Electromagnetic Study of MW-Class HTS Wind Turbine Generators

doi:10.4236/epe.2013.54B072

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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

Email: [email protected]

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

Finally, complete content and organizational editing be-

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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

ErBln





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

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 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.

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



, 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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