“Fewer conventional power plants in the grid mean less inertia in the system—a safe supply is at risk if this is not countered,” says project leader Dr. Boris Fischer from the Fraunhofer Institute for Energy Economics and Energy System Technology, IEE. “On paper, kinetic energy in wind turbine rotors is perfectly suited to compensate for the loss of inertia. Our research project clearly shows that this is also possible in practice: the turbines can even provide power system inertia. We have thus done pioneering work for maintaining grid stability in the future,” Fischer states.

Complex interactions between the grid and the turbine

The provision of system services such as power system inertia is part of a paradigm shift in the way wind turbines operate: whereas grid status and turbine operation have been largely unconnected so far, there is now constant interaction. Complex feedback occurs between the grid and the turbine. As a result, requirements regarding turbine generator control are also changing. For example, to support the frequency at short notice, the active power of the wind turbines must be changed with high gradients. “This can cause vibrations that lead to increased mechanical loads,” Fischer explains. The drivetrain with its rotor blades, shafts, gearbox and generator is affected just as much as the tower.

Focus on power system inertia

In the “GridLoads” research project, Fraunhofer IEE from Kassel and its partner MesH Engineering GmbH from Stuttgart investigated how grid-supporting control methods affect the drivetrains and towers of wind turbines. The scientists focused on power system inertia because providing it can place particular strain on the mechanical structure of the turbines.

They used complex simulation methods to determine the effects of the novel vibration phenomena that arise when system inertia is provided. The particular challenge here was to bring together two different worlds: the turbine domain and the grid domain. Thus, the researchers configured high-resolution multi-body models in the mechanical domain and coupled them with transient network simulations in the electrical domain. This way, they were able to precisely identify the electromechanical vibration modes of a reference plant.

Adapting the necessary controller modules

“Our investigation shows that moderate demands of grid-stabilizing power do not represent a critical load on the mechanical components,” Fischer summarizes the situation. The systems also cope effortlessly with wide-area oscillations or the switching of step transformers. The amplitudes of the resulting vibrations are so low that the components are not damaged.

“The reduction in system inertia caused by the increased feed-in of wind turbines can therefore be compensated for by the turbines themselves without any problems in the vast majority of situations,” Fischer emphasizes. However, this only applies if the turbine manufacturers have previously adapted their control modules for the power electronics to the new tasks. The researchers are also demonstrating how this is possible in the “GridLoads” project.

“Our controllers offer an optimal compromise between the mechanical requirements on the plant side and the electrical requirements on the grid side,” says Fischer. In this respect, the Fraunhofer researchers were able to benefit from their extensive experience in the field of generator and converter control.

Heavy load in rare exceptional cases

Only in the case of a few specific, rare grid faults can excessive mechanical stress be placed on the drivetrain and tower, the research project shows. These include the occurrence of large power deficits or surpluses due to the failure of a power plant, or the fault-related, spontaneous formation of an island network (“system split”). In these cases, wind turbines have to provide sufficient compensation power immediately. They do so by increasing the generator torque very quickly, resulting in a mechanical impulse to the drivetrain—a potentially critical situation if the turbine is operating at full load. As Fischer explains, “You can’t deal with such extreme situations even with the most intelligent control system.”

However, there are several other ways to handle this type of extraordinary load. For example, it would be possible to expand the overload capacity by oversizing the electrical components and the mechanical drivetrain. Likewise, additional components could be installed such as batteries or even supercapacitors for short-term storage. They are able to provide the required high power within a very short time.

Another alternative is to use the power reserves of the turbines: after all, they only run at full rated power for 10 to 20 percent of their service life. In addition, many turbines are operated in derated mode during periods of high wind. “Discussion is still required on what advantages and disadvantages these options have in economic terms,” Fischer comments.

The “GridLoads” project was funded by the Federal Ministry for Economic Affairs and Energy based on a resolution by the German Bundestag.