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There are claims that this drive solution, typically with 400-500rpm- rated generator speed, offers the best compromise between complexity and reliability, and optimised cost of energy (CoE)—especially compared to a PMG- based direct drive. One of the current key CoE drivers is the amount of magnet required for a given power rating.

Drive experts at German gear maker Winergy argue that compared to direct drives their latest medium-speed HybridDrive gearbox-generator system needs only around 20% of the quantity of magnets.And at a 2010 Husum fair,a presentation by power-engineering giant ABB indicated that relative magnet mass of direct drive generators is a factor of ten higher than a high-speed geared system, both rated at 3MW.

ABB offers the wind industry a wide range of product solutions, including generators, power-electronic converters, transformers, motors, circuit breakers/ contractors as well as high-voltage DC (HVDC) systems for long distance electric power transport. The various electric drive solutions encompass traditional high- speed geared, medium and low-speed geared, and direct drive (no gearbox). Available generator solutions incorporate doubly fed induction (DFIG), induction- type (asynchronous), and synchronous generators either classic-built with electrically excited pole magnets or designed as PMG. Induction and synchronous generator topologies both require a full (100%) power converter when operating in variable speed mode.

A large majority of onshore turbines use high-speed geared drive trains and nearly every offshore turbine does as well. There is a strong perception across the industry that gearboxes are unreliable and prone to failure—this despite statistics on turbine and drive-system performance showing that gearbox failure is not a major issue anymore. One source of such findings is the 2011 EU-funded Reliawind study.

With high-speed wind systems,four-pole, and, for larger power ratings six-pole, DFIGs remain a popular wind industry choice.A key benefit of DFIG is that it only requires a small converter rated at around 20-34% of nominal generator power, resulting in substantially lower equipment cost with reduced internal efficiency losses.According to ABB, these factors together contribute to a technically and economically viable solution for grid-code compliance and high total system efficiency.

Induction generators are semi-standard technology in numerous industrial applications but less common in modern variable-speed wind systems. Siemens “uåX˜äŠÊ˜·³Ç is the single biggest wind- industry user of these high-speed induction generators and employs them in its 2.3MW and 3.6MW geared turbines. In both sizes, common four-pole configurations are used.

UK-based Condor Wind Energy’s planned 5MW turbine is one of the few medium-speed models planned for offshore use that comes with with an eight-pole induction generator enabling 700rpm generator speed—roughly half that of 4-pole machines.

Classic synchronous wound-rotor generators are characterised by electrically excited generator poles. For many decades, they have been standard equipment in large conventional and nuclear power plants. In the wind industry, Germany’s Enercon is the biggest single user of classic synchronous direct-drive generators in its turbines, which are now available with rated capacities up to 7.5MW.

Turbines designed to operate with variable rotor speed and fixed generator speed (50Hz/60Hz) are still comparatively uncommon and are used in only a small portion of high-speed geared wind systems, typically combined with classic synchronous generators. Because the generator is directly connected to the grid, a power converter can be eliminated.

At the same time, choosing medium- voltage can sometimes under specific circumstances eliminate the need for a medium-voltage transformer.The variable-to-fixed-speed functionality can be achieved by incorporating full
mechanical, mechanical-hydraulic, full hydraulic, or other devices. Examples of turbines with full hydraulic transmission in development include Mitsubishi’s 7MW SeaAngel and a hydraulic-drive technology developed by Chapdrive of Norway.

Synchronous generators with permanent rotor magnets can be more compact and substantially lighter than classic equivalents.These PMGs can either be designed with an outer rotor or inner rotor depending on such variables as the choice of cooling system or other in-house technology preferences. PMGs are also known for superior partial-load efficiency because normal rotor-excitation losses of about 30% of total internal losses are eliminated. For all these reasons, direct- drive PMGs have become a preferred technology choice with most new entrants in both the onshore and offshore market, and by big names including Siemens, GE and Alstom.

Generators with a full power converter completely decouple the generator from the grid, enabling a wider variable speed range compared to DFIG.Additional claimed benefits over DFIGs include superior grid compliance to expected network-integration rules for wind turbines, superior mechanical drive train dampening and enhanced aerodynamic efficiency through full speed range.

ABB has played a pioneering role in developing PMGs for the marine and wind industry since the mid 1990s.Around 2000, it supplied a 3kV/4kV medium-voltage PMG for a Dutch direct-drive Zephyros prototype, developed to operate at 1.5MW rated onshore and offshore with higher rpm and increased 2MW output.The Zephyros turbine was likely the world’s first multi-megawatt class turbine fitted with a medium-voltage PMG, and its successor, by Chinese manufacturer XEMC, is a major success in that country.

Another novel innovation by ABB was a low-speed PMG it supplied to Finnish wind turbine manufacturer WinWinD, incorporated in a 2001 1MW prototype fitted with a single-stage planetary gearbox from Metso Gears (now Moventas). In 2004,WinWinD installed a larger 3MW sister prototype, again with a PMGNew idea The Fusiondrive jointly developed by Moventas and The Switch potential methods to integrate a medium- speed design into a wind turbine: fully integrated, semi-integrated, and non- integrated.

Fully integrated drive systems typically share the same main casting or other support frame,bearings and shaft.The solution requires joint development between gearbox and generator manufacturers.The Multibrid-type turbine models are good examples of such fully integrated drive systems.A key benefit is that it enables a compact, strong and stiff single load-carrying structure with minimised risk of component misalignment, and favourable weight. A main disadvantage is that when a major mechanical failure occurs the complete drive system must be exchanged for repair.

Semi-integrated drive systems are typically based on a modular design approach with the planetary gearbox and generator being separate units with or without similar outer diameter and integrated by a flange connection.This strategy allows a choice between different gearbox and generator (supplier) combinations, mounting flanges and coupling.

Modular design, due to modest individual component sizes and masses, also enables rather uncomplicated dismounting for inboard repair and/or full exchange. Separate gearbox and generator modules further offer increased options with regard to choice of cooling system, while drive system development and turbine time-to-market could potentially be shortened as well.

By contrast, the HybridDrive is a typical "single-source" product. German engineering consultancy Aerodyn Energiesysteme has also developed a semi-integrated, single-source drive system with 3MW and 6.5MW power ratings called Super Compact Drive, or SCD, marketed as a license product.

A medium-speed drive system alternative is a non-integrated solution with all main components positioned in a line arrangement similar to a common high-speed drive train design.

Medium-speed drive systems must always accommodate a brake disc.With both single-stage and two-stage planetary gearboxes, the generator is mounted at the centrally located output shaft.The latter is usually designed as a hollow central shaft stretching between nacelle and hub pitch system, and for connecting power cables (electric pitch) or hydraulic hoses (hydraulic-pitch).With three-stage gearboxes the third high-speed stage has
designed by ABB. Both the WWD-1 and WWD-3 WinWinD

sister models are conceptually based upon a technology license of a patented, fully integrated drive system called Multibrid. The latter was developed by German engineering consultancy aerodyn Energiesysteme as a novel hybrid solution between high-speed geared and direct drive.A German company rather confusingly also called Multibrid in late 2004 installed a 5MW Multibrid M5000 offshore turbine prototype featuring a single-stage gearbox and PMG.

There are still only a few operating turbines incorporating a hybrid-type drive train, and nearly all are Multibrid-type systems incorporated in both WinWinD and (now AREVA) Multibrid turbines.And similar to these latter turbine models, PMGs are again a near sole focus for new hybrid-type turbines up to 7MW+ in development.
In the new 7MW offshore class, DSME of Korea has announced a two-stage medium-speed turbine and Vestas is developing a three-stage medium-speed model offering 400rpm generator speed.

Winergy’s first 3MW HybridDrive, a compact semi-integrated two-stage gearbox with flanged-on PMG will be fitted into a new 3MW Fuhrla¨nder turbine model. HybridDrive systems will become available up to 6-8MW for both onshore and offshore application.

Finnish companies Moventas and The Switch jointly developed a functionally comparable system called Fusiondrive for use in a 3MW DeWind turbine.They say they have developed a 7MW version as well.
A differentiation between low-speed and medium-speed drive systems can be useful.With conventional planetary-type 6 single-stage gearboxes, the maximum step-up ratio is limited to around 1:6.3.A distinct single-stage gear technology variation is a "one-and-a-half-stage" gearbox, by which a special gear arrangement enables a larger step-up ratio of about 1:10.This system is applied in the M5000, enabling stepping up 15rpm rated rotor speed to 150rpm rated generator speed (15x10).

With two-stage gearboxes, the maximum achievable step-up gear ratio is about 1:40 (about 6.3 squared),with 15 rotor revolutions now corresponding to 600rpm. Choosing between single-stage versus two-stage gearboxes impacts drive systems technologically — especially with regard to complexity, mass and relative demand for copper and rare earth metals.

Two factors are — Low speed: less complex and cheaper single-stage gearbox and a relatively large, heavy and more expensive slow-running generator; Medium-speed: more costly two-stage gearbox with increased complexity combined with a smaller, cheaper and faster-running generator.With PMG application, this solution significantly reduces demand for rare earth metals compared to a single-stage, similarly rated gearbox-generator design.

In its technical documentation, ABB calls direct drive "low-speed" and does not differentiate between single-stage and two-stage gearbox systems. Both are referred to as "medium speed" with rated generator speeds "up to 500rpm".These systems come in power ratings between 1-8MW, and at voltage levels between 690V to higher than 3.3kV.

The company has determined threean off-centre output shaft for generator mounting, while at the same time the hollow central shaft is left free for cable and/or hose transport.

Due to their compactness and favourable mass, integrated and semi- integrated medium-speed drive systems offer significant potential for reducing head mass compared to similarly rated classic non-integrated, high-speed drive trains. However, several wind experts independently point out the real potential for substantial head mass saving is with a completely new approach to turbine nacelle design. Simply adapting existing designs for accommodating a usually shorter integrated or semi-integrated medium-speed drive system might by contrast require additional measures and introduce sub-optimisation with only limited mass-saving benefits.

This article originally appeared in Wind Stats.