COVER STORY
Winds of Change hydraulic turbines generate green energy By Michelle Berdusis, Wind Energy Business Development Specialist, and Mark Barnes, Senior Vice President, Global Business Development, Des-Case Corporation
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ith an increasing drive toward renewable energy, wind power is now globally accepted as a leading technology to deliver “green” energy. Well established in Europe for the past 30 years, other countries worldwide are embracing wind energy as a clean, reliable source of power. China has committed to generating 50% of domestic power from wind energy. At the same time, Denmark is close to being self-sufficient in wind power generated offshore in the North Sea. However, as more countries become reliant on wind power, there’s been an increased focus on turbine asset reliability to ensure peak power production at low overall operating costs. Conceptually, wind turbines are simple in design, consisting of a rotor that converts rotational energy into electrical energy through a generator. Rotational motion transfers to the generator through the drive train, which often, though not always, consists of a gearbox to increase shaft speeds of the rotor to drive the generator. Because wind turbine gearboxes are critical, significant research, engineering, and design have gone into their reliability with good effect. In addition, turbine owners, operators, and OEMs developed several maintenance strategies focused on increasing gearbox reliability. These practices include gearbox design, metallurgical improvements, specialized lubricant formulations, optimized fluid filtration, and cooling. However, the same focus has not been spent on wind turbine hydraulics. Not every turbine uses hydraulics, though it is estimated that approximately 50% of them have some form of hydraulic system, a trend that is likely to grow as the size of rotors increases, requiring greater torque (force) to change the angle (pitch) of the blades.
Hydraulics in wind turbines Hydraulics performs several roles in wind turbine operations. The most common is brake and pitch control. Pitch refers to the angle of the blade. To maximize power production efficiency, blade pitch angles are constantly adjusted through a proportional valve-controlled system. The brake refers to a braking mechanism used to slow and lock the rotor from turning under adverse conditions, for example during high wind speeds. This is a safety mechanism that is required to prevent overspeed of the turbine, which could result in structural failure and damage. Some turbine designs use hydraulic rotor and yaw systems to control the movement of the nacelle to yaw (rotate) in and out of the wind. This is to optimize production and prevent damage or failures during extreme conditions. While wind turbine gearbox repairs and replacements can cost up to $500,000, hydraulic failures cost far less, ranging from $20,000 to $30,000 to replace failed pitch cylinders. However, as we become more reliant on wind power to provide base power loads, avoiding downtime through optimum maintenance of the gearbox and hydraulic system becomes a strong focus, particularly for self-performing owner-operators responsible for maintenance and operational costs. Many wind farms operate under
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power-purchase agreements that contain severe fines and penalties for unscheduled power interruptions. The complexity of performing maintenance 80-120 meters (270-400 feet) in the air means that any “up-tower” maintenance task comes with additional challenges. The work requires specialized training and safety procedures, a necessity exacerbated by the growth of offshore turbines with drastically greater safety hazards.
Fluid cleanliness Like any industrial hydraulic system, the main components of a wind turbine hydraulic system are pitch cylinders (actuators), accumulators, seals, a hydraulic reservoir and supply lines, pumps, hydraulic valves, and the control panel. Like any hydraulic system, the reliability of one in a wind turbine is tied to the health and cleanliness of the hydraulic fluids. While design certainly plays a role, an estimated 50% to 70% of hydraulic system issues in wind turbine applications relate directly or indirectly to fluid health and cleanliness. With contamination, common causes include solid particles ingested from the outside or internally generated due to wear, moisture from humidity in certain regions, and degraded fluid from poor lubricant management. Perhaps the most damaging is solid particle contamination. When a particle trapped between two surfaces removes material from one of those surfaces, the result is what’s known as three-body abrasion (see figure 1). As much as 66% of lubrication-related wear is attributed to three-body abrasion. Hydraulics are particularly prone to it because of the tight clearances found in high-pressure components. Even if outright failure does not occur, particles trapped between the moving surfaces of a valve can result in an effect known as stick-slip, in which the valve momentarily locks while the particle is lodged between the surfaces. Stick-slip can result in suboptimal pitch controls. Another common problem with wind turbine hydraulics is seal failure and resulting fluid leakage. For an industry that prides itself on delivering
FIGURE 1: Three-body abrasion causes cutting wear in sliding contacts.
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