The hydraulics of wind power

In the last few years wind turbine technology has changed. Previously the wind turbines were stall machines and their position would shift only once every 10 minutes. Such turbines have been superceded by continuous pitch systems, where the pitch, the position of the nacelle and angles of the blades, constantly changes in small amounts once every rotation. That could be on average 15 times per minute.

“While this optimised the production of energy from the turbine, for us, the actuator manufacturer, it presented a real challenge,” says export sales manager, AVN Energy, Silkeborg, Poul Kristensen. “Instead of hydraulics producing six long strokes per hour, they now had to give 900 short strokes in the same period. And it’s not just the pitch which is continuous, it is also the turbine’s operation, with the actuators needing to initiate those strokes 24 hours a day, seven days a week.

Simulation programs are extensively used by AVN to specify the best hydraulic and actuation system for each design of wind turbine. Following on from this though, automated physical testing is a necessity. The conditions within the wind turbines are very specific to the application. This means that AVN needs to build test rigs to their own designs that can as closely as possible replicate the situation within the nacelle and hub.

“We know that the hydraulic system can only ever be as strong as its weakest link, and early on we realised that the reliability of the sealing configuration was highly dependent upon the quality of its counterparts,” says Johnny Fruekilde from AVN’s Research and Development department. “So one area we have focused on is the interaction between the surface finish of the rods and shafts of the actuators and the sealing components. A special rig was constructed specifically to test this and operates 24/7.”

The seals within the hydraulics are integral to its performance, and optimising their life is critical to the long-term effectiveness of the total system. Several other specially built rigs are used to measure sealing characteristics, as the dynamic demands of the application are extreme.

“The requirements for sealing of the actuator for wind turbine applications were unique,” says Per Hvidberg, sales engineer from Trelleborg Sealing Solutions, Denmark. “Never before had I been faced with a demand for a sealing configuration on a cylinder that produced relatively rapid short strokes continuously. And not only was there linear pressure from the rear, there could be side load too.”

“Within the actuators is a complex arrangement of seals ranging from O-rings to specialist Turcon PTFE-based geometries and Slydring in Orkot,” says Per. “The unique configuration is specially engineered to enhance lubrication, optimise friction characteristics and maximise service life, while preventing any external leakage. Some of the seals are expected to achieve the 20-year target, but it is impossible to guarantee this.”

“As this was the case,” says Johnny, “the hydraulics were designed for easy exchange of the seal set. This is mounted in a module that can be quickly bolted on and off. The minimum life expectancy of the sealing configuration, allowing for the seal that has the shortest predicted life, is seven years, but replacement is recommended after five. Other than this, and routine rod replacement, the actuators should run without maintenance except for the systematic checking, that the operators do, for any leakage or loss of pressure. We feel that this arrangement gives the ideal compromise between minimum required maintenance and guaranteed long-term performance.”

“Cleanliness of subcomponents is another important factor,” comments Poul. “Before assembly the system is flushed through to ensure there is no metal from machining or other debris such as dust or sand within the cylinder. Any residual matter such as this has been found to cause wear on the seals, shortening seal life and consequently total system life.

“It’s not quite like the cleanrooms used in semiconductor or chemical processing, but it’s advanced in our type of manufacture. The cleanroom will be completely enclosed with barriers between it and the outside world and an extraction system to eliminate media that could potentially enter the actuator’s hydraulic system before it is enclosed.“

Challenging requirements

In Denmark, the wind power actuator and its sealing system must be capable of operating at 250 bar with constant pressure on the rod from behind and differential side loads that control positioning. Seals must give minimal wear and facilitate dynamic movement that is continuous in short strokes, on average 900 times per hour.

Temperature resistance is needed down to -30 °C as standard and to -40 °C in the Arctic. Below these temperatures the oil within the cylinder cannot function and requires warming with heating elements. Maximum temperature is 60 °C. Beyond this the system is cooled, otherwise the oil becomes stressed, its viscosity is too low and it carbonises.

In addition, the actuators must withstand high humidity, salt spray and the rigours of wind and rain. Corrosion is prevented with advanced coating technology.

It’s hard to imagine when you look at a wind turbine that the nacelle, or the structure that houses the turbine’s generating components for the blades, is large enough for a man to stand up in. It has to be, because for maintenance the engineer has to enter this either through the side, but more commonly by climbing to the top of the tower, and down into the nacelle from there. That’s not easy on land and even more daring when the turbines are up to 100 km out at sea.

www.tss.trelleborg.com