SGRE uses offshore experience to take 5.X onshore platform past 6MW

Launched in 2019, Siemens Gamesa's 5.X modular turbine sets new onshore wind boundaries for rating and rotor size, and has secured orders before installing the two planned prototypes. Eize de Vries takes a close look at the platform's key technology features.

Two versions of Siemens Gamesa Renewable Energy’s (SGRE) new 5.X platform will be initially available. The SG 5.8-155 has flexible 5.8-6.6MW (nominal) power levels and a 155-metre rotor for medium/high IEC II wind conditions. It offers today’s highest onshore rating — in the temporary maximum 6.6MW power mode — since Enercon has discontinued its 7.5MW E-126 EP8 turbine. The SG 5.8-170 offers 5.8-6.2MW ratings with a 170-metre rotor for low/medium-wind IEC III sites and sets the current record for the wind industry’s largest onshore rotor diameter.

SGRE first introduced flexible ratings with the 4.X onshore series and the 11MW SG 11.0-200 DD offshore model. OEMs have moved increasingly towards offering a broad portfolio in terms of options and features to enable optimal siting and to make their technology and their clients’ projects as competitive as possible.

First "true" post-merger product

The 5.X platform is the first product SGRE developed as a truly merged company through international collaboration and benchmarking, selecting best-practice technologies from both former standalone companies, plus selected third-party solutions.

"We have integrated several proven offshore solutions, including automated lubrication of the drivetrain, and the latest remote condition-monitoring software and hardware built on a comprehensive track record," says deputy programme director Umesh Kumar Getta.  

Due to the 5.X’s higher nacelle mass compared with the previous 4.X series, the SGRE team chose to go with high-capacity offshore yaw drives. The hydraulic pitch system, on the other hand, had already been individually pursued by both Siemens and Gamesa prior to the merger. The overall product development focused on achieving the lowest possible levelised cost of energy (LCoE) by optimising Capex, Opex and annual energy production (AEP) while setting new standards for onshore cost-effectiveness.

The turbine features a hinged roof section to provide easier crew access during turbine installation and operations and maintenance (O&M) activities, supplemented by an onboard movable crane for up-tower repair and parts exchange. The turbine only requires a main service once a year and uses the latest insights regarding working safety and cyber security, reflecting the platform’s focus on reliability and serviceability, according to Getta.

Because the 5.X has been developed as a global platform, the turbine’s dimensions and individual component masses had to be limited for easier transportation. The nacelle cover height, for instance, is such that it allows "undisturbed" road transport in major wind markets.

"In the US, individual transport loads above 68 tonnes are a major cost driver. In other markets, there might be limitations in high-capacity installation-crane availability," Getta says.

"We offer multiple modularisation solutions, usually limited to three or four modules, but it can be up to six if required. We always consider the specific market circumstances and focus on optimised project cost. Increasing the module count is inevitably linked to higher on-site assembly complexity, and thus requires more time and overall effort."

Visual difference

A prominent visual difference between the 4.X and 5.X is the transformer room hanging underneath the nacelle at the rear behind the tower. "We introduced this solution with the 6MW direct-drive offshore turbines back in 2011, but functionally as an enclosed space with a (now) 66kV transformer at the lower level inside the nacelle behind the tower," says 5.X platform chief engineer Mikel Butragueño.

"Moving the MV-transformer much closer to the tower centre reduces vibration and cable transport losses inside the nacelle, which means cost savings, too. We also switched from a dry transformer to a compact oil-cooled unit as used in Siemens legacy offshore turbines."

A final benefit linked to moving the transformer outside the "main" nacelle is that the coolers could now be incorporated inside, resulting in easier transport logistics and a simplified overall design. The transformer room is dismounted and stowed inside the nacelle during transport.

The 5.X retains a high-speed non-integrated drivetrain with DFIG, as deployed in the 4.X and other Gamesa-legacy onshore platforms, but with one key difference. Instead of a "standard" four-point gearbox support (main shaft with two bearings), the 5.X now incorporates a compact cast main bearing unit for rotor support and linkage to the three-stage gearbox. A comparable bearing solution has been used since 2008 in legacy Gamesa medium-speed turbine models, and is also popular in several competitor designs for onshore and offshore.

The main-bearing unit comprises a stationary outer housing, a hollow main shaft and two pre-tensioned taper-roller bearings. "This main-bearing concept is a proven, reliable solution and enabled substantial cost and mass reduction," says Butragueño. "We retained the rigid gearbox flange connection for fast, uncomplicated assembly and on-site exchange. We kept the familiar gearbox torque arms support to guarantee essential drivetrain dynamic flexibility."

Sourcing strategies

The 5.X gearbox portfolio contains different in-house and external units, and some SGRE designs can also be manufactured by selected suppliers. Internal drivetrain-technology benchmarking further showed that a high-speed gearbox plus DFIG is still a winning combination in terms of Capex as well as LCoE through reduced power losses in the partial converter.

Butragueño says this translates into better AEP. He is convinced that DFIG-based hardware and software will be able to meet even more stringent future grid demands. The Gamesa-legacy common low-voltage level is retained to benefit from the existing large, mature global components supply chain, he explains. The top-rated 6.6MW DFIG deployed in the 5.X platform beats the previous record held by Senvion’s 6.XM152 offshore model (2014).  

The relatively lightweight glass and carbon-reinforced epoxy blades of the SG 5.8-155 and SG 5.8-170 are equipped with SGRE aerofoils. These conventional structural blade designs of 76 metres and 83.5 metres each comprise an upper and lower shell with pultruded carbon caps, enabling lighter blades.

The blades’ "Dino Tails Next Generation" add-ons, designed for optimised power output and reduced aerodynamic sound pressure level at all wind speeds and combining serrations and porous trailing edges, are a distinctive feature.

"These add-ons work in two ways," says Butragueño. "They limit the maximum sound emission to 105dB(A) for the SG 5.8-155, and 106dB(A) for the SG 5.8-170, when operating in maximum performance mode. At the same time, another feature allows the turbines to run at rated tip speeds above industry-common levels for onshore wind. This is an ideal value in specific markets without noise restrictions, enabling maximum power and increased AEP."

Segmented blade

However, for noise-sensitive sites, the sound pressure level could be reduced to 99dB(A) and even further down to 97dB(A) if required, without impacting AEP too much. SGRE is developing a segmented 83.5-metre blade for specific markets, which will become part of the product portfolio in future.

The turbines come with a range of tower options, including "standard" tubular steel towers and hybrid concrete-steel towers of up to 1.65 metres from different third-party suppliers. "We also developed a wide-base segmented steel tower in-house," says Getta. "We’re not the only ones, and even though many patents have already been awarded, several solutions only have regional coverage and there is always room for new ideas and concepts. A second ongoing in-house development is full concrete towers with up to 135-metre hub height."

Testing programme

The test programme focuses on turbine reliability and involves three distinct stages.

The first is component testing, having the scope to check the overall performance of a specific part, which includes design validation, certification and design-lifetime. The next stage is sub-system testing, which again involves the same three main aspects. The final step is completing design validation and certification for the prototype and eventually the 0-series, now focused on the overall performance of each complete platform turbine model. 

Even without operating prototypes, SGRE has already received several orders for the new platform, including one for 35 6.6MW-rated 5.X-155 turbines going to the 231MW Skaftasen project in Sweden.

The SG 5.8-155 prototype is planned for the second half of this year, followed by the SG 5.8-170 at the end of the year or in early 2021.