTS generators are only analyzed using stator winding methods.
Fig. 4
shows the electromagnetic torque waveforms of three types of HTS generators. The torque ripples of the fully aircore types are lower than the partially ironcore types and fully ironcore types because the magnetic field of the stator parts in fully aircore types is uniformly distributed. Of the ironcore types, the partially ironcore type with SDW and the fully ironcore type with FDW have the lowest values of torque ripples. An optimal design of stator slot shape will be required to reduce the torque ripple in the future.
Electromagnetic torque waveforms of the HTS generators
The HTS wire is influenced by the perpendicular magnetic field because the shape of the HTS wire is the tape type. Therefore, the operating current is selected by the value of the perpendicular magnetic field. The values of the maximum magnetic field and perpendicular magnetic field of the HTS generators are illustrated in
Fig. 5
.
Magnetic field values of the HTS generators
Table 4
represents the losses and efficiencies of the HTS generators. The stator coil losses of each stator winding type are equal due to the same total stator coil area. The efficiencies of fully aircore types are the highest at about 98.5%, and the efficiencies of fully ironcore types are a little lower than the partially ironcore types due to the iron loss of rotor body.
Losses and efficiencies of the 10 MW HTS generators
Losses and efficiencies of the 10 MW HTS generators
 4.2 Comparative analysis of the HTS generators
The HTS generators must be designed to use the least possible amount of HTS wire because the cost of the wire accounts for a substantial proportion of the manufacturing cost. However, in order to design the overall wind power system, the weight and volume of the HTS generators must also be considered.
Fig. 6
shows the total length of the HTS wire and the volume and weight of generators. The volume and total length of the HTS wire in the fully ironcore type generators with SCW and FCW are the smallest and shortest. The weight of the fully aircore type generators with FCW and FDW are the lightest because the mass density of the nonmagnetic material is lower than the magnetic material.
Total length of the HTS wire, volume and weight of the 10 MW HTS generators
All items considered, the design of fully ironcore type generator with FDW is recommended for the 10 MW class wind power system because weight and volume of three types of the generators are not significantly different, however, the total length of the HTS wire shows a huge difference over 200 km between the fully aircore types and fully ironcore types. Among the stator winding methods of fully ironcore types, FDW has the lowest torque ripple.
Table 5
shows the specifications of the 10 MW fully ironcore type generator with FDW. The maximum magnetic flux density of the fully ironcore type generator with FDW is about 7.89 T at HTS field coil as depicted in
Fig. 7
, and the
Fig. 8
shows the magnetic flux density of HTS rotor coil in fully ironcore type generator with FDW
Specifications of the 10 MW fully ironcore type generator with FDW
Specifications of the 10 MW fully ironcore type generator with FDW
Magnetic flux density of 1/24 model of the 10 MW fully ironcore type generator with FDW
Magnetic flux density of 1/2 model of rotor coil in the 10MW fully ironcore type generator with FDW
5. Conclusion
This paper deals with designs of large scale aircore and ironcore HTS wind power generators to reduce their weight and volume and the total length of HTS wire. The three types of HTS generators such as fully aircore, partially ironcore, and fully ironcore are designed considering the various stator winding methods. As a result, the THD values of the HTS generators are less than 5%, and the torque ripple of aircore type HTS generator with FDW is the lowest. In the case of the partially ironcore type with SDW and fully ironcore type with FDW, the torque ripples of these stator winding methods are lower than other stator winding methods. The efficiencies of the three types of HTS generators are 98% similar to each other. The volume and total length of the HTS wire in the fully ironcore type generators with SCW and FCW are the smallest and shortest. The fully aircore type generators with FCW and FDW are the lightest in weight. All items considered, the design of fully ironcore type HTS generator with FDW is recommended for the 10 MW class wind power system. The results will be effectively utilized to design a largescale HTS generator for wind power systems.
Acknowledgements
This research is financially supported by Changwon National University in 20132014.
BIO
HaeJin Sung She received B.S. and M.S. degrees in electrical engineering from Changwon National University in 2012 and 2014. Her research interests are wind power system, superconducting generator.
Minwon Park He received B.S degree in Electrical Engineering from Changwon National University in 1997 and his Master’s and Ph.D. degrees in Electrical Engineering from Osaka University in 2000 and 2002, respectively. His research interests are the development of the simulation model of power conversion equipment and renewable energy sources using EMTP type Simulators.
InKeun Yu He received B.S degree in Electrical Engineering from Dongguk University in 1981 and his M.S. and Ph.D. degrees in Electrical Engineering from Hanyang University in 1983 and 1986, respectively. His research interests are electric energy storage and control systems, PSCAD/EMTDC and RTDS simulation studies, and renewable energy sources.
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