Fail-safe design approval for future turbine growth

Fraunhofer´s nacelle test rig uses an artificial network with 44 MVA installed inverter power

By Britta Rollert,
Director Marketing and Communication, Fraunhofer IWES

Before the market launch of newly developed or modified prototypes for offshore applications, the need for extensive tests and feedback from experience gathered in the field is essential. Testing is an opportunity to improve the turbines and technologies in use further and faster in the future. It increases their reliability and thus reduces inherent development risks and the cost of offshore wind power utilization further.

Fraunhofer IWES’ work focuses particularly on component and system tests on large-scale test benches. These validation services accelerate the market introduction of innovative products and enhance certification processes.

Nacelles, drive trains, and direct drive generators
Larger rotors require more powerful drive trains; this is a matter, first and foremost, not just of adapting the nominal output, but of improved concepts, detailed solutions, and component development. Due to the exponential growth of the static and dynamic drive train loading, the reliable and economical dimensioning of parts and assemblies is approaching the limit of the scope of current calculation models and practical experience.

The dynamic nacelle testing laboratory (DyNaLab) and its implemented, dedicated hardware-in-the-loop system are designed for realistic test scenarios (turbine operation). On this system test bench all current offshore turbines as well as next-generation turbines can now be subjected to extended function tests, development tests as well as accelerated tests or stress screenings.

These kinds of tests facilitate the concrete analysis of the dynamic interaction of individual components on the real nacelle mainframe, with the original power cabling as well as using the real wind turbine controller. Its core components are: a highly dynamic direct-drive in excess of 10 MW nominal and 15 MW peak output and a force-controlled servo-hydraulic load application system for the main shaft hub interface of up to 2 MN and 20 MNm.

Support structures
The support structures and foundation systems are often the most expensive parts of an offshore wind turbine. Monopiles currently have a market share of around 97%, reaching diameters of up to 11 meters. The disadvantage is that there is no universal dimensioning basis for XL monopiles, as the standard p-y method [DIN EN 19902, 2014] was calibrated for “long slender jacket piles with diameters of less than 1 meter” [DNV, 2014]. As such, this method is not applicable without prior validation. Since the field tests required for this are very costly, reproducible large-scale tests can be an economical alternative. One solution to this issue is to revise the existing p-y curves both under static and cyclic loading. For this, Fraunhofer IWES runs a test center for support structures in Hanover, in which new dimensioning methods can be developed or existing ones can be validated.

Rotor blades
IEC 61400/23 requires that newly developed rotor blades must undergo both static and dynamic testing for structural durability. For this, after preliminary static testing in four directions, the rotor blades undergo cyclic testing in two load directions for a certain number of load cycles by excitation at their natural frequency. Finally, static tests must be conducted once again. Fraunhofer IWES currently has two blade test benches performing this kind of certification testing. To date, 20 new designs measuring up to 83 m in length have been tested for various clients.

Full-blade testing with natural frequency excitation in dynamic tests minimizes the energy requirements and generates economic benefits.

Testing should be seen not as a burden, but rather as an opportunity to improve the turbines and technologies in use further and faster in the future, to increase their reliability, and thus to reduce inherent development risks and the cost of offshore wind power utilization even further.

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