Direct drive superconducting wind generators - Wind Power Generation and Wind Turbine Design - page 320

6.4 Design challenges

The direct drive wind generator presented a number of design challenges, which
were identifi ed as risks or potential stumbling blocks at the start of the project.
However, the conceptual design stage identifi ed solutions to all of them. A number
of these challenges are described below.

6.4.1 Rotor torque transmission
The very high rated torque of this generator needs to be transmitted from the HTS
coils at cryogenic temperature to the shaft at near to ambient temperature, without
conducting an unmanageable quantity of heat from the warm parts to the cold
parts. A typical cryocooler that can extract 100 W at 30 K would require in input
power to the compressor at approximately 10 kW, although cryocooler effi ciency
is expected to improve over the next decade.
The conceptual design used a torsion rod system that could transmit rated and
fault torque with only a little over 20 W of heat conduction to the cold parts.

6.4.2 Managing mechanical forces
The generator is a large machine, with a very high rated torque, operating at high
magnetic fl ux density (>4 T in parts of the HTS coils). The large physical size
means that stresses due to differential thermal contraction must be carefully mod-
elled and managed to prevent excessive stress in rotor components, particularly in
the HTS coils where excessive strain in the HTS wire could lead to a reduction in
the critical current of the wire, which could lead to a quench, when the wire returns
to its non-superconducting state.
The high operating current density in the HTS coils in combination with high mag-
netic fl ux density, leads to very high Lorenz ( J

B ) forces acting on the HTS wire.
Although the generator torque acts on the coil by this force, it represents only 10% of
the total Lorenz force on the HTS wire, the remainder due only to applying rated fi eld
current. The force density on the HTS coil for this condition is shown in Fig. 12.

×

Max

Min

Figure 12: Force density on the surface of the HTS coils.