The Big Wind Energy Drive Train Technology Debate

In the wind power industry, the debate on which generator and converter option makes for the best modern wind turbine drive trains is still raging. Numerous technology experts and industry commentators promote the use of the double-fed induction generator technology, whilst expressing doubts about the advantages of the permanent magnet generator (PMG) approach. The Switch wants to set the record straight and explain why PMG with a full-power converter (FPC) is by far the best technology, regardless of which key decision-making criteria you use.

Aiming for the Lowest Levelized Cost of Energy

The big challenge is to get the industry to fully understand the advantages of advanced drive trains that lead to better performance and lower LCOE.  Permanent magnet technology represents permanent performance through high reliability, overall efficiency and high-quality electricity. 

There are still markets throughout the world where the initial investment decision-making criterion is price. These include emerging markets just getting into wind, for example. Still, we see a clear market for premium turbines that feature a higher upfront cost, but much better total lifecycle performance. PMG has been proven to provide the lowest levelized cost of energy (LCOE).

In the first phase of wind turbine technology development, for example, local Chinese turbines were mainly built based on licenses from Europe. Therefore, induction generator based turbines were built originally. The first ones were based on a standard asynchronous machine and a soft starter; later on, drive trains advanced to feature a double-fed induction generator (DFIG) and converter. Finally, along with the shift into the new millennium, came a shift in technology towards new solutions represented by permanent magnet generators (PMG) and full-power converters (FPC).

In 2004, there were no PMG-based turbines in production in China, yet some open-minded companies had that technology on their drawing boards. Over the past ten years, we have witnessed the PMG revolution in China. In fact, all Chinese wind turbine manufacturers that have really been successful during the past ten years, and are still on top today, have selected PMG and FPC based drive trains in their turbines. They have been lucky to be able to leapfrog ahead to the most modern technology, since they were able to select their own turbine designs from scratch once the license phase was over.

The main reasons for PMG and FPC drive train success are high reliability and availability, combined with the ease of handling existing and future grid connection requirements, such as low voltage/high voltage ride-through. Drive trains with FPC can also handle requirements for feeding reactive power. DFIG machines cannot reach the same level without an external power conditioning device.

Full-power converter via The Switch

Finally, the development of rare earth material prices experienced considerable fluctuation a few years ago. This scared off many companies from moving faster into the use of PMG. For approximately the past two years, the price of rare earth magnets stabilized with the entry of new suppliers in various parts of the world. This has led to a health balance of supply and demand.

It is clear that the old designs will remain. But the majority of new designs in China are based on PMG and FPC. And these are now leading the way — and the world of wind power.

Higher Overall Life Cycle Cost-effiency

It is sometimes claimed that permanent magnet generator (PMG) and full-power converter (FPC) drive trains are more expensive than double-fed induction generator (DFIG) drive trains. However, when every investment and operational factor is taken into account, PMG + FPC drive trains work out to be a cheaper, more cost-effective option over the total life cycle of the turbine, despite slightly superior upfront costs.

Specifically, the lower grid connection costs of PMG-FPC equipped turbines represent a significant advantage over DFIG models. It’s an important element to keep in mind when one knows that efficiency and grid compliance are the top demands when it comes to generator selection. While partial converters may have improved somewhat in grid code compliance, full-power converters remain the preferable option.  These factors, along with high AEP and reliability, are very important from an investment point-of-view.

DFIG technology now complies with the grid codes by adding hardware and software at the expense of extra costs. This is a simplistic answer to the problem, and is more of a quick fix than a concrete solution. With fault ride-through and power factor capability, the DFIG converter becomes similar in size and cost to the full converter. A key benefit of PMG-FPC drive train technology is the fact that it already includes features such as reactive power generation and low voltage ride-through (LVRT). These inherent benefits level the playing field when making comparisons with the cheaper upfront costs of double-fed induction generator drive trains, which also need additional VAR support to make a connection to the network.

But the real long-term advantage comes from the extra energy produced with the higher power curve efficiencies of PMG technology in low or medium wind conditions. The PMG’s maximized energy production is what gives significantly higher income and profitability. This is especially true when operating at partial power, where the highest number of operational hours of a wind turbine’s lifetime is spent.

PMG-FPC drive trains actually improve efficiency over the full operational range of the turbines. Although some claim that DFIGs are more efficient than PMGs at full load generation and in high, steady winds, in reality, the efficiency of the PMG + FPC and the DFIG + partial converter are similar when operating at 100 percent power. However, we know that this situation rarely occurs, and in general working conditions, PMG drive trains have proven to be more efficient.

Generator losses are always lower with PMG than with DFIG, since there are no excitation losses. The fact that DFIG only needs a partial converter reduces the difference in total drive train efficiency between these two concepts — especially at nominal loads. At partial loads, however, there is a significant difference — and this is where a wind turbine operates most often with fluctuating wind speeds. That’s why a PMG drive train results in higher AEP.

PMG designs also enhance reliability and serviceability, leading to lower O&M costs. It is important to bear in mind the required servicing and total reliability of the drive train. On this account, PMG designs have a comparatively low number of electrical faults and failures. The annual service costs of PMG technology can be up to 30 percent lower than the service costs for DFIGs.

Design Simplicity Leads to Superior Reliability and Availability

One of the common miss-conception in the industry is that PMG-FPC solutions require lots of potentially unreliable electronics. This commonly admitted myth states that full-power converters are not very reliable because they rely greatly on power electronics, which would be more prone to faults than gearboxes. This is inaccurate. In reality, the amount of electronics used in PMG+FPC drive trains is comparable to DFIG systems. More importantly, it is worth noting that there are no electronics used in the PMG generator itself, only in the converters.

Medium-speed permanent magnet generator via The Switch

Furthermore, when considering a multi-megawatt system, a DFIG solution most likely consists of only one converter for the rotor connection, while a full-power converter system can consist of several parallel power threads. As semiconductors do fail, it is better to have still healthy power threads in operation despite a failure in one of them, allowing the turbine to run at limited partial power, rather than having the whole turbine at a standstill due to a failure in power electronics.  Permanent magnet generators have high reliability and low maintenance costs due to better heat performance than the DFIG, as well as being able to function without slip rings or encoders.

PMGs require the use of NdFeB magnets, which are sensitive to corrosion and heat. For this reason, some industry commentators claim that electrical losses could climb rapidly due to excessive heat. They also wrongly assume that there is a risk of reversed polarity or permanently losing magnetic field strength.

Such statements seem to ignore the fact that NdFeB magnets are always coated, which helps to protect them from corrosion very efficiently. Hermetic sealing is also applied when assembling the rotor, which also helps in this regard.

With PMG+FPC Drivetrains out-performing DFIGs from both a full-cycle cost-efficiency and reliability perspective, the turbine manufacturing industry needs to revise some its old assumptions and obsolete turbine technology choices, in order to embrace the generator technology that leads to better AEP.   

Lead image: Wind turbine via Shutterstock

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Jussi Puranen is the research and development manager for electrical design at The Switch.

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