Energy and Power Engineering, 2013, 5, 373-376
doi:10.4236/epe.2013.54B072 Published Online July 2013 (http://www.scirp.org/journal/epe)
Electromagnetic Study of MW-Class HTS Wind Turbine
Generators
Yongchun Liang
School of Electrical Engineering, Hebei University of Science and Technology, Shijiazhuang, China
Received January, 2013
ABSTRACT
High temperature superconductor (HTS) technology enables a significant reduction in the size and weight of MW-class
generators for direct-drive wind turbine systems and reduce the cost of clean energy relative to conventional copper an
permanent-magnet-based generators and gearbox. Using MAXWELL, we studied MW class superconducting synchro-
nous machines. By comparison the weight, we concluded that HTS wind turbine with rotor iro n is th e heav iest and HTS
wind turbine without rotor iron and stator teeth is the lightest. By comparison the flux density, HTS wind turbine with-
out rotor iron is the least and HTS wind turbine without rotor iron and stator teeth is the largest. By comparison the cost,
HTS wind turbine with rotor iron is the highest and the other two is almost the same. HTS wind turbine without rotor
iron and stator teeth is the best type.
Keywords: FEM; High Temperature Superconductivity; Electromagnetic; Wind Turbine
1. Introduction
The challenges of future energy demand and the possible
global warming due to fossil fuel consumption have
boosted the wind turbine generator’s global market. Total
installed wind capacity is expected to increase to be-
tween 577 GW and 3000 GW by 2050[1]. In China, total
installed wind capacity is expected to increase to 49 GW
by 2030[1-3].
Most present turbines are operated on-shore, but the
interference with the residents and higher wind speed at
sea is the motivation for building off-shore wind farms.
A major fraction of the cost of off-shore farms is due to
the foundations of the turbines, the grid connection and
maintenance. The cost of foundations and connections
can amount to 70% of the first cost. Large wind turbines
would reduce this cost. Compared to the geared drive
concepts, direct-drive concepts may be more attractive
due to the advantages of simplified drive train and higher
overall efficiency, reliability and availability by omitting
the gearbox. Therefore, a break-though technology to
develop light weight and compact direct-drive wind tur-
bine generators, such as 10 MW, is surely expected in
recent years. However, the generator's mass and size in-
crease with the generator capacity. The optimum weight
for a 10 MW direct drive PM generator is greater than
300 metric tons including support structures with an air
gap diameter greater than 10 meters [4-6].
To solve this problem, compact and high-power den-
sity wind turbine generators are required. High tempera-
ture superconducting technology is one of the solutions
for this problem. With recent progress of fabrication
technologies, 2G HTS tapes with high critical current is
promising to develop high magnetic field HTS coils for
power apparatuses. HTS conductors with critical current
densities are as high as 200 A/mm2 at 78 K and zero
magnetic fields are now available. Whereas in conven-
tional water-cooled windings, it is not realistic to use
current densities in excess of 10 A/mm2[5]. By applying
HTS to the wind turbine generators, it is expected to pro-
vide the light weight and compact design, since the
magnetic field can be higher compared with the conven-
tional generator, so that the iro n core can be considerably
reduced or removed [7,8]. Therefore, it is considered that
the application of the HTS technology to the wind tur-
bine generator is one of the key issues to break the tech-
nical power limit of the conventional wind turbine gen-
erator.
For future design of 10 MW wind turbine system, we
focus on apply high temperature superconductivity for
the 1000 KW class wind turbine generators in this study
[9-12]. The electromagnetic character of three models is
studied by finite element method. Th e first model is with
iron rotor. The second model is without iron rotor. The
third model is without iron rotor and stator teeth. The
flux density and output power of these three models are

Y. C. LIANG
374
discussed.
2. Model of 1000 kW HTS Wind Turbine
The electro-magnetic design of 1000 KW wind turbine
generator with HTS field windings is performed by using
FEM analysis. In this study, the three-phase generator is
considered. The number of the revolution is 10 rpm. In
this study, we build a original type with iron rotor and
calculate the flux density of the whole generator, the ro-
tor, the stator, the air gap between rotor and stator teeth,
and the armature windings. In order to reduce the weight
more, we release the iron rotor and compare these two
models. As an example, Figure 1(a) shows the schematic
illustrations of the cross section of the wind turbine gen-
erators with the HTS field windings with iron rotor and
stator teeth [9,10]. The model is for the case of 4-pole
generator. HTS field windings are shown in Figure 1(b ).
For the HTS field winding, the high magnetic field de-
sign is necessary to effectively use the advantage of HTS.
DC field coils of the rotor are made of HTS tapes such as
Bi-2223 or YBCO typ e superconductors. In this analysis,
it is considered that the HTS field winding is operated at
20 K and the rated current density is set at 1.68 × 108
A/m2. This value is reasonable for both HTS tapes such
as Bi-2223 or YBCO in higher magnetic field [12,13].
The cross section of HTS field winding is 126mm ×
126mm. The length of HTS field winding is 1.5 m.
In this design, the slot numbers per phase and per pole
are 3. The cross section of armature winding is 400 mm2.
The conductor number per slot is 16. The packing factor
is 0.5.
(a) Cross section
(b) Structure of HTS field coil
Figure 1. Structure of 1000 kW HTS wind turbine.
3. FEM Analysis
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2D FEM is a useful tool to simulate the magnetic field
of HTS turbine generator [15-18]. Figure 1 represents
the FEM analysis model for the fully superconducting
wind turbine generator. Because the computer is enough
to simulate a full model, the full superconducting wind
turbine generator is calculated. By comparison with in-
duction generator and permanent magnet generator, the
fully superconducting generator has a reduced mechani-
cal air gap between field coils and armature windings. In
this design, we assumed that an 80 mm air gap between
field coils and armature windings. Thermal insulation
layer around HTS coils is also considered.
We add 1.68 × 108 A/m2 in the HTS field windings.
The magnetic flux density distribution of the whole
model is shown in Figure 2. The maximum magnetic
flux density is about 9 T and located around the field
windings.
Figure 3 represents the magnetic flux density distribu-
tion of the armature windings of the HTS wind generator
with iron rotor. This figure refers that a maximal mag-
netic flux density at the armature windings was about 5 T.
The magnetic flux density under the poles is higher than
other plac e .
Figure 2. Magnetic field distribution of HTS turbine.
Figure 3. Magnetic field distribution of stator windings.

Y. C. LIANG 375
Figure 4 shows the magnetic flux density distribution
of the air gap of the HTS wind generator with iron rotor.
The maximum magnetic flux density is about 6.6 T and
located at around the HTS coils and the magnetic flux
density in the air gap is higher than in the armature
windings.
The output voltage and power is determined by the ra-
dial flux density which passes through each armature
winding turn. Because the flux density distribution is
uneven as shown in Figure 3, the radial flux density of
each armature winding turn can be substituted by the
average value. Figure 5 shows one cycle of the average
radial flux density waveform of one turn of armature
windings. The rms of the radial flux density of one turn
of armature windings is about 1.02 T.
Assuming that spatial distribution of the magnetic field
is sinusoidal with the amplitude of 0, the induced
phase voltage (phase-to-ground) in the armature winding,
is proxi mate ly exp ressed by
B
0
E
0
00
2 1
60
2a
N
ErBln


0
qP
(1)
where 0 is the rated revolution (rpm), a is the mean
radius of the armature winding, l is the length along the
generator axis which is effective for the power conver-
sion, P is the pole number, 0 should be an even num-
ber because of the 2-layer winding. q is the slot numbers
per phase and per pole.
N r
n
For the model shown in Figure 1, 0 = 15, a =
1.446, l = 1.2 m, P = 4, q = 6 and 0 = 12. 0 = 1.02
T was used. The induce phase voltage 0 = 565.8 V.
By assuming 3-phase balanced resistance 1.03
N
E
r
n B

, the
output power is 936 KW. The output power fits the de-
mand.
Figure 4. Magnetic field distribution of air gap.
Figure 5. Radial Flux density curve of armature winding.
4. Analysis
In order to reduce the weight, we removed the iron rotor.
The other parts are the same as the model in Figure 1.
The radial magnetic flux density waveform of one turn of
armature windings of HTS turbine generator without iron
rotor is shown in Figure 6. The rms of the radial flux
density of one turn of armature windings is about 0.8 T.
The induce phase voltage 0 = 443.8 V. By assuming
3-phase balanced resistance 1.03, the output power is
586 KW. The output power is less than demand.
E
From Figures 2-4, we can know that the maximum
magnetic flux density in th e stator teeth is more than 2 T
which exceeds the saturation value of the magnetic field
in the conven tional magnetic core material. To secure the
higher magnetic field, we improve the model structure.
We remove the stator teeth and the armature windings
are supported in the non-magnetic material and the back
yoke made of the magnetic material is used for the pur-
pose of magnetic shield. The size is the same as the
model as shown in Figure 1.
Figure 7 shows one cycle of the average radial flux
density waveform of one turn of armature windings of
HTS wind generator without rotor ion and stator teeth.
The rms of the radial flux density of one turn of armature
windings is about 1.16 T. The induce phase voltage 0
= 643.5 V. By assuming 3-phase balanced resistance
1.03
E

, the output p ower is 1232 KW. Th e outpu t power
fits the demand.
The model shown in Figure 1 is the heaviest model
because of the iron rotor and the model without iron rotor
and stator teeth is the lightest. Furthermore, the cost of
Figure 6. Radial Flux density curve of armature winding
without iron rotor.
Figure 7. Radial Flux density curve of armature winding
without iron rotor and stator teeth.

Y. C. LIANG
376
the model shown in Figure 1 is the largest. By compar-
ing the magnetic flux density, the model without iron
rotor and stator teeth produce largest magnetic flux den-
sity and output largest power.
5. Conclusions
This paper simulated the flux density and output voltage
and output power of MW class superconducting wind
generator. Three kinds of generators were calculated and
compared. The simulation results show that the model
with iron rotor and the model without iron rotor and sta-
tor teeth can meet the output power requiremen t. But the
HTS wind turbine generator without rotor iron and stator
teeth is the lightest, the cheapest, the largest induced
voltage and the largest output power. This model is the
best choice.
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