Advanced Wind Turbine Program Next Generation Turbine ... - NREL

The advantage of this system is that from 70%–80% of the power is produced by the stator,
which is connected directly to the electrical grid without need of power conversion. Only the
20%–30% of the power derived from the rotor requires power conversion. This arrangement allows
for variable speed operation of the wind turbine rotor, thereby maximizing energy capture,
while allowing the use of a power converter sized for only 20%–30% of the wind turbine rated
power. Partial power conversion improves the system efficiency (i.e., energy production) while
significantly reducing the cost of the power electronics. Furthermore, 4- and 6-pole induction
generators and motors are widely manufactured and relatively inexpensive.
In moving away from the baseline drive train configuration, the designer can work with five key
parameters:
• Generator synchronous speed and the corresponding required gearing
• Type of generator
• Number of generators
• Speed regulation (variable speed or fixed speed)
• Power conversion options.
Table 1 summarizes the configuration options associated with each parameter and the anticipated
cost and performance trade-offs that have motivated the studies of these options. A total of 216
permutations of the configurations shown in Table 1 exist. However, many of the permutations
obviously can be eliminated. For example:
• The direct-drive option only works with one generator. The use of multiple generators
requires multiple output shafts from a gearbox.
• Only the wound rotor induction generator can utilize partial power conversion.
• Fixed speed operation requires no power conversion.
Table 2 summarizes the 36-drive train configurations examined in the present study. Every configuration
in Table 2 does not need to be examined separately, because several of the design parameters
can be changed independently. For example:
• Changing from variable speed to fixed speed operation will impact the aerodynamic energy
capture similarly regardless of the drive train configuration.
• Changing from variable speed to fixed speed operation will eliminate the power converter
regardless of whether the generator is direct-drive, medium speed, or conventional speed.
• Use of multiple generators should not significantly impact the cost of the power converter,
as the cost of the latter is mostly a function of the total power rating.
• Use of multiple generators should not significantly impact the energy capture.
As a result of these assumptions, the field of candidate configurations for study have been narrowed
to 11 categories, as defined in Table 2. These 11 categories of drive train configurations
are each discussed in the following subsections 2.1.1 through 2.1.3.
A summary of the COE impacts on proposed design changes is summarized in Section 2.1.4.
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