By the mid-1990s, power engineers considered the doubly-fed induction generator (DFIG) obsolete with no advantages and little prospects. But by the late-1990s, the technology had become a wind-industry success story.
This was kick-started by pioneering German firm Tacke Windtechnik (now part of GE), which introduced a DFIG-fitted 1.5MW variable-speed prototype in April 1996. A few months later, DeWind (now Daewoo) launched a 500kW prototype.
In the next few years, several small suppliers introduced DFIG technology, including US-based Zond (now GE) and Germany's Jacobs Energie (now Repower), Vestas, Nordex and NEG Micon (now part of Vestas).
DIFG's new lease of life came when engineers fitted power electronics to the wound rotor.
"The solution enabled variable-speed operation in a speed range from roughly 60% to 110% of the rated speed," says Henk Polinder, associate professor of electrical machines and drives at the Delft University of Technology in the Netherlands. "But the key cost and efficiency benefits are that only a partial converter - about 30% of rated power - is required as the generator stator is directly connected to the grid."
By comparison, both variable-speed synchronous and asynchronous induction generators require a 100% power convertor. Polinder adds that improvements to generator field-brush lifespan and bearing insulation also made DFIGs a more attractive option.
Permanent-magnet generators (PMGs) are posing a challenge to DFIG. A number of major suppliers, including GE and Vestas, have partially switched from DFIG to PMGs.
PMG benefits include superior partial-loads efficiency, a more compact generator design and the possibility to apply the turbines in 50Hz as well as 60Hz wind markets without requiring hardware changes. PMG also more easily complies with the latest turbine-grid rules requirements.
One source of debate over the respective technologies is the comparison between total efficiency figures for different generator-converter combinations measured over their entire operational range. How does the slightly more efficient PMG measure against the DFIG when set against higher losses for a full converter as opposed to a partial converter?
Several sources speak of comparable total efficiency figures for both solutions while others say a DFIG with a partial converter produces slightly better results.
Overall, wind industry opinion seems to favour PMGs. One potential impact on future PMG development, however, is the availability of rare-earth elements required for PMGs' field magnets.