In order to achieve their design potential as credible alternative providers of energy, wind turbine systems have to employ highly efficient, reliable and robust components. Among the most important of these components are gearboxes; in particular, planetary gearboxes. One of the manufacturers of planetary gearboxes is Brevini. This article describes the company’s activities within this field.
Planetary gearboxes manufactured by Brevini are used in three key areas of turbine operation: the rotor, the yaw system for altering the orientation of the rotor in the wind, and the electrical generator system. Both of the latter systems are housed within the nacelle, which is the heart of the turbine and also accommodates the main rotor shaft, a disc brake for use in emergency stop conditions or during maintenance operations, and the control system and switchgear cabinet for the wind turbine.
The most common type of rotor consists of three blades (usually made of glass-fibre reinforced polyester) assembled onto a cast iron or steel rotor hub, complete with pitch single ball race bearings and internal gearing. The presence of the pitch bearings means that one or more planetary gearboxes (for each blade) are involved, together with electric brake motors as the prime mover. This set of components is defined as the ‘pitch system’.
The pitch system employs three-stage slewing planetary gear units (maximum torque: 5,800–10,000Nm) from the company’s RPR range. The RPR units turn the blades on their axes (usually up to 90 degrees) in order to regulate the rotor’s performance, in the range from 10 to 30rpm, under different environmental conditions. In the case of excessive wind speeds (higher than 25m/s) this system is used as an ‘aerodynamic brake’, where blades can be swivelled fully perpendicular to the direction of rotation, in order to decelerate and stop the generator.
The RPR series slewing drives are an essential component in keeping weight and size to a minimum on modern wind turbines. Their planetary structure means that they can withstand very high torque values while maintaining reduced dimensions. The slewing drives are designed with an extended output housing to withstand high radial loads: overall, they provide a higher radial load capacity when compared to standard gearboxes. This advantage translates into extended bearing life, a critical component of machinery reliability. In addition, reduced radial loads within the slewing drive itself enable it to tolerate higher radial loads from the equipment that it is driving.
Brevini slewing drives - © Brevini
The rotational movement of the turbine rotor is connected to the electrical generator via a three-stage, overdrive gearbox, whose transmission ratio depends on the rotor diameter (the larger the diameter, the higher the transmission ratio). In particular, the gearbox must increase the rotation speed from 30 to 60rpm (which is insufficient for producing electrical energy) to 1,500rpm. The unit has gears generally in parallel on the input side and a planetary gear stage on the output side. The gearboxes are shaft-mounted units with a double torque arm. They are selected based on a requirement for 20 years of continuous life, and are usually equipped with a cooling system including oil filters and an oil recycling unit. The use of a shaft-mounted unit offers several benefits: it eliminates the cost and complexity of couplings, and also the time and labour involved in ensuring correct alignment of the gearbox and the equipment it is driving. In addition, the direct drive provided by shaft mounting avoids radial loads imposed by chain drive arrangements.
To ensure maximum power output from the generator, large wind turbines are equipped with a yaw system that allows the nacelle, via a combination of an anemometer and the turbine control system, to constantly adjust its position according to the direction and force of the wind. The yaw system comprises one or more planetary gearboxes, depending upon turbine size, with electric brake motors as prime movers. The gearboxes are usually RPR slewing drives with maximum torques in the range from 10,000 to 70,000Nm. They are mounted in the vertical position at the top of the tower, with their output shafts and pinions facing downwards and engaging with the yaw ring (a single or double ball race with internal gearing that provides the connection element between the nacelle and the tower).
Yaw system: the gearboxes are usually RPR slewing drives in different
configurations with maximum torques in the range from 10,000 to 70,000Nm
The pitch system employs three-stage slewing planetary gear units
(maximum torque: 5,800–10,000Nm) from Brevini’s RPR range. - © Brevini
In most designs of wind turbines the yaw ring is fitted with a hydraulic brake system having one or more brake calipers. During yawing operation these are only partly released in order to achieve a smooth movement. When there is no yaw operation the nacelle is kept securely in position by the hydraulic brake system and by the brake motor on the planetary gearbox yaw drive.
The application of the planetary gearboxes to all the key operating areas of wind turbines is the result of a number of factors. Firstly, planetary gear trains are able to deliver high reduction ratios in small packages, and to transmit several times the torque of similarly sized, conventional gear units. Secondly, they are compact and lightweight, and require little installation space. Thirdly, the planetary drive is up to 98% efficient and, critically, is able to provide extremely low speeds without any loss of efficiency.
Finally, there is the facility of modular design, based upon the epicyclic cell, to provide ultra-compact multiple reduction units. The advantages of this arrangement to the designer are twofold. The planetary stages can be combined to match the increasing torque being transmitted through a gearbox. They can also be assembled to provide reduction ratios right up the scale: to a value of 15,000:1 if required.
Yaw system: wind turbines are equipped with a yaw system that allows
the nacelle to constantly adjust its position according to the direction
and force of the wind. - © Brevini
This article appeared in the volume 1, Nr. 4 / 2005 issue of Windtech International and is republished with permission of the authors and publisher.