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Question of the Week: Will permanent magnet generators replace DFIG?

While some manufacturers are moving away from the traditional geared doubly-fed induction generator (DFIG) machines, most turbines produced worldwide still follow this model.

Siemens has plumped for direct-drive designs
Siemens has plumped for direct-drive designs

Question: Will permanent magnet generators become the standard for wind turbines?

Jussi Vanhanen - Head of product management, the Switch

Although the debate around which generator and converter option makes for the best modern wind turbine drivetrains is still raging, we believe permanent magnet generator (PMG) with a full-power converter (FPC) will emerge as the clear drivetrain technology of choice for modern wind turbines.

When analysed from either a total lifecycle cost-efficiency angle or a reliability angle, a PMG with a full-power converter is by far the best technology, regardless of which key decision-making criteria is used.

Wind turbine manufacturers are switching over to FPCs throughout the world to be able to support fault ride-through and fulfill the strictest grid code requirements.

Once this decision is made, it is easy to see that PMG-FPC drivetrains work out to be cheaper and more cost-effective over the total lifecycle of the turbine. PMG-FPC simplicity leads to superior reliability and higher availability.

The long-term advantage of a PMG-FPC drivetrain comes from the superior annual energy production (AEP) that is possible in the medium and long term, thanks to the higher power curve efficiencies of PMG technology.

Together, PMG and FPC simply lead to the lowest possible levelised cost of energy and long-term permanent performance.

Feng Zhao - Head of wind energy, FTI Consulting

Before 1995, the dominant concept for wind turbine generators was the squirrel cage induction generator (SCIG) connected with a multi-stage gearbox for a fixed speed wind turbine. However, as the turbine technology evolved, this was overtaken by the doubly-fed induction generator (DFIG) design. This enabled a degree of speed flexibility, significantly reduced the power converter rating (by up to 70%), and allowed for greater active/reactive power control. The DFIG solution became a huge success and developed into the standard component for the industry.

Although DFIG is still the mainstream technology in the market if we look at megawatts installed, the direct-drive concept represented by Enercon's turbines has steadily been gaining popularity. Direct-drive PMGs really took off in 2009 when two Chinese turbine vendors, Goldwind and XEMC, also started producing them on a mass scale.

The market share of direct drive PMG turbines increased from 1.9% in 2008 to 18.3% in 2013, the concept has its advantages against the geared solution and a number of large manufacturers including Vestas, Gamesa and Areva have opted hybrid PMG for their medium-speed multi-MW offshore turbines, so it is not a surprise that the market share of turbines with PMG is continuing to grow.

It is still, however, hard to see the PMG concept replacing DFIG as the mainstream turbine technology in the near or medium term for a number of reasons. Currently, more turbine manufacturers are pursuing variable speed turbines with DFIG designs than direct drive. There is also a shift in technology back to the DFIG, led by GE Wind. The DFIG has been standard in wind turbine models for 15 years and recent improvements in the basic technology has proven its availability and reliability.

While there is a trend for using direct-drive PMG machines at sea, mainly driven by Siemens and Alstom, this concept has not so far been tested offshore at a large commercial scale as long as the geared machine.

Uncertainty of supply for rare earth minerals — which are key elements in the PMG's construction — adds another challenge for non-Chinese turbine manufacturers. On top of this, turbine drivetrain technology is still evolving. Hydraulic drivetrains with a synchronous generator are expected to gain market share in the future, possibly pushing out PMG to some degree.

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