As wind power stations continue to increase in size, the demands for more sophisticated technology is also increased in order to ensure a high degree of efficiency and thus maximise profitability for operators. This primarily affects the automatic adjustment of blade angles, for which some particularly interesting solutions have been developed.
For centuries now we have been utilising wind power to perform vital tasks, such as the grounding of grains or the irrigation of fields. While the windmills that we now admire in the Dutch countryside or when approaching Mallorca by air still have a distinct charm, today’s industrial wind turbines (from 1 MW upwards) are highly complex systems, equipped with state-of-the-art technology, which will provide hundreds of households with electricity for many years to come. In the meantime, wind power stations have become a major business and, alongside solar, biogas and water power stations, are amongst our most important alternative energy sources.
As the current boom in wind power emerged, a great deal of effort was made to utilise existing generator technology to the greatest possible extent in order to ensure that systems were ready for the market in the shortest possible time. This often resulted in substandard systems, however, in spite of their enormous potential to reap the benefits from one of our most important renewable energy sources. Further developments in wind power technology have not only significantly increased system capacity, but have also improved reliability. These key factors alone made systems of this kind a worthwhile, profitable investment for operators.
In all types of wind turbines, adjustment to the respective prevailing weather conditions plays a crucial role. While millers had to shorten the coverings on their sails if the wind became too strong, this adjustment is now performed on older wind turbines by stalling them, which is determined by the shape of the rotor blades and occurs automatically according to wind strength. However, this passive technology is far too inefficient for modern systems, as this method leaves a large proportion of the potential energy available unused. The solution is to actively adjust the angle or pitch of the rotor blades, a central function for modern wind turbines from a particular size. The pitch control of these gigantic rotors serves two functions: to optimise the electric power yield for the respective prevailing wind strength and to safely stop the systems during strong winds or in the case of an emergency. Modern systems for adjusting blade angles either take the form of a simple, collective design in which all blades are adjusted using a common linkage, or they take the form of more complex solutions with individual blade adjustment, where each rotor blade can be adjusted on an individual basis.
The search for a drive system
Engineers therefore had to set out to find drive solutions for the blade adjustment. Older systems were predominantly equipped with hydraulic drive systems, which have subsequently proven to be too large, heavy and inaccurate as well as requiring a high level of maintenance. This is why modern systems work with electromotive blade adjusting drives. With suitable electronics these solutions can be controlled to a significantly higher degree of accuracy, reduce maintenance costs and, furthermore, do not involve the risk of hydraulic oil leaking into the sea or onto agricultural land, which could result in fatal consequences at typical locations of systems of this kind.
Until recently, even state-of-the-art pitch control systems have used relatively traditional technologies such as AC induction motors or brushed DC motors with accompanying electronic components for speed or position control. But in the same way that turbine technology shifted from dual-fed induction generators (DFIGs) driven by large, complex gearheads, over to directly driven permanent magnet synchronous machines, the use of higher-performance, brushless, permanent magnet motors in pitch control is also on the rise. The arguments supporting this technology range from its compact design and high level of dynamic accuracy in comparison with AC induction motors, to its low maintenance costs when compared with brushed DC motors.
Just as important as selecting a suitable motor is the matter of optimising power transmission. Dynamics, reliability and service requirements are again the main point of focus when it comes to selecting the gearheads. On the one hand, the gearheads must work to a high level of precision, as rotor blades are only adjusted by a few degrees for an optimum wind yield. On the other hand, incredibly fast motions are required if the rotor has to be stopped during a storm or other emergencies. In these cases, the rotor blades are brought into the so-called feather position and taken out of the wind within a matter of seconds. In addition to this, the gearheads must also work to a reliable standard in adverse environmental conditions: whether located in the humid climate of Texas, the coldness of Siberia or the salty air of the North Sea. Systems should also be maintenance-free to the greatest possible extent, as each standstill is costly and there is a great deal of expense involved in terms of both time and money in accessing systems in elevated positions.
Further components of a wind turbine, in which robust, weather-resistant drive systems are required, can be found, for example, on the maintenance door. The door is often so heavy that it is difficult to open it manually – especially if it is frozen over with snow and ice. For this purpose, supporting drive spindles or linear actuators are used here. The same technology is applied at the rotor brake, which reliably holds the rotor in its parked position if wind conditions do not permit operation or when maintenance or inspection work is performed. If a drive does not function as it should, this can result in catastrophic consequences, both for the entire system and the technicians or for the area directly surrounding a wind turbine.
A tailor-made solution
Thomson, one of the world’s leading providers of drive systems, is now offering special products and solutions, which are tailor-made for usage in wind turbines. For the pitch control of wind turbines, UltraTrue planetary gearheads from Micron come highly recommended. Micron, the planetary gearhead specialist, is part of Thomson, which manufactures a wide range of innovative, linear drive technology products. In the case of Micron UltraTrue gearheads, development focuses upon two central requirements: maximum possible performance and moderate costs. These essential features are facilitated by helical planet pinions, housing and shafts made from stainless steel as well as tapered roller bearings and spherical roller bearings. This results in considerable savings for wind turbine operators, both in terms of purchase and failure-proof operation, while significantly lowering maintenance costs in comparison with many other systems.
What’s so special about the Micron planetary gearheads is the ingenious pinion arrangement, in which three so-called ‘planet gears’ are rotated by a drive pinion or ‘sun wheel’. The planet gears rotate within a gear ring, which is normally milled directly into the inside of the housing. A particularly rigid design emerges from this, ensuring a high degree of torsional stiffness for the entire gearhead unit. As the load applied to the output shaft is distributed onto planet pinions in equal measure, a planetary gearhead has a higher loading rate than a helical gearhead of the same size. In addition to this, several gearheads can be accommodated in a limited space, meaning that extremely high transmissions are possible. While standard systems provide maximum transmissions of 100:1, with its rectangular- shaped gearheads Micron offers transmissions of up to 500:1 as standard.
Kollmorgen, a Thomson sister company, provides tailored solutions for pitch control based on robust, tried and tested products such as brushless EC motors (electronically commutated motors). These motors are resistant to even the harshest of environmental conditions. They have been specially developed and tested to fulfil the highest standards with regard to resistance to temperature, impacts and vibrations, meaning that they can even be used in military applications. The EC motor is the result of decades of development work by Kollmorgen in the field of brushless motors in the highest performance range. Depending on customer requirements, these motors can be combined with various tried and tested control electronics platforms. Kollmorgen offers pitch control solutions in both the high-voltage (230-460 V AC) and the low-voltage range (24-80 V DC). Kollmorgen motors can also be combined with various gearhead versions in line with specific customer requirements.
Thomson also provides linear actuators and drive spindles for use in wind power stations. Its extensive product range offers extremely robust precision solutions that can cope with loads of up to 50,000 N. They work effortlessly in the salty air of sand storms, in places where there are extreme temperature fluctuations, in crisp cold conditions or under high loads resulting from vibrations and impacts. The E150 actuator from the Electrak Pro product range, for example, is frequently used as the drive for the rotor brake. These modules are designed for axial loads of up to 9,000 N, provide maximum dynamic load speeds of 38 mm/s and have a robust load holding brake. Thanks to their resistant and corrosion-proof aluminium housing (IP66 protection), they can be used virtually anywhere where there is a high ingress level of dust, pollution or water and continuous, reliable and maintenance-free operation is an absolute must.
Positive future prospects
Equipped with these high-performance drive components, the next development phase in the field of pitch control has already been scheduled and has to a certain extent already begun. This, for example, includes the implementation of highly developed control strategies for blade adjustment in the form of a multivariable control system. Technology of this kind opens up additional opportunities for further increasing the yield of wind turbines. It may also be possible to integrate load data from the status monitoring system of the rotor blades and the turbine tower into a continuous control cycle for adjusting the blades. A system would therefore be able to work closer to its upper operating point without the risk of damages. Thanks to this kind of optimisation, the pitch system is able to maximise the torque supplied to the generator and thereby increase its output power without exceeding the permitted loads on the rotor blades or the tower. As a next step, a computer model of the occurring vibrations and unbalances could be incorporated into the control algorithm to continually adjust the individual pitch of each rotor blade regardless of its actual position.
The field of renewable energy resources is currently experiencing an extremely interesting phase. Continual developments in both the fields of wind power technology and other renewable energy resources will in the long term enable us to provide future generations with power generation procedures, allowing us to effectively protect valuable resources.