Designing for Wind Turbine Reliability

DN Staff

May 5, 2011

10 Min Read
Designing for Wind Turbine Reliability

Motion controltechnology is addressing two of the key issues that the wind power industryfaces as it matures: moving offshore and the use of bigger turbines. But theclear focus is on system reliability, which has become the number one designchallenge that wind turbines must address.

In addition to a technology focus on gearboxes and generators,suppliers are looking at the entire drive train and how to use technologiessuch as gearbox health management tools, condition monitoring, ac brushlessservo motors, and precision cooling systems to provide robustness andserviceability that the next generation of this alternative energy demands.

"Instead of focusing solely on the gearbox as in the past, OEMsare focusing on the complete electromechanical drive train," says DheerajChoudhary, business unit manager for Global Renewable Energy at Parker Hannifin. "If we look at the windturbine drive train from where the rotor shaft connects to the gearbox and onthe other side where electrical energy comes out of the turbine, there aretremendous opportunities for boosting system reliability and serviceability."

Choudhary says that turbine design teams are now taking apragmatic approach on the need for reliability and serviceability in the drivetrain of offshore wind turbines. The focus is on larger wind turbines (4, 5, 7and up to 10 MW) developed for offshore operation, and on ways to keep thedrive train stable by deploying optimum conditioning and cooling for thevarious key components such as bearing, gearbox, generator and converter.

"It is a well-known fact that ifthe bearings and gearbox are well-lubricated, cooled with clean oil, have nomoisture or humidity present, and employ a good filtration package, the gearboxis less likely to fail," says Choudhary. "If we can monitor the gearbox using acondition monitoring package that includes trends from temperature, pressures,vibration and cleanliness of the oil and moisture, we can reliably predictfailure or prevent failures by knowing what will happen in the gearbox over atime horizon."

Remote Monitoring

With the trend movingtoward larger turbines and offshore applications because of the availabilityand consistency of the wind, an important focus is on the serviceability andservice intervals of these offshore installations. This is driving suppliers tobetter control of the temperatures in the gearbox and to the optimization aswell as monitoring of the cleanliness of the oil in the system. To get servicepeople on board in an offshore turbine, it normally means you need a ship orhelicopter. As a result, suppliers are improving the lubrication systems andincreasing the lifetime of the filters, so it's a maximum of once a year toservice the unit; and sometimes the service interval is increased to 18 months.

Users can employ an online connection to assess when they need toschedule maintenance on the turbines. Some of these systems, includingParker's, constantly measure the pressure differential over the filter elementand communicate the information over industry-standard electronic protocolsthat can be accessed remotely. With ability to program these devices, reachinga threshold of pressure level indicates it is time to change the element.Remote monitoring also allows for preventative and scheduled maintenance eventsthat reduce the cost of service.

Parker also offers systems tomonitor the condition of the gearbox lube oil with respect to the amount andtype of dirt in the bearings or gearbox. If everything is running smoothly andthere is no extensive wear on the bearings, it is normally OK to use an18-month service interval, but performance is also dependent on the gearboxitself, vibration, alignment, loads and other factors.

"We have also introducedmodularity into the system, so instead of using one large filter we arespecifying two or three optimum size and mesh filters on the gearboxlubricating system to better perform service and further condition the oilwhile keeping pressures consistently low in the system," says Olli Rantanen,marketing manager at Parker's Filtration Group. "The lifetime of the componentsis vital along with serviceability. We know that, in many gearboxes, people areusing a lot of connectors and hoses to connect different components which meansthere is always a possibility for leakage, not to mention the drop inefficiency of the package."

Parker's design approach tothese applications is to integrate multiple functions into a patenteduni-package which combines the process and the control parts of the totalsystem, plus optional functions including bypass filtration if that is needed.

A third important area iscondition monitoring. Normally if there is any oil condition monitoring at allin these systems, it is metal residue monitoring - which means that only thelarge metal particles are monitored, not for size but for count.

"We know that if a system isstarting to degenerate, it produces a lot of smaller particles before the bigparticles start to appear," says Rantanen. "Instead of measuring particles thatare 100 microns or bigger we can start trending micrometer-sized particleswhich the human eye can't see. ?You almostneed dedicated laboratory equipment to view particles of this size."

Parker has introduced new tools for this type of monitoring anddeveloped a system to do this online with the gearbox without taking samples toa lab. The key is that measurements can be trended on a continuous basis andspecific commands can be used to prevent further damage or optimize the use ofthe filtration system.

Trend to AC Brushless Servo Motors

"The key in wind turbineapplications is reliability and availability, which means the wind turbine must be available toproduce energy whenever there is wind," says Mauro Gnecco, North American windmarket manager for MoogInc. "The challenge for system suppliers is to make sure that componentsare reliable and robust enough to support that environment which meansbalancing features, performance and cost."

Electric pitch control systems are typically located in therotating part of the turbine (hub) and include one rotary actuator (electricmotor and gearbox) connected to each blade. Power and control electronics allowthe motors to develop the right torque and follow a motion profile provided bythe turbine controller. A power back-up system allows the motors to bring theblades into a safe position, even in case of power loss, and a set of encoders(one per blade) supplies blade position feedback.

With wind turbine applications, whether they are offshore or not,the environment is harsh because system components are essentially outdoorswithin a hub that is not weatherproof. Ambient operating temperatures rangefrom extremes of -40 to 50C. This means that the actuators and motors needspecial attention paid to sealing, and the ability to meet performancerequirements throughout the full temperature range.

Moog offers new ac brushless servo motors rated for IP65. Thebody of the motor is a single piece with two end caps to create fewerconnections and places for water to permeate the motor. ?The grease and lubrication systems have to be selectedto deal with the wide fluctuation in temperature ranges and, in very coldtemperatures, the motors are designed to self-heat via the controller onstart-up.

"The rigorous environmental conditions apply to the controls aswell, plus all of the components in the control cabinet. Up in the moving rotoras well, the system must be designed to withstand the vibration and shock of itbeing continuously rotated," says Rob Nicholl, engineering manager at Moog Inc."Reliability is key because the motion system functions as the key safetysystem of the wind turbine."

Moog is focusing on supplying highly reliable integrated electricpitch systems to the wind industry. Even though they offer pitch systems usingdifferent types of motor technologies (dc brush type motors, ac inductionmotors and ac brushless servo motors), the focus is on systems
using ac brushless servo motors.

"Traditionally, dc motors have been applied to pitch systems butthere is a current trend to applying more ac synchronous motors to eliminatethe brush failure mode," says Nicholl. "Throughout the rest of industry andaerospace applications, there is a general belief that ac motors are inherentlymore reliable, even though they haven't beenwidely accepted in the wind industry yet."

Differential Gearboxes

"The trend in the windindustry we have seen is a continuing move toward larger-size turbines," saysParveen Gupta, director of Wind Energy - U.S. sales for Bosch Rexroth. "Until lastyear, the most common size installed in the U.S. was 1.5 megawatts. But thetrend is larger and larger systems, and turbines in the 2 to 2.5 megawatt rangewill be more or less a standard for onshore wind turbines over the next severalyears."

Gupta says a clear trend is more systems going offshore oractually "near shore" where they are typically installed between three to fivemiles off the coast. The advantage is a smooth flow of wind, the turbine is notin anybody's backyard, and the noise issue is gone. For all those reasons, windturbines are moving to offshore locations but a major problem is regularperiodic maintenance which has the potential to become prohibitively expensive.

"Because of quality and reliability issues, people are talkingabout using a direct drive system rather than a gearbox for large offshoreapplications," says Gupta. "We see some benefits in that but; by the same tokenwith developments to increase reliability and improved quality, we believe youcan have a very good solution with a gearbox in the system even up to 5 or 6megawatts."

With the demand for higherpower density and compactnessgrowing, Rexroth's differential gearbox, Redulus GPV-D, offers more compactdrives and minimizes the weight by providing a multiple power-split gearboxdesign solution.

Gupta says that unlikeconventional generator gearboxes in the multi-megawatt class, four or moreplanetary gears do not revolve around the sun wheel in the input stage.Instead, the Redulus GPV-D offers two input stages each fitted with threeplanetary gears, and the advantages of a static determination paired withfreely adjustable sun pinions.

Another key design feature isa thin outer diameter with only a slightly greater total length. With turbinecapacities increasing, it offers up to a 15 percent weight advantage over gearbox concepts currently in use without a decrease in reliability.

"There have been tremendous improvements in terms of planning anddoing preventative maintenance rather than repairs after a catastrophicfailure," says Gupta. "Along with the trend toward larger turbines typicallylocated offshore, there is a rise in condition monitoring solutions that enableusers to predict and plan for maintenance."

Lubrication is the key to the reliability and operating life ofgearboxes, and this applies in particular to gearboxes for wind energy plantsdue to the difficult conditions in which they operate. This is why Rexroth isoffering main gearboxes featuring their own cooling lubrication system withparticle monitoring and water content sensors. Another development from Rexrothis a blade control monitoring system that detects damage and ice build-up byanalyzing the natural frequencies of the elastic bodies.

Gupta says that while engineers are generallyconcerned about the reliability of the gearboxes, often what actually limitsbearing life is lubricating fluid or oil which becomes too dirty or too hot.The push to condition monitoring is one way to manage these important assets.

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