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

2.1.1 Conventional-Speed Generators and Multi-stage Gearboxes
These options are most closely related to the baseline configuration. Possible changes are:
• Use alternative converter topologies.
• Use generators other than wound rotor induction machines.
• Operate at fixed shaft speed.
• Use more than one generator.
Each of these proposed changes is discussed further below.
2.1.1.1 Alternative Converter Topologies.
The baseline Z750 turbine uses an IGBT-based current source inverter with an active bridge rectifier
(AR-CSI). The 6-pole-generator on the baseline turbine has a synchronous speed of 1,200
rpm at 60 Hz. In the Z50 configuration, it is operated from approximately 850–1300 rpm, which
means it is operating both sub-synchronously and super-synchronously. As a result, the power
converter must be designed to be bi-directional, that is, it must both feed and take power from
the rotor.
Unidirectional Current Source Inverter
GE investigated using a unidirectional CSI with a passive (uncontrolled) diode rectifier (DR­
CSI) by operating the wound rotor induction generator only sub-synchronously. This concept
probably represents the smallest change to the baseline configuration of any concept studied in
the project. The proposal was to operate such a generator from approximately 720–1100 rpm,
remaining entirely sub-synchronous. Although operating at 720 rpm would represent higher slip
and more rotor current than operating at 850 rpm, it would probably be more cost effective to
only construct the converter to handle unidirectional power.
However, the study showed that the higher rotor currents would not only require larger rotor
windings, but they would likely produce greater problems with generator cooling and overall
losses. Although they did not conduct a detailed design, engineers at one of GE's generator suppliers
briefly examined the concept and concluded that they would have to increase the frame by
one, if not by two frame sizes, and that it could produce intractable cooling problems. Another
GE-commissioned study concluded that the increase in rotor power would increase the generator
volume, weight, and cost. Furthermore, this work indicated that the rotor power increase was
also a more significant cost driver for the converter, and estimated that the cost of a unidirectional
power converter for subsynchronous generator operation, assuming the same aerodynamic
rotor speed range, would cost more than the bidirectional arrangement.
Results indicated this concept was not worth pursuing further.
Voltage Source Inverter Topologies
GE also examined voltage source inverter (VSI) topologies, including:
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