New semi-integrated medium speed PM generators maximize life cycle profit from multi-MW offshore turbines

2012-06-29 - ABB produced the first MW class Medium Speed Permanent Magnet Generators (MS PMGs) in 2000. They offer a proven way to reduce the top head mass of offshore turbines without compromising reliability. With the highest system efficiency, especially at partial load during low wind, and low O&M costs, they help to maximize annual production and life cycle profit.

Three ways to realize an MS drivetrain

As a result of lower operating speeds – when compared with mainstream, high speed solutions - the MS drivetrain is generally regarded as being more reliable and subject to less wear. There are three ways of implementing an MS design, in which the generator scope of supply can range from stator and rotor parts only to complete generator units.

Fully integrated
In this design (sometimes referred to as a “hybrid” design), the gearbox and generator share the same frame, bearings and shaft. Fully integrated solutions require joint development work between the gear and generator manufacturers, with a well defined division of responsibilities and scope of supply. Very compact, low weight designs are possible. This solution has already been in use for more than 10 years, since the first serial produced turbines using the Aerodyn “Multibrid” concept were launched in 2000 with ABB generators. Today, ABB delivers 1, 3 and 5 MW generators of this type.

This new solution delivers significant additional benefits. In a 7 MW offshore turbine, for example, the generator and gear are separate units that are partly integrated via a flange connection. This enables easy dismounting for servicing, low lifting weights and the use of serial produced and individually tested plug-in generator and gear components. Coordination between the generator and gearbox manufacturers is primarily related to the mounting interface flange and the coupling that connects the two shafts. Both units are complete modules with their own shafts and bearings. This results in easy logistics and further savings in installation, as no special crane is needed. The use of separate units also enables better bearing current protection than is possible with the fully integrated type. As only the mounting interface flange and coupling need customer specific engineering, turbine OEMs can select this design to gain a fast route to serial production and the offshore markets.

In this design the generator is a separate, independently mounted unit, with a shaft connection to the gear – the drivetrain is therefore similar to a high speed three-stage gearbox solution. This configuration offers reliability and cost efficiency, but no other significant additional benefits.

Design features

One factor common to all three types of MS drivetrain solution is that the shaft has to be in a straight line from the hub to the generator non-drive end. It cannot be offset as in a standard high speed three-stage gearbox system. This is because the generator shaft functions as part of the “pitch tube” - it is hollow to allow cables and/or hydraulic hoses for the blade pitch control system to pass from the nacelle into the rotor hub.

The size of the generator is influenced by the nominal speed. In conven­tional designs it can be selected from about 150 to 400 rpm using a single or two-stage gear. Future gear concepts could enable nominal speeds of 500 rpm or even more, for smaller size generators with slightly higher efficiency.

Increasing demand for MS PMGs

Offshore turbines need reliable, cost-effective solutions that allow cost to be lowered in all project phases. These solutions must enable serial manufacturing and easy installation and operation of wind parks to maximize the energy production. MS PMGs meet these needs.

“The new semi-integrated MS generators offer a proven solution, ensuring the best life cycle profit for the wind park,” says Timo Heinonen, Sales Manager. “One of our modern 7 MW MS PMG concept designs, for example, with a nominal speed of around 400 rpm, can be about 3 meters in diameter and weigh under 30 000 kg. It offers over 98% efficiency - the highest of any commercial wind generator design - with partial load efficiencies of 98% even at 20% load. This means the highest annual production of electricity. Reliability is also very good, with the mean time between failure (MTBF) estimated at some 25 years.”

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