To most casual observers, the forecast of sustained growth across the worldwide wind energy sector in the years ahead would seem quite sunny. Down on the wind farm, however, where operators are striving to generate a consistently competitive power source, storm clouds related to reliability issues can drastically darken the horizon.
There’s no getting around it. Over its typical 20-year service life, a wind turbine may be exposed to some of the most extreme operating conditions on the planet. Equipment problems WILL arise. Costs from unplanned shutdowns and maintenance fixes can be staggering, not to mention compounded by accessibility issues. That’s because a turbine’s nacelle—a veritable “command central” that contains a gearbox, low- and high-speed shafts, generator, controller and brake—can be perched hundreds of feet off the ground and/or situated miles out at sea. When equipment fails, wind farms must deal with exorbitant crane mobilization expenses, lost energy production, soaring costs per kilowatt-hour and untimely delays in obtaining replacement parts in a burgeoning industry where the demand for necessary components routinely outstrips supply.
While wind farms cannot avoid the uncertainty of the changing wind and weather, operators can act to reduce uncertainties regarding the reliability of equipment. Proactive maintenance activities hold the key to unlocking optimized capacity and long-term profitability.
Among technologies successfully cultivated from applications in other industries, condition monitoring systems enable early detection and diagnosis of potential component failures Maintenance. In addition, automatic lubrication systems deliver accurate and timely lubrication with minimized maintenance support to keep all points properly lubricated and components performing as anticipated.
Monitoring for timely maintenance
Condition monitoring is a strategy whereby physical parameters (such as vibration, temperature, lubrication particles and others) are measured regularly to determine equipment condition. This procedure makes it possible to detect machine and component problems before they can result in unexpected downtime and the high costs associated with maintenance and interrupted production.
An integrated on-line condition monitoring system within a typically difficult-to-reach wind turbine nacelle (like the one shown in Fig. 1 on the next page) offers a powerful tool for managing day-to-day maintenance routines and consolidating risky, costly maintenance activities. These systems pay off for wind farms by allowing operators to monitor and track deteriorating component conditions in real-time— which leads to maintenance decisions based on actual machine conditions instead of arbitrary maintenance schedules.
A condition monitoring system developed and dedicated for wind turbines allows for round-the-clock monitoring of key turbine components. (A capability for remote monitoring via the Internet or GPRS provides a solution for offshore turbine operations.) By tracking component performance, maintenance activities can be coordinated across the wind farm; service calls can be better planned and combined; and operators can take advantage of planned shutdowns to service several turbines at the same time, since machinery conditions are known from the monitoring. All contribute economies and efficiencies for the wind farm operation.
The monitoring process for a wind turbine can effectively reduce lifecycle costs and extend service life. Implementing necessary repairs when problems begin to surface, for example, proves easier and much less expensive than running a turbine to catastrophic failure. Conversely, as demonstrated at the U.K. wind farm (see Sidebar on previous page), data can prompt repairs for the most opportune time—without risking additional damage or failure.
Today’s monitoring systems can handle any number of turbines and multiple data points. Using vibration sensors mounted on a turbine’s main shaft bearings, gearbox and generator, systems (in tandem with software) will continuously monitor and track a wide range of operating conditions for analysis. Wireless capabilities allow operators to review data from any location with a computer or hand-held device with Internet access (which can shorten lead-time from alarm to solution). The collected data also can be figured into root cause failure analysis, which can then be applied to eliminate recurring failures.
Among the operating conditions that can be targeted for early detection, diagnosis and remedial action:
- Unbalanced turbine blades
- Shaft deflections
- Mechanical looseness
- Foundation weakness
- Bearing condition
- Gear damage
- Generator rotor/stator problems
- Resonance problems
- Tower vibrations
- Blade vibrations
- Electrical problems
- Inadequate lubrication
Monitoring systems can play vital roles in highly reliable maintenance forecasting, which is an essential requirement for improving turbine reliability and availability. This is made possible by continuously recalculating fault frequencies and delivering accurate values based on reliable trends, which facilitates alarms at various speeds and loads, including very low main shaft speeds. (The trend data also enables trend-based root cause failure analysis.)
Ultimately, a tailored condition monitoring system can assist wind farm operators in performing appropriate maintenance at the right time and set the stage for Condition-Based Maintenance activities, whereby maintenance, inspection and overhaul of plant machinery are scheduled largely on the basis of machine condition. In this approach, rollout of maintenance relies upon condition data instead of the calendar.
As a result, wind farm operators can extend maintenance intervals, consolidate maintenance initiatives, cut operating costs and costs per kWh, reduce the risk of unplanned shutdowns, prevent lost energy production due to breakdowns, and predict remaining service life by turbine.
Turning to automatic lubrication
Just as condition monitoring technologies can optimize resources for timely and appropriate maintenance deployment, centralized automatic grease lubrication systems can contribute their own reliability benefits. Systems engineered for bearings, pitch and yaw gears and other locations in a wind turbine can efficiently deliver exact and clean quantities of appropriate lubricant at the right positions at the right time. The maintenance benefits: Timely and effective lubrication helps to reduce wear, minimize lubricant consumption, maximize efficiency and curb unscheduled downtime.
Automatic delivery of lubrication also lifts a heavy burden from the shoulders of the maintenance staff. According to industry averages, 10-20% of the uptower time involved in servicing a turbine is spent on relubrication (technicians crawling around in the cramped nacelle and hub to grease lubrication points numbering from 10 to more than 80 with several different greases in each turbine). And, in the case of conventional manual lubrication methods, over- or under-greasing (leading to potential failure) always is an unwanted possibility. Lubrication intervals may be sporadic or ill-timed, contaminants can inadvertently be introduced and equipment performance may be compromised.
With centralized lubrication, every point receives the proper lubricant in an accurate amount with the objective to minimize wear and promote longer service life. The problems associated with excessive lubrication can vanish; lubricant consumption can fall over time; maintenance time, energy and costs can diminish; more informed and timely decisions can be made for lubricant purchases; and operational reliability can be improved. (The only requirements: Refill the lubrication reservoir and occasionally inspect the connected lubrication points.)
Advanced systems additionally offer the capability to provide central monitoring of all feeder outlets, if desired, at relatively low cost, and can incorporate lubricant collectors attachable to open geared wheels and lubricated pinions for pitch and azimuth drive wheel.
Centralized lubrication systems can be applied to all bearings at a turbine’s rotor shaft, blade pitch and azimuth positions, as well as non-rotating applications inside the turbine.
Decision-making for the most appropriate system will depend, in general, on the application and, in particular, on a range of other parameters, such as the operating conditions (variations in the operating temperature and lubricant viscosity); accuracy requirements for lubricant quantities; turbine system geometry (size, dimensions and symmetry); and monitoring demands, among others.
When planning, installing and—subsequently—implementing a centralized lubrication system inside a wind turbine, remember to:
- Determine the number of lube points.
- Choose the proper lubricant for the temperature, speed and load conditions.
- Calculate appropriate dispense rates and quantities for the application.
- Choose pumps consistent with the type of actuation and system capacity.
- Consider monitoring systems that can be integrated with the lubrication system.
The previously mentioned condition monitoring and lubrication technologies represent “umbrella” approaches for achieving consistent reliability and uptime in wind turbines. But they’re not the only moneymaker strategies available. Other things can help wind farm operators generate profits, too.
As a few examples, customized bearing housings for main shaft applications can be modified to fit the frame and shaft dimensions and incorporate high-quality labyrinth or lip seals to reduce the subsequent need for maintenance. Hydraulic couplings can be specified to accommodate the limited space of a nacelle and enable easy mounting and dismounting in a fraction of the time (and labor) required for mechanical couplings. Insulated or hybrid ceramic bearings can keep maintenance at bay by protecting generator bearings against the passage of damaging electric currents.
All such solutions suggest that partnering with a services provider experienced in the many interrelated aspects of wind turbine technology can provide operators with the most current engineering resources to help keep the blades turning productively. MT
Kevin George is manager of Wind Energy Business Development for SKF USA Inc., based in Marietta, GA. An active member of the American Wind Energy Association www.awea.org, he holds a BSME from the Georgia Institute of Technology. Telephone: (770) 591-8747; e-mail: Kevin.S.George@skf.com
Fortunately for today’s wind farm operations, condition monitoring information even can be used to control or postpone repairs. This was the case at a U.K.-based wind farm where one of SKF’s WindCon condition monitoring units was deployed. The unit was installed on a wind turbine that had already experienced damage to the low-speed part of the gearbox (and the gearbox replacement already was planned). The system not only registered the damage, but also determined that the damage was stable enough to postpone the gearbox replacement and keep the damaged turbine in operation.
After monitoring the damaged part for almost 12 months, the system eventually detected a rapid increase in the damage pattern, and only then was the turbine taken offline for gearbox replacement.
By postponing the gearbox replacement for a year, the wind farm was able to accrue interest on the capital needed for the overhaul and efficiently plan for parts delivery, shipping, personnel and cranes for the job. The alternative would have been a rushed operation accompanied by unnecessary costs, several weeks of downtime and lost productivity.