As the demand for wind power has been growing consistently over the years, efforts have been made to accelerate technological developments to keep pace with this growth. Wind turbine power capacities have increased by a factor of 120, from 25 kW to 3,000 kW and beyond, in the past 20 years, with their blades, rotor and hub heights increasing in size accordingly. Most commercially available and widely used turbines across the world today are taller than 100 metres, with their rotors having diameters of more than 120 metres. Thus, the drivetrain, the powerhouse of a wind turbine, has also evolved to suit the increase in size and rated power capacity of wind turbines. Drivetrain design has been constantly upgraded for greater efficiencies in cost as well as performance. Since the drivetrain is one of the most complex components of a wind turbine with a large number of moving parts, it is quite prone to breakdowns and faults. Thus, much of the advancements in drivetrains are focused on having fewer moving parts and weight reduction to have more compact, lighter and simpler systems.
Geared and non-geared wind turbines
With time, the number of possible drivetrain configurations has increased, based on various factors such as loading and costs. The most prominent difference that has emerged over the years is whether a wind turbine has a gearbox or not. Thus, broadly, two different categories of turbines dominate the market. The first one is dependent on a mechanical transmission system to increase the rotational speed of the shaft of the turbine rotor so as to drive a generator. In the second category, the generator directly uses the high torque to generate power without any mechanical transmission for increasing speeds.
In the case of geared wind turbines, the speed conversion ratios are higher, which means higher operations and maintenance (O&M) requirements due to various gearbox components. On the contrary, slow rotating electric generators are larger and heavier. Thus, these days many manufacturers opt for hybrid drivetrains, which have a gearbox that transfers the slow rotational speed of the shaft to a medium or high speed generator, coupled with a full converter. Whether a drivetrain is geared or non-geared or hybrid depends on financial factors as well as other considerations such as weight, reliability and the expertise of the manufacturer. Moreover, there is no consensus regarding the type of drivetrain that is best suited for all project sites in all geographies.
Various drivetrain configurations
According to a research article titled “Technological Evolution of Onshore Wind Turbines – a Market-based Analysis” by Javier Serrano-González and Roberto Lacal-Arántegui, overall, there are six major turbine types, based on drivetrain configurations. These are a mix of geared, direct drive and hybrid drivetrain arrangements.
- Squirrel cage induction generator (SCIG): This has a gearbox and is a robust configuration with a simple construction. This does not have a power converter or speed regulator and is a low speed design with the rotational speed of the asynchronous generator dependent on that of the blades.
- Wounded rotor induction generator (WRIG): This configuration has a gearbox with higher control flexibility than SCIG turbines. In this design, it is possible to control the speed of the asynchronous generator by a variable resistance that can modify the rotor current. However, WRIG is prone to high electric losses.
- Doubly fed induction generator (DFIG): This configuration has a partial converter connected to the rotor of the generator to control its current. Thus, the gearbox converts slow rotational speed into high speed suitable for generators. This configuration has lower electrical losses and better response to grid requirements.
- Direct drive machines: This configuration avoids the use of gearbox by directly coupling the synchronous generator to the main shaft through a full-power converter. This converter adapts the frequency of the generated wind power according to the grid and enables total control on the rotational speed of the generator. The direct drive wind turbines can have either electrically excited synchronous generators (EESGs) or permanent magnet synchronous generators (PMSGs).
- Full converter and synchronous generator: This configuration has a gearbox as well as a full converter, connected to a medium or high speed synchronous generator. While both EESG and PMSG are used, the use of the latter is more widely prevalent.
- Full converter and asynchronous generator: This configuration also has a gearbox as well as a full converter. However, in this case, the converter is connected to a high-speed asynchronous generator. In most cases, the simple yet robust SCIG is used.
Amongst PMSGs and EESGs, the former has fewer moving parts, leading to lower maintenance requirements and higher realisable efficiencies. However, the rare earth materials like neodymium and dysprosium used to make these permanent magnets have highly variable prices, which impact the turbine cost. Moreover, the majority of the rare earth materials come from China, which has a high concentration of these elements, creating uncertainties in supply chains.
Recent advancements
There is a lot of research concentrated on improving drivetrain configurations with new alternative designs aiming for greater cost efficiencies and lighter turbines. For instance, the medium speed permanent magnet generators have lower costs than standard PMSGs as they use less amount of rare earth materials. Moreover, the operating voltage of these advanced generators is lower, which reduces cooling requirements and facilitates the use of less number of cables. Another innovation is the use of advanced high efficiency power electronics in wind power turbines, which helps increase reliability and capacity, efficiency and decrease in cost.
Other advancements include the continuously variable transmission that avoids the power converter by directly connecting synchronous generators with the grid. Turbines are also being manufactured with hydraulic transmission, which is cheaper than the usual gearboxes but enables higher variation. The magnetic parts and electric generator are brought together to form a magnetic pseudo-direct drivetrain, to enable a significant reduction in size than standard direct drive machines with PMSGs. Finally, superconducting generators are also being explored as an alternative to the widely used PMSGs. Superconductors would help to reduce the volume of the turbine by a significant margin, as they have almost zero resistance, which can increase the current in coils and also air-gap flux densities.
Outlook
The drivetrain is the most critical part of a wind turbine as this is where the actual conversion of kinetic energy into electricity takes place. Thus, it is of paramount importance to design components, that can ensure higher efficiencies of this part of the wind turbine. However, drivetrains are massive and heavy and have to be mounted almost at the top of a wind turbine. The weight of drivetrains and associated components becomes even more significant in the case of offshore wind turbines, where turbine sizes are significantly higher than onshore wind turbines. Hence, to enable ease of handling, transportation, storage and installation and also have lower costs for each of these, it becomes important to make drivetrains lighter.
Further, considering a long-term view over the entire project life cycle, wind turbines and their critical components such as drivetrains must be designed in a manner so as to optimise O&M costs. Thus, the focus of wind turbine manufacturers is to also decrease the complexity of conventional drivetrains, as in many cases these manufacturers are in charge of the O&M of wind plants. All these factors are leading to rapid innovations in the drivetrain space as the race to design the most optimal, efficient and lighter drivetrain continues.
