With the direction of the wind energy market moving toward bigger wind plants and bigger turbines, the role and sophistication of motion control is growing. With offshore installations expected to grow to five megawatts in the next few years, wind turbine OEMs are moving to individual blade control, advanced sensing and control techniques plus electromechanical systems to assure reliability, durability and precision.
"Five years ago, the average size of a turbine was somewhere between 750 kilowatts and one megawatt. In 2008, the average size was approaching 1.6 to 1.8 megawatts," says Dheeraj Choudhary, business unit manager for Global Renewable Energy at Parker Hannifin Corp. "Over the next five years there will be a lot of turbines installed in the 2, 2˝, 3 and 5 megawatt range."
Choudhary says that with wind turbines of this size, the diameter or span of the blades is more than 100m. Each blade is close to 50m and as long as 60 to 65m for the high end of the turbine size. No matter how lightweight the blades become, they need to have the structural integrity to last 20 years or more. And because each blade weighs anywhere from 10 to 20 tons, the engineering goal is effectively moving and controlling these large, awkward "wings" in high winds.
"These blades almost never rotate in a smooth circular fashion. While spinning, they move around in response to wind gusts, side loading and torsional loading," Choudhary says. "So the challenges that we see in pitch control systems are not just reliability, durability and precision, but also controlling these blades so that the overall life of the turbine is preserved."
Dan Foster, engineering manager for Moog Industrial Group, says that "as the systems continue to grow in diameter, the loads on the blades, rotors and hubs have more variation. Increased loads on the blades, the rotor shaft and whole turbine translate into component wear, premature breakdown, imbalances of loading and torque on the towers and even the tower structure itself."
Foster says that this problem statement is what the industry needs to address. One of the biggest trends moving forward is the potential benefits of individual blade control and how it might help this situation. Today, most turbines are using independent pitch control with each blade independently controlled by a servo actuator device. However, all of the blades respond to the same command as they go through their cycle.
"What individual pitch blade control provides is real-time feedback from blades or monitoring devices. An approach we are exploring is to embed sensors into the blades for real-time load feedback," says Foster. "The system closes the loop at the turbine level using that feedback to really significantly reduce the load variation from blade to blade."
The result is an ability to handle peak gusts better and more quickly. And also, as blades rotate 360 degrees, the system is able to provide dynamic control based on system parameters at any angle. At the blade's highest point, it has a significantly different load versus the wind flow as it's sweeping past the tower or toward the ground. It clearly creates a gradient, but use of dynamic control both increases the efficiency of the turbine and provides more useful wind-generating power.
Wind turbine OEMs are split between electric or hydraulic solutions, and sometimes the same OEM will offer both technologies. With the move to the larger units, the turbines are either hydraulically or electromechanically controlled but, with areas such as China where 90 percent of the wind plants are electromechanical, there is a general trend toward electromechanical solutions.
Bosch Rexroth recently added a new electromechanical pitch axis to its product line and is starting to put the first systems in the field. The new product is a complete pitch-axis control system featuring ac technology, redundant controllers and new power back-up technology using high-capacity capacitors.
"We are not only providing the motor and gear box, but there is a need to mechanically actuate the blade and we are also providing the complete system," says Till Deubel, manager for Market Segment Energy for Bosch Rexroth.
The system offers three controllers, a dedicated controller for each blade, which are networked together so they can redundantly replace each other if needed. If one of the controllers has a failure, the other two controllers can take over so that the operation of the wind turbine can continue.
The system also features a new uninterruptible power supply with high-capacity capacitors. Until now, most systems have used batteries but the capacitors need less maintenance and are more durable over time. The system also connects with sensor technology to sense vibrations and load spikes. A condition monitoring system checks the electromechanical pitch axis at all times.
Parker Hannifin classifies pitch control technology into three different segments. Electromechanical rotary systems use a geared ring with a gear box, an electric motor, drives and battery packs. Linear electromechanical systems use a push-pull type approach using an electromechanical cylinder and a rod while employing a similar control strategy as a rotary system. A third class of systems uses hydraulic cylinders and a power pack that produces power for typically double-acting cylinders or a set of push-pull cylinders, to move the blades.
"As the wind turbines get larger, the rotary systems don't offer an optimum torque-to-weight ratio and are limited over time, with respect to precise positioning of the blades," says Choudhary. These rotary drives use an electric motor with a brake attached to a 100 to 1 gear reducer that connects to the slew ring gear using a pinion arrangement. The torque required to move these exceptionally large blades goes up disproportionally, along with the size and weight of the complete system.
Over time, due to the vibration in the blades and constant grinding of the gear arrangement in the same range of motion, the precision of a rotary pitch system declines and causes more vibration in the blades. "So, essentially, it is a death spiral of progressively worsening pitch performance," says Choudhary. To avoid such issues in large and expensive turbines, Parker has elected to pursue linear hydraulics and linear electromechanical technology in its portfolio.
"Using a linear approach, we eliminate the slew ring gear, position sensors and limit switches, pinions and the greasing of gears that is required," he says. This push-pull system has much better torque-to-weight ratio and is an intrinsically safe system with a self-locking mechanism. Both the electromechanical and hydraulic linear systems minimize involuntary rotary motion in the blades, minimizing the shocks from migrating to the drivetrain and preventing further degradation of performance.
"What we are providing our OEMs is a technology platform that can be used with either a hydraulic or an electromechanical design approach for blade pitching, with the benefits of lower weight, higher precision, better durability and safety while influencing the life of the turbine in a positive way," says Choudhary.
Bosch Rexroth's Deubel says hydraulics can offer an advantage in implementing newer ideas in pitch control. Today, usually all of the blades have the same pitch angle and the pitch angle is adjusted over time. But new ideas for individual pitch control would change the pitch angle depending on the position of the individual blade to compensate for the fact that there is more wind in the high position of the blade than in the downward position of the blade. There is a gradient where it is lower at the bottom, and ideas are under investigation about changing the pitch angle as the blade turns to compensate for that.
Another possibility is changing the pitch angle at the moment the blade passes the tower to eliminate the wind shadowing effect which can generate both a noise and a small jolt to the whole wind turbine. Deubel says that using individual pitch control is a factor which should favor hydraulics because of the high dynamics and very quick response times required.
Dan Foster of Moog says the key to individual blade control is both the development of algorithms and the robustness of the complete system. Because the complete system including the controller, servo drives, servo motors and sensors is spinning, reliability must be extremely high.
Moog is also developing sensor technology for individual blade control using optic fibers that are embedded into the fiberglass blade when it's laid up. These high-speed strain gauges use an optic fiber technique that is placed in specific locations on the blades to monitor strain. And since the system is using this feedback to close a loop, they have to be extremely reliable.
If you think of a blade sweeping through its rotation, at every single angle, the strain is different because of the angle, rotational position and the weight of the blade itself. It can be different with varying wind characteristics and, with the three blades working together, there is a need to unload the central piece because it reacts to all three independent blades.
Foster says the engineering challenge is how to optimize system performance while minimizing the total load on the system. Availability is divided into two components, one being the robustness of the system which is achieved by using reliable components and redundancy. The second part is that ideally the system should generate energy over the widest possible range of wind conditions, so the turbine can be available for the greatest amount of time. That's another area where, with independent blade control, the turbine may be able to operate with higher and lower wind speeds because of its ability to optimize the set-up for a particular range of wind conditions.
Increasing the efficiency of the turbine is mostly related to the speed of the blade, and the rotational speed of the turbine which is a generator. The key is what output load is it seeing and balancing the output load to get best operation. By controlling speed based on the load the generator is seeing, the system is working toward an optimum set of conditions to create efficiency in the generator.
For most systems today, the blades are at a specific angle for certain types of conditions and output. If the wind gets too high, the system has to shut them down. And if the wind gets too low, they shut down because they can't spin the turbine to generate power. And so the goal with individual blade control is to optimize and push that window further out at both ends. Availability goes up because you are able to operate over a wider range of wind conditions.
"With the move to larger turbines, it's a continuing progression and blade sensing technology is an advantage," says Foster. "Blade monitoring packages, ice control and blade monitoring for vibration and damage identification all focus on availability. If the system has vibrations or imbalances that need to be proactively addressed, it's possible to discover the root cause and prevent damage that can create significant downtime."
Individual blade control is an area where the industry is headed. For core products such as motors, drives and electromechanical actuators, it means larger frame size devices. It's driving power stages to provide larger current ratings and higher output.
Individual pitch control, in order to have lower overall loads on the system including the loads on the blades, hub, chassis, bearings and shafts, must also provide a much more active control than what is used today with independent blade control. The control is more active and the duty cycle is higher, so the challenge is actuators that are smaller, more reliable and robust.
"The problem with bigger gearboxes is that it makes everything bigger and heavier, but today that is already a problem for the maintenance crew because the hubs are small and it's tough to move things up and down them," says Foster. "The focus on brushless technology and what we can do with our gear systems to make them handle the more aggressive duty cycle are the areas we are focusing on."