As increasing numbers of wind turbines move out of their manufacturers’ warranty period, more and more maintenance contractors will be required to provide independent advice to wind farm operators. In particular, third party technical capacity will be required for component inspections, repairs and refurbishment, especially for critical parts such as gearboxes and blades.
With hydraulic systems representing a substantial part of each turbine, it is important that both operators and potential maintenance providers are aware both of the potential problems that could arise as parts wear, and how to implement an effective preventative maintenance programme.
The latest hydraulics technology can commonly be found in the nacelle of a wind turbine to control the pitch of the blades and the yaw of the nacelle, optimising the turbine’s power generating efficiency and protecting the equipment in high winds. Another use of this equipment is in low maintenance hydraulic gearboxes that are helping wind farm operators to reduce costs considerably.
In pitch control systems, the angle of the rotor blades is altered slowly and precisely to achieve maximum generating output. The blades are either turned into the wind to increase rotational speed if the wind speed falls, or out of the wind if wind speed increases, causing the rotational speed of the blades to decrease in order to protect the turbine from damage. This is typically achieved by installing three pitch control systems in the hub of the turbine, one for each blade. These systems employ hydraulic cylinders that vary the angle of the blades through a cam action.
Similarly, the yaw, or rotational position of the nacelle is continually adjusted to obtain maximum efficiency from the turbine. In the same way that output is optimised by tilting the angle of the blades so that they are facing into the wind, the nacelle too must be rotated horizontally about the axis of the tower in response to the changing wind direction. Hydraulically powered yaw control systems are able to offer a simple, compact direct drive, reducing the frequency of maintenance required in comparison with other methods, such as electromechanical control.
As it is these systems that are key to the efficiency of wind turbines, the hydraulics must be protected from potential particulate and water contamination – one of the main causes of failure in lubricated and hydraulic equipment. In essence, what is needed is a range of effective filtration and condition monitoring solutions if engineers are to achieve consistently reliable operation, and therefore minimised operating costs. This is particularly true when you consider that the hydraulic fluid, if maintained and filtered properly, can help turbine operators overcome the rising oil costs as the closed system means that it recycles virtually all of the oil it uses.
Effective inline filtration is particularly crucial to remove particles before they reach sensitive components such as electromechanical equipment and gearboxes, as failures in these systems often account for the majority of unplanned maintenance work. This is simply achieved by using the latest filtration technologies that can capture particle and water contamination to just a few microns from hydraulic fluid.
Without the use of this technology, precision engineered system components, including cylinders, accumulators or valves, will suffer from reduced levels of performance and premature failure, ultimately reducing the efficiency of wind turbines and inflating costs for operators. Equally, the specification of such technology requires careful consideration to prevent the flow of fluid from being restricted, which will reduce the efficiency of the hydraulic circuit.
Indeed, it is vital that filtration systems are correctly designed and sized to match the overall operating conditions and eliminate the risk of pressure loss across filters, which can reduce the dynamic performance of hydraulic systems and lead to problems, such as increased energy consumption and heat generation. Just as importantly, filter packages should be correctly selected to ensure that the required frequency of intervention, such as for cleaning or replacement, matches that of the overall equipment maintenance strategy to minimise the costs and disruption associated with filter care.
To address this issue, the leading manufacturers of filtration systems for wind turbine hydraulic equipment have developed a new generation of products that offer maximum performance, including unprecedented strength and dirt holding capacity, without the drawbacks that were traditionally associated with the technology, such as resistance to flow. For example, Parker’s Microglass III material has been developed to reduce dynamic pressure across the filter by eight per cent while improving contamination loading by 15 per cent typically, compared with conventional filter media.
Although this innovative fluid filtration technology effectively removes contaminants that find their way into a hydraulic system, contamination levels should also be recorded and analysed to enable essential maintenance to be scheduled before problems occur. Until recently, inline monitoring was complicated and expensive, while taking samples from a hydraulic system in a wind turbine to a remote laboratory for testing was impractical; however, wind farm operators can now use the latest particle counting technology, such as that developed by Parker Hannifin, as a low cost, real time, onsite solution.
Particle counters or portable analysers, such as Parker’s icountPD, offer a fast and accurate measurement of contamination levels in hydraulic fluids. These devices often use a process called light obscuration, light blockage or light extinction, where the shadow of any particles suspended in a fluid passing across a laser light source causes a voltage drop across a light sensitive diode; the signal generated as a result of the shadow is dependent on the size of the particle and the speed at which it passes across the light.
The icountPD can also be built into the hydraulic system, including the lubrication or power transmission circuit, along with remote monitoring devices to provide end users with a real time look at solid contamination levels in accordance with ISO cleanliness codes. In particular, this high performance device can identify particles down to four μm in size, providing an early warning of wear and potential component failure, as well as the option for an integral moisture sensor to detect water contamination without requiring a separate stand-alone unit.
Furthermore, the latest generation of particle counters feature robust constructions, making them ideal for use in the hostile environments in which wind turbines are typically located. Their compact, portable nature makes the devices ideal for field use, while their powerful internal computers offer results that are consistently accurate, removing the subjectivity associated with manual, laboratory based testing methods.
Thanks to these recent advances, engineers can now implement effective preventative maintenance strategies that use a combination of the latest condition monitoring technologies and a practical approach to filter replacement. As a result, problems can be identified early enough to prevent severe damage to key components, eliminating the risk of degraded performance and, in the worse cases, catastrophic failure. Perhaps most importantly, this can help operators realise significant cuts in operating margins, boosted performance and reduced total cost of ownership for maximum profitability.
Shaun Skilton is product sales manager for the HFDE Condition Monitoring Business Unit of Parker Hannifin. With annual sales of $10 billion in fiscal year 2010, Parker Hannifin is the world’s leading diversified manufacturer of motion and control technologies and systems, providing precision-engineered solutions for a wide variety of mobile, industrial and aerospace markets.
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