Wind turbines are powerful structures with the capability to convert strong winds into electricity, to power households and industries. Although the structure of a turbine may look simple from the outside, it houses a powerful technology known as drivetrain. A drivetrain broadly consists of the main bearing, shafts, gearbox, generator and power converter, which work together to generate electricity. Along with larger blades, innovations in wind energy technology have largely been centred around improving efficiencies in drivetrain technology. Further, as wind power is increasingly being set up offshore rather than on land, there is more scope for developers to increase both the size and power of the wind turbine and plant. The capabilities of drivetrains have evolved considerably over the years to account for these innovations, and there is scope for even more growth as global wind turbine deployments increase.
A recent paper titled ‘Wind turbine drivetrains: state-of-the-art technologies and future development trends’ by Nejad et al., published by the European Academy of Wind Energy presents some of the key innovations and future trends in drivetrain technology around the world. Renewable Watch highlights some of the key aspects of the paper…
Innovations in drivetrain technology
One of the common trends in the drivetrain market is the pursuit of lighter and more compact drivetrains to reduce nacelle mass and, hence, costs, especially in offshore and floating systems. Similarly, there have been efforts to increase the mechanical integration between the main bearing, gearbox and generator. However, there is a need to review the innovations and concerns for each component.
Main bearings: The main bearing plays a critical role in supporting the turbine rotor. Currently, main bearing designs use rolling element bearings. It is difficult to move past such designs, as other existing bearing technologies such as hydrostatic, air and magnetic bearings require very rigid support structures or are limited to smaller diameters, contrary to how modern turbines are designed. However, recent studies estimate main bearing failure rates to be as much as 30 per cent during the operating period of a wind plant, calling for improvements in design. With larger turbines, main bearing replacements become more difficult and expensive, as they require the removal of the rotor. As a result, main bearings are increasingly regarded as part of the load-carrying structure, with larger cost implications for failure. One solution is the development of coupled approaches to the modelling and assessment of wind turbine drivetrain systems. Newer designs such as novel main bearings, asymmetric spherical roller bearings, and plain bearings are also being developed and tested.
Gearbox: Gearboxes transform slow speed, high torque wind turbine rotation to the higher speed required by the generator. The gearbox system consists of many elements, including rotating shafts, gears and bearings, and is designed for a minimum life of 20 years. Its reliability is linked to the reliability of its components, which most standards do not account for. Similar to main bearings, this has serious implications on O&M costs, especially as wind turbines get bigger and more powerful. The additional factors that must be considered while estimating the reliability of gearboxes are gear tooth surface durability, bending strength and shaft fatigue fracture. Further, requirements for materials, processing and manufacturing can also be incorporated into these standards. In terms of innovation, plain bearings in the gearbox are under development because they offer advantages in terms of torque density. These devices are already becoming common in new gearboxes. To achieve further cost reductions through economies of scale, modular gearbox designs have also been introduced.
Generator: Wind turbine drivetrains can be either geared or direct-drive generator systems. Geared generator systems can be further divided into either doubly fed induction generators (DFIGs) with partial power converters or brushless generators with full-power converter (GFPC) systems. The DFIG system is widely used for medium-size turbines ranging from 3 MW to 6 MW. On the other hand, GFPC solutions are used at power levels up to 10 MW. In terms of direct-drive systems, rare-earth permanent magnet synchronous generators (PMSGs) are appealing for offshore applications. In terms of the power conversion system, PMSGs with full-power converter systems are becoming more common than DFIGs with partial-power converter systems. While PMSGs can handle higher-powered wind turbines, they have been shown to have a higher failure rate, especially in large wind turbines. Further, concerns over the availability of raw materials – namely rare-earth elements such as asneodymium, praseodymium and dysprosium, which are used to manufacture PMSGs – have led to the development of alternative technologies. Breakthroughs in alternative superconducting materials could shape the future of generators in the wind market.
Research has shown that the reliability and availability of wind generator systems have a strong impact on the levellised cost of energy. Improving the reliability of these systems is vital, as the size and power of wind turbines are growing both onshore and offshore. One potential solution is linked to improving system reliability by considering interactions with other components. Second, in large wind turbines, multiphase windings with modular converters can be used to improve the generator system availability. Significant cost savings can be realised by developing effective stator winding cooling systems that can foster the development of higher-power PMSGs of substantially smaller diameters, while not adversely affecting the electromagnetic performance of the generator. Additionally, multiphase, modular designs are solutions for some of the challenges, and they have even been used in commercial systems.
Power converter: The power converter is responsible for controlling the output power of the generator with regulated voltage and frequency. For MW-scale generators, however, the low frequency torque pulsation and high total harmonic distortion can potentially become very harmful to the generator. As a result, designing a power converter is very challenging. Further, temperature swings induced by wind speed variations in power converters also cause significant wear and tear to the system. While integrating storage technologies presents a potential solution to curb ageing, it is currently not economically feasible for many wind power plants. Alternatively, the kinetic energy in the wind turbine’s rotor can be used as energy storage by rotating the speed control to suppress the power fluctuation in the power converter and thereby reduce temperature swings. Further, variable switching frequencies using an altered grid filter design can also reduce temperature swings, which is an attractive option as a control approach that does not add hardware costs, but may be challenging to execute. Adopting liquid cooling systems can also extend the life of the power converter.
There are also concerns of grid instability with increased wind penetration. Energy storage systems or synchronous condensers can be linked to wind turbines to offer inertia and to reduce the burden of the grid without costly grid reinforcement options, but these components are still expensive. Another promising approach is to apply grid-forming mode control to the grid-side wind in such a way that the inertia from the wind turbines can be used to support the grid and enhance grid stability and reliability.
Other emerging areas
As floating wind turbines become more prominent, drivetrains in these set-ups will have to be modified as they are exposed to damage from waves in addition to wind loads. These wave-induced motions can have a drastic impact on the life of the main bearing – particularly on the one carrying axial loads. Studies have also found that the air gap stability of generators is also threatened in a floating wind plant, highlighting the need for air gap management for direct-drive generators on floating platforms. Some studies have also suggested looking into the coupling effects between the structure and the drivetrain when constructing floating wind projects.
Another area that efforts must be directed towards is minimising the impact of the wake effect, or the reduced wind speed induced by the turbine as they extract from the wind. The wind speed reduces behind a given turbine and impacts the efficiency of adjacent turbines. The wake effect also causes increased turbulence and thus can affect the loading on the drivetrain. To address this concern, optimised plant control design is necessary to both maximise the wind plant power intake and minimise the degradation of the drivetrain. Moreover, a wind plant must be designed to distribute accumulated fatigue evenly over the drivetrains of different turbines to improve the reliability of the plant.
Finally, the integration of digital technologies and real-time data management can support wind turbine drivetrain analyses at both the system and component levels during its life cycle. To this end, digitalisation targets the sensors and actuators installed on the drivetrain as well as the other turbine systems that are connected to the turbine and the plant’s control and monitoring systems, to improve reliability, availability, quality of service and user experiences. Digitalisation also enables digital twin models that can support the drivetrain’s design and operation. While it is difficult to execute such a model, through cloud computing it is possible to break the digital twin into simple sub-problems with less computational complexity. Internet of things-based technologies can then provide real-time access to data.
The drivetrain and its components are crucial to the smooth functioning of a wind power plant. While there have been innovations in each of these components, the overarching conclusions are that there is a need for improved standards in assessing the reliability of the drive chain, as well as a need to explore the integration of the various components. Moreover, these developments in drivetrains must be in line with the other innovations in the wind industry. While digitalisation, particularly for drivetrain technology, is still at a nascent stage, it is important to explore how it can be used to address the current concerns in drivetrain systems.