Image credit: HAWE Hydraulics

The muscle that pitches wind-turbine blades can come from either a hydraulic or electric device on most turbines rated at and below 2.5 MW. But for turbines over 3 MW, the job of pitching blades more often falls to hydraulics. And hydraulics can handle more.

“Hydraulics in wind turbines usually refers to the assemblies for brake control and regulating the blade setting through pitch control,” said Bob Pettit, Corporate Technical Director at HAWE Hydraulics.

In a nutshell, the process of turning wind into electricity involves a drivetrain turned by a rotor of two or three blades and related equipment. While small turbines often have fixed rotor blades – they do not pitch – larger turbines require blades that pitch, and so are mounted on bearings.

To drive each blade to its best pitch position requires a hydraulic pump, motor, reservoir and associated equipment. For example, the pump and motor are usually mounted in the nacelle while hydraulic pistons are mounted in the hub. A hydraulic rotary joint allows passing hydraulic fluid from the stationary side to the rotating side. Pitch control then varies the pitch of the blades to maintain nearly-constant rotational speed at the generator.

Aside from wind velocity changes, the pitch of the blades may vary even during a 360-degree rotation of a single blade. Such control is necessary because wind velocity at the 12 o’clock position may be significantly different from its velocity at the 6 o’clock position, according to Afzal Ali, Director of Marketing at Deublin. Typically, the pitch of each blade varies continuously and independently.

In an emergency, hydraulic pitch control can operate without an external power supply thanks to an accumulator, a sort of hydraulic battery. Hydraulic actuation provides the shortest stopping time, a wider range of operating temperatures than alternative systems and it’s backlash free.

The rotor brake, also hydraulically activated, engages during emergency stops as well as service work when the turbine is manually shut down. Another hydraulic system, one that activates yaw brakes, is comprised of several hydraulically-activated brake calipers which act on a break disk at the top of the tower. During normal turbine operation, brake calipers are under maximum pressure to keep the nacelle facing the wind direction.

Hydraulic equipment usually includes an accumulator as a way to store energy in case of an emergency shutdown. The system is usually designed to fill the accumulator during off-demand periods, when pump flow is not allocated to system actuators. The pressurized fluid stored in the accumulator can then pitch the turbine blades to a safe position where they can stay until power is restored or the halting condition is corrected.

Researchers are considering the advantages of replacing gearboxes in some turbines with hydraulic transmissions.

“For instance, there is some compressibility in the fluid that prevents shock loading to the generator,” said University of Minnesota and Mechanical Engineering Professor Kim Stelson. “Hydraulics equipment is lighter than the conventional equipment it replaces. Also, control is somewhat simplified. Controls in a conventional turbine are based on torque, a quantity that can be difficult to measure, although it can be approximated with current. But in a hydraulic transmission, it is measured with pressure from the pump and that is more convenient.”

Other researchers are also devising ways to replace gearboxes in large turbines. While working on the development of a variable-displacement hydraulic machine in the 1980s, engineers at Artemis Intelligent Power found a way to improve hydraulic efficiency. The company replaced the mechanical valves and swash-plates of conventional variable-displacement hydraulic machines with computer-controlled high-speed solenoid-valves. This opens new markets for hydraulics because Digital Displacement machines are efficient at all load levels and have super-fast response to computer control. This development has led one Japanese turbine OEM to build a multi-MW unit with a hydraulic drive that is now working off the coast of Japan near Fukushima.

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Several trends here are toward more reliable designs in new turbines and in replacement units. In the aftermarket, custom designs meet client specs, slip rings are available for non-supported machines, and combination devices for transmitting power, signals, and fluids. “New solutions are needed to replace older technology slip rings in turbines with long-life, high-reliability designs that can operate for years without maintenance,” says Moog’s Senior Business Development Manager Steve Black.

For pitch functions, the ideal slip rings would be installed and forgotten. Many designs in use today require periodic maintenance to clean away wear debris, flush out oil and dirt, and relubricate. In pitch slip rings, design trends are on at least two fronts. “First, as turbine designs increase in power output and size, the demand on the pitch slip ring is for higher power transfer,” says Black. “Second, data handling and signal demands are increasing to provide additional condition monitoring in the hub. It is critical that the contact technologies be durable enough to handle power peaks at the extremes of operation.” More complete pitch control is another trend. These would include hub controls, pitch-drive motors, power back-up, blade monitoring, and slip-ring channels to transfer the power and control signals through the rotary interface.

A quest for higher reliability has encouraged some companies to devise non-contacting slip rings. At least two companies are at work here developing inductively coupled power transfer devices. One manufacturer says its slip rings work at high efficiencies by using wireless power transfer that can harmonize itself in the field. The company says this makes the systems resistant to changes in environmental conditions and load variations. Power transmits as a single load while a built-in, data-communications system switches power, for example, to meet different device requirements of wireless receivers.

Collaboration is another trend. One ISO is partnering with a slip-ring manufacturer to design, manufacture, and distribute slip rings. Manufacturer UEA says it provides its slip rings as completed, ready-to-mount assemblies with optional pre-wired harnesses. The goal is to reduce up-tower time so the company makes a wide selection of circuitry available, with many combinations of amperage and voltage, ac or dc. The compact design is made possible by stacking brushes on alternating sides. The slip rings come in bore sizes from 0.500 to 14.00 in. UEA President Mark Hanawalt says his company’s custom-designed wind turbine slip rings match customers specs and can be delivered in a matter of weeks.

UEA recently designed an improved slip ring for Clipper wind turbines. It’s more reliable, says Hanawalt, with improved sealing, removing the exposed external wiring, and enhancing the communication circuits. For instance, a new ball bearing design significantly improves bearing life over the original design, which required more delicate handling.

Trending offshore — Manufacturer Deublin says its off-shore wind turbine union includes a slip ring and encoder. Units for off-shore and on can deliver up to 250A, transmit Ethernet and Profibus signals, resist shock and vibration, and provide a protection class IP 54-IP66. Add a rotary union and device accommodates hydraulics, electrical power, EtherCAT, and rotary position information. “Offshore turbines will require more reliable components due to their remote locations. Cost and time of reaching the offshore turbine is significant compared to most of the onshore turbines” says Jerry Lichter, Senior Product Engineer at Deublin.

Lastly, custom designs and combination units — Building blocks allows solving design challenges such as RF shielding, mixed signal handling, high-frequency-impedance matching, reduced temperature generation, and miniaturization, says Deublin. The company adds that its plug-and-play slip ring and union combination reduces installation and maintenance time because electrical and hydraulic connections are plugged or unplugged as part of the installation or removal process rather than having to disconnect those cable connections and hose fittings separately. Other options include absolute or incremental encoders, optical rotary joints for fiber optical connection, even an internal heater for use in cold weather.

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The multi-passage soft-seal rotating unions add to Deublin’s broad range of product offerings.

A new product line of multi-passage soft-seal rotating unions is now available from Deublin Company. Rotating unions are components in a broad range of equipment applications that require connection between stationary supply lines, and rotating equipment including cylinders, rolls, spindles and clutches.

While rotating unions are used to convey virtually all types of liquid or gas (steam, water, coolant, hydraulic or cutting fluids, inert gas or vacuum), multi-passage rotating unions are used when more than one media must be conveyed simultaneously using different passages. Soft-seal refers to the Plastomeric seal technology, which is a combination of plastic and elastomer, with a proprietary chemistry and geometry that differs by application.

Applications for Deublin multi-passage soft-seal unions include machine tools, ladle turrets used in steel continuous casting, and metal coil winding, plastic and rubber manufacturing, and more.

Deublin offers standard, semi-custom, and custom options depending on the requirements of each application. Standard features include 2-8 passages, while custom passages for the largest applications can be 22 or more. Port types include NPT, BSP, or SAE.

A new brochure is available in print (request MPSS142 US), or via the Deublin website: <http://www.deublin.com/product-support/request-a-catalog/> .

DEUBLIN Company
www.DEUBLIN.com

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