Age is no barrier to performance

Paul Boughton
Matt Fielder considers the challenge of extending turbine life beyond OEM warranties.

Demand for wind turbines is growing rapidly year on year, with the speed of new turbine installations being forecast to increase at a healthy rate in coming years. Yet this fast growth, in what is still a relatively immature market, is creating fresh and perhaps unexpected challenges for manufacturers and wind farm operators. In particular, earlier generations of machines are coming off-warranty, giving rise to concerns over ongoing operating costs, while the success of new turbines that are expected to offer ever higher levels of output and availability, with lower capital and operating cost per MW hour, will depend on the performance of new motion and control technologies.

There is currently a target set by the European Commission to produce at least 20 per cent of the region's energy from renewable sources by 2020, a figure that we are well on the way to meeting. There has also been considerable lobbying from various political and trade bodies for further targets to be set beyond this date; the European Wind Energy Association (EWEA), for example, is keen for a target of 45 per cent to be reached by 2030.

Long term targets or not, the sector is thriving. EWEA reports for 2010 show a 51 per cent increase over the previous year in the number of installed offshore turbines, adding 883MW of additional capacity. This brought the total number of operational offshore turbines in Europe to 1,100, with an average wind turbine size of 32MW, capable in a normal wind year of generating 11.5TW of electricity.[Page Break]

Although onshore the picture is slightly different - there was actually a 10 per cent decline in rate of new installations in 2010 - the overall picture is extremely positive with new turbines increasing in number, size and capacity. However, as more and more turbines come on-stream and then begin to age, we are going to see a dramatic increase in the number of units that are no longer covered by OEM warranties, with potentially a significant rise in cost per MW hour as maintenance and repair account for a growing percentage of downtime.

There are essentially two issues here: the need to manage existing turbines to maximise availability; and the need to develop new turbines that by default require less long term maintenance to minimise both warranty and operating costs.

Hydraulics technology forms a substantial part of each turbine, so it therefore represents an important area in which to concentrate improvements in performance and reliability.[Page Break]

Conventionally, mounted within the hub of the nacelle there will be a dedicated three-way pitch control system - one for each blade - incorporating hydraulic cylinders and cam mechanisms that vary the angle of the blades. Similarly, turbine braking is modulated hydraulically using a series of valves to prevent excess torque being transmitted through gearbox components, while the same hydraulic power packs used as part of the pitch and braking systems can also be used to adjust the yaw of each nacelle.

Hydraulic power will be generated via dedicated pumps and motors, which in the latest systems incorporate advanced monitoring and control sensors and drives, with accumulators being added to act as an energy reserve and improve speed of response. This integrated system typically provides load sensing, so that pressure and flow are matched exactly to demand, and can play an import role in enhancing overall levels of efficiency. In addition, the use of accumulators can enable the size and weight of other hydraulic components to be reduced, thereby providing further savings.

Other recent developments in hydraulic technology can also significantly improve the reliability of turbine systems. For example, Parker cylinders now feature integrated manifolds, control valves and position sensors to ensure that rotor blades are quickly feathered if a problem occurs; similarly, Parker hoses offer improved durability, ozone and UV resistance, non-conductivity and chemical resistance, with low volumetric expansion and pressure drop.

Regardless of the sophistication of the latest hydraulic systems, it remains essential to protect them from one of the primary causes of failure: oil that has become contaminated through the ingress of particles and moisture. This can originate from a variety of sources including the initial system construction, poor maintenance procedures, damaged seals and wear between moving internal surfaces.[Page Break]

Efficient, long life filtration is therefore critical. It's also worth noting that the same point applies equally to turbines where most control elements are electromechanical rather than hydraulic, as lubricating oils must still be correctly filtered to avoid damage to moving parts, especially in gearbox units. And as most turbines' failures to date have been traced to mechanical gearbox faults, it makes sense to ensure that all filtration systems are working efficiently.

The latest filtration equipment uses extremely advanced dynamic flow and separation technologies to eliminate the problems caused by contamination, being capable of removing particulates and water droplets down to just a few microns in size. Developments in filter media have played an important role, with modified glass fibre materials being used to offer improved strength and dirt holding capacity with reduced flow and pressure resistance. Parker's Microglass lll material, for example, can reduce dynamic pressure across the filter by 8 per cent while improving contamination loading by 15 per cent, compared with conventional filter material. Perhaps as importantly, these developments also help to reduce weight and size, and can extend service life.

The developments described above will all help to improve the operating life of hydraulic turbine systems. One of the most important tools, however, for lowering MW cost/hour, is the ability continuously to monitor the condition of the hydraulic systems, both through remote feedback from sensors and drive controls but also through analysis of contamination levels in oils or lubricants. The latter is critical, as early detection of contamination will enable impending problems to be identified and appropriate action taken in a planned and therefore cost effective manner. In most instances remedial action can probably be scheduled as part of routine maintenance programmes; even if the problem is more pressing it may still be possible by adjusting turbine operation to delay the point at which it needs to be taken offline.[Page Break]

Condition monitoring for hydraulic systems involves measuring the particle and moisture content of oils. Measuring instruments can either be handheld devices, used during routine maintenance, or fitted online to provide continuous readings to a remote monitoring station.

Modern analysers, such as Parker's icountPD, offer fast and accurate measurement of contamination levels using a process called light obscuration, light blockage or light extinction. Essentially, the shadow of any particles suspended in oil passing through a light beam from a laser source causes a corresponding voltage drop across a light sensitive diode mounted opposite the source; 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.

These analysers can easily be built into hydraulic circuits, including the lubrication or power transmission circuit, to provide operators or maintenance contractors with a real time readout of solid contamination levels, which in the case of the icountPD is in accordance with ISO cleanliness codes. Typically, the latest analysers can identify particles down to 4µ in size, with integral moisture sensors being used to detect water contamination without the need for a separate stand alone unit.[Page Break]

Hydraulics is a proven technology - indeed it's been in common use in industry since the eighteenth century - yet still provides an effective method of delivering motion and control.

For wind turbine designers and engineers, the continuous evolution of hydraulic components and systems, allied to effective filtration and condition monitoring techniques, has the potential to reduce significantly the operating cost of both the current and future generation of turbines.

Matt Fielder is Industrial Market Development Manager for Parker Hannifin (UK) Ltd, Dewsbury, UK.

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