Direct drive superconducting wind generators - Wind Power Generation and Wind Turbine Design - page 305

The requirements of the offshore market differ from those of the onshore market.
Since the installation cost, and the cost of access for maintenance is signifi cantly
higher for offshore turbines, there is a preference for fewer higher power turbines
in order to reduce the number of installations. The higher towers would also result
in higher average hub height wind speed. Most of the offshore wind farms put into
service around the UK to date have used 2, 3 or 3.6 MW turbines. The cost of
turbine foundations is a signifi cant proportion of the offshore wind farm, and since
the mass of the nacelle has a signifi cant impact on the cost of the most common
monopole foundation, a low nacelle mass is important.
Due to the cost and limited availability of access to offshore turbines, reliability
is most important. If a failure occurs in a turbine in the North Sea in the winter, the
time of maximum energy production, it may be weeks or months before a suitable
weather window provides access to enable major equipment repair.
2.2 Case for direct drive
Early wind turbines had low power ratings (100 kW or less) and typically used
a fi xed speed induction generator (a standard industrial induction motor), driven
though a speed increasing gearbox. The turbine power was limited in high wind
speeds by progressive aerodynamic stall of the blades. As turbine ratings increased
to more than a few hundred kilowatts, the advantages of using a variable rotor
speed with blade pitch control became apparent. The most popular solution was
the doubly fed induction generator (DFIG), in which the stator is directly con-
nected to the grid, and the rotor power (30-50% of the total power) is fed to and
from the grid through sliprings and a variable frequency power converter [6]. This
arrangement had the advantage of a smaller power converter at a time when the
cost of power electronics was high. With turbine ratings of 2 MW or more, and
with wind energy beginning to contribute a signifi cant proportion of the grid gen-
erating capacity in some countries and the falling cost of power electronics, the
fully fed generator became attractive. The DFIG began to be replaced with either a
cage induction generator or a synchronous generator, and all of the power was trans-
ferred to the grid via a variable frequency converter. This system offered a number of
advantages: the generator no longer had slip rings that required regular maintenance,
and the fully fed converter made it easier to implement ride through grid fault capa-
bility and continue generation once the fault had cleared - essential for the security
of power supply when wind contributes a signifi cant proportion of generating capac-
ity. Unlike the directly connected stator windings of the DFIG, the converter isolated
the fully fed stator windings from the grid, so offered greater protection from grid
faults for the generator, as well as the turbine mechanical components.
The speed increasing gearbox is a complex mechanical system requiring good
mechanical alignment for reliable operation. It also has lubrication and cooling
systems requiring maintenance. Gearboxes have been responsible for reliability
problems in many wind turbines in the past. One investigation [7] found that gear-
box development had not kept pace with the increasing size of wind turbines,
results in more reliability problems with the newer larger turbines. The 2006 annual