The development of the REpower 3.XM by turbine producer REpower Systems AG has been described by the company as more evolutionary than revolutionary. Building on its experience gained from erecting more than 1,400 MD/MM-platform systems with 1.5 or 2 MW rated power, the first prototype of the 3.XM104 was erected in late 2008 on a site near Husum, Germany. A second turbine will follow in the first quarter of 2009.

Developing the 3.XM

The development of the ‘MMx’ Project – the ‘x’ stood for something beyond 2 MW – began in the first quarter of 2005. At this time, the product management team began specific market analysis, trying to forecast what size – in terms of dimensions and rated power – would be demanded for future onshore wind turbines. By the end of 2005, a general product definition had been agreed upon, fixing the turbine’s general features.

At the beginning of 2006, the product development department started the concept phase, during which it evaluated available and potential turbine and component technology.

The new turbine was to achieve the highest efficiency in the multi-megawatt class, be the biggest possible onshore turbine with similar logistical requirements to the 2 MW class, as well as maintaining a low sound power level.

The company also wanted to retain its core research and development philosophy and company strategy, including the continuous use of a platform strategy; premium quality in system design and components; a conservative design approach with focus on long term reliability; and a focus on outstanding serviceability.

By the end of 2006, the product development team and the product management team came up with a consented technical concept and entered the design phase. After almost two years of detailed development, the 3.XM onshore turbine platform was officially launched at the Husum WindEnergy fair in September 2008.

Evolving turbine technology

The first turbine in the 3.XM series is the 3.XM104, which features a rated power of 3.3 MW, a rotor diameter of 104 metres and an IEC IIa certification, including a 78–80 metre hub height steel tube tower.

A conclusion drawn from the concept phase was that the MD/MM platform would still offer some advantages for the multi-megawatt class, but that a new platform would have to be designed due to size and weight issues. The 3.XM and the MD/MM platform both feature a gearbox design with a doubly-fed asynchronous generator, which delivers 80 per cent of the power directly to the grid and up to 20 per cent via the inverters. This design helps to keep weights and costs down, while maintaining optimal performance. The turbine also offers extended grid features to contribute to the fulfilment of the rising grid requirements worldwide.

The 3.XM also includes a reliable electrical single blade pitch in a ‘fail-safe’ mode. Based on industry feedback, a water cooled converter in the nacelle (used in the REpower 5M and 6M offshore turbines) has been introduced for the onshore application of the new turbine.

Another new feature of the 3.XM is the use of the 50.8 metre blades designed and produced by REpower. These blades offer aerodynamic efficiency, whilst keeping the sound power level to a minimum. For higher sound reduction requirements, optional sound management products will be available, allowing the turbines to run in a noise reduced mode when required.

As masses and dimensions are a crucial point for this turbine, the turbine producer has focussed on its logistics and erection. Importantly, it is possible to install the nacelle and the drive train separately. This cuts the maximum weight of the mass to be lifted down to less than 60 tonnes – around the same weight of a standard 2 MW turbine nacelle, including the drive train. The service side has also been given special attention: the inside of the nacelle is designed in a generous way, offering good accessibility to all components. All rotating parts are covered to avoid injuries. The hub has been designed in a unique way to allow access from within the nacelle, enabling service staff to communicate and pass tools and/or spare parts to and from the hub.

Turbine testing and future prospects

The installed 3.XM104 prototype in the in the North of Germany is undergoing an extensive testing program to validate the internal calculations and comply with the required testing for certification according to the IEC 61400 series. The serial production of this turbine is scheduled for the second half of 2009.

Looking to the future, REpower is working on further variants of the new turbine, which will be optimised for IEC I and IEC III conditions. In addition, variants for 60 hertz markets are expected to follow in the future.