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offer compelling advantages to COE. The conclusion, therefore, is that short of some other significant
justification for using multiple generators (e.g., a significant change to the main frame
which lends itself to multiple generators), GE could not justify moving in this direction on the
basis of COE considerations.
2.1.2 Medium-Speed Generators and Single-Stage Gearboxes
A more significant departure from the baseline is represented by the use of a medium-speed generator
coupled to a single-stage gearbox. This concept was originally proposed as an alternative
to the direct-drive generator concepts discussed in Section 2.1.3. The direct-drive generators
tend to be extremely large to the point of seeming almost impractical. On the other hand, the
conventional arrangement of multi-stage gearbox and high-speed generator leads to a configuration
in which the gearbox is substantially larger than the generator. Therefore,, it was speculated
that a compromise between the two approaches employing a single gear stage and a mediumspeed
generator might provide for a more optimal matching of those two components. In fact, it
was proposed that they could be integrated into a geared generator contained in one housing.
The concept was originally proposed in conjunction with an innovative new tubular nacelle concept
in which the single gear stage and the generator could be integrated into the tubular nacelle,
eliminating the gearbox and generator casings. This concept is discussed further in Section
2.1.3.3, but for the purposes of the present analysis, a more appropriate evaluation of this concept
would result from considering stand-alone gearbox and generator components or a stand-alone
geared generator.
2.1.2.1 Medium-Speed Wound Rotor Induction Generator
GE developed a design for an integrated, geared, medium-speed, wound rotor induction generator
(Figure 2). Previous work on the direct-drive generator as reported in Section 2.1.3.2 showed
it was not possible to design an induction generator operating at 20–35 rpm, the stator of which
could be directly connected to the electric grid at 60 Hz, allowing for only partial power conversion.
As a result, no great advantage exists for the induction design for direct-drive applications.
For medium-speed applications, however, interest remains in demonstrating a wound-rotor induction
machine that could be directly connected. As with the conventional-speed generators,
such a machine would offer the advantages of variable speed operation with only partial power
conversion, but with reduced gearing.
Several alternative generator/power electronics configurations were identified, including 32–,
40-, and 50–pole machines operating both sub- and supersynchronously as well as subsynchronously
only. The study showed that the bidirectional systems (40zB and 50zB) could be constructed
to cost no more than the conventional speed system. In both cases, the converters cost
slightly more than the conventional converter, while the generators cost slightly less than the
conventional system. The latter result was surprising. However, one consequence of the increase
in pole count was a loss of approximately 2%–2.5% of system efficiency. This represents a significant
loss in energy capture detracting strongly from any cost savings that might result from a
reduction in gearing.
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