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

possibly an improvement in generator efficiency. As a result, GE did not give significant consideration
to this option.
GE, instead, examined the option of connecting squirrel cage or synchronous machines directly
to the grid and eliminating the need for power converters. Squirrel cage asynchronous generators
and wound rotor synchronous generators can be purchased for approximately 60%–70% of the
cost of a wound rotor asynchronous machine. The converter cost is eliminated. The cost of a 4pole
or 6-pole permanent magnet synchronous machine was not thoroughly investigated by GE,
as it was decided that the machine would be more expensive than a wound rotor synchronous
generator. On the performance side, however, some energy capture is lost from fixed speed operation.
This energy capture was estimated by GE personnel at approximately 3%. The impact
on COE is negligible, as the reduction in cost is offset by a reduction in energy capture. As a
result, GE decided not to pursue these fixed speed options further. These fixed-speed options
also lack the ability to provide dynamic VAR control offered by the baseline system.
2.1.1.4 Multiple Conventional-Speed Generators
GE investigated the use of a two-stage hybrid planetary/helical gearbox driving multiple generators.
Figure 1 illustrates the proposed drive train layout. Although originally proposed with
wound rotor asynchronous generators in mind, the concept would apply equally well to squirrel
cage machines or synchronous generators. The motivation to consider this option is the possibility
that reduced generator frame size and increased part count could result in reduced generator
cost.
Use of multiple generators might also provide for improved energy capture, as the wind speed,
shaft speed, aerodynamic rotor power, and generator power can be more optimally matched with
multiple generators than with only one generator. Two generators have been used in the past,
most notably in the U.S. by Kenetech.
The perceived cost advantages of this
concept did not materialize upon
closer inspection. For the volumes
being purchased by an OEM customer
such as GE, the cost per kilowatt
rated for wound rotor induction
generators is nearly constant as generator
rating changes. The size at
which wind turbines are now being
manufactured means that few economies
of scale were found in cutting
the generator rating in half or by a
third. Examination of the volume
costs of off-the-shelf squirrel cage
generators showed similar trends.
Furthermore, the benefits to the energy
capture were believed to be generally
small. As a result, the use of
multiple generators does not seem to Figure 1. Multiple Generator Configuration
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