TURNING TO CONDITION MONITORING FOR WIND TURBINES

Wind turbine technology has traveled a long way in a relatively short span of time, with today’s machines generating increasingly higher levels of efficiency and the industry growing at every turn. While the projected 20-year life cycle for a wind turbine suggests long-term endurance, these complex systems integrating thousands of components can experience many of the same operational and maintenance issues associated with any machinery. Equipment can fail, unexpected downtime can follow, costs can rise, and potential revenue from lost electricity sales can fall. Ultimately, operators have practical incentives to “prevent failure” instead of “running to failure.”

When components in a wind turbine fail, the ripple effect can be staggering. Operators must deal with crane mobilization expenses (as high as $100,000 per incident), lost energy production, excessive costs per kW-hr, and untimely delays in obtaining replacement parts in a burgeoning industry where the demand for necessary components routinely outstrips supply. The costs from unplanned shutdowns and maintenance fixes can further be compounded by accessibility issues, particularly when the nacelle of a wind turbine is 100 m off the ground or situated offshore miles out at sea. Worker safety, too, is always a cause for concern.

While wind farms cannot avoid the uncertainty of the changing wind and weather, operators can act to reduce uncertainties regarding the reliability of equipment and, in turn, influence and control both scheduled and unscheduled maintenance costs. Proactive Condition-Based Maintenance (CBM) initiatives can hold the key to unlocking optimized capacity and long-term profitability down on the wind farm. As a result, wind turbine OEMs, third-party repair contractors, and reliability equipment and service providers are increasing their focus on providing support to this emerging need for wind farm operators.

GETTING UP AND RUNNING
Most large, modern wind turbines are horizontal-axis types, whose primary components include blades or rotors (which convert the energy in the wind to rotational shaft energy), drive train (usually including a gearbox and generator), tower (support structure for the rotor and drive train), and other equipment, including controls, electrical cables, and ground support and interconnection equipment.

Depending on their intended application, wind turbines have been engineered in a variety of sizes and power ratings. The larger utility-scale machines can range in size from 750 kW to 3 MW, reach a height of 20 stories, integrate blades longer than a football field, and each can produce enough electricity to power 1,400+ homes. Specialized companies typically will handle the installation of commercial wind turbines and will possess the required expertise to transport the huge parts to the designated site and assemble the structures.

While the overall cost of wind power has decreased in the past ten years, the technology necessitates a higher initial investment than fossil-fueled generation power plants. The machinery in a wind park accounts for roughly 80 percent of the initial cost with the balance for site preparation and installation. Once a wind turbine is up-and-running, the costs associated with operation and maintenance (O&M) can vary widely and, in most cases, can be difficult to predict or pin down in advance. But operators recognize an overarching need to minimize time, money, and labor involved both in scheduled and unscheduled maintenance activities.

When a wind turbine fails, on-site technicians often will be the first responders. However, the resolution to remedying the failure will vary by wind park. Operators may rely upon OEMs, independent repair and maintenance contractors, their own in-house technicians, or a combination. Local distributors will be enlisted to supply components for out-of-warranty repair. (OEM warranties run today from two to ten years.)

While failure modes are contingent upon the different designs of machines and their duration in service, a 10-year study by the German Wind Energy Measurement Programme (Stuttgart, Germany) shed light on which components will likely fail more often. This study tracked the performance of 1,500 wind turbines in Germany from 1997-2006, accumulated 15,400 turbine years of operation, and developed a detailed picture of failure probabilities. The findings: Electrical equipment represented the most common cause of stoppages (approximately 5.5 incidents every 10 machine-years) and gearboxes were next, accounting for 1.5 incidents every 10 machine-years. As in any complex system, however, the failure of a critical component, such as a gearbox, can then cascade to cause damage to other components. Advance warning can make all the difference.

MONITORING FOR TIMELY MAINTENANCE
Among technologies successfully transferred from applications in other industries, Condition Monitoring Systems (CMS) enable early detection and diagnosis of potential component failures and serve as a platform for implementing Condition-Based Maintenance practices. CMS also can detect wind turbine problems from causes other than component failure, such as rotor imbalance due to icing and electrical faults.

Condition monitoring is a strategy whereby physical parameters (such as vibration, temperature, lubrication particles, and others) are measured regularly to determine equipment condition. This process makes it possible to detect machine and component problems before they can lead to unexpected downtime and unplanned costs from maintenance and lost production. An online Condition Monitoring System offers a powerful tool for managing day-to-day maintenance routines inside a wind turbine and consolidating risky, costly maintenance activities. By equipping operators to monitor and track deteriorating component conditions around-the-clock in real-time, maintenance decisions can be based on actual machine conditions instead of arbitrary maintenance schedules. Mounted sensors and enabling software do the work and pinpoint the problems. And, along the way, costs can be saved and unscheduled downtime can be minimized.

The principles of condition monitoring are not new, but this proactive approach has gained significant interest in the industry due to the increasingly sophisticated computational interpretation and analysis capabilities for measured data. CMS data can be applied to adjust scheduled maintenance intervals and strike an ideal balance between the cost of maintenance and the cost of unscheduled fault repairs. Monitoring systems additionally can enhance and optimize maintenance-forecasting planning by continuously recalculating fault frequencies and delivering accurate values based on reliable trends. This can facilitate the assigning of alarms at various speeds and loads, including very low main shaft speeds, and form the basis for trend-based root cause failure analysis.

Most monitoring systems today can accommodate a large population 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 expand system potential by offering the capability to review data remotely from any location with a computer or hand-held device with Internet access. This can shorten lead-time from alarm to solution. Among the operating conditions that can be targeted for early detection, diagnosis, and remedial action:
(1) Unbalanced turbine blades
(2) Misalignment
(3) Shaft deflections
(4) Mechanical looseness
(5) Foundation weakness
(6) Bearing condition
(7) Gear damage
(8) Generator rotor/stator problems
(9) Resonance problems
(10) Tower vibrations
(11) Blade vibrations
(12) Electrical problems
(13) Inadequate lubrication

For example, many wind farms worldwide have incorporated SKF® WindCon condition monitoring systems as part of their standard maintenance program. Mounted inside the turbine’s nacelle is an “Intelligent Monitoring Unit” featuring 16 different channels where multiple measurement points can be connected. The typical wind turbine configuration would incorporate the main bearing (one channel); gearbox (four channels); generator (two channels); and tachometer (one channel). In addition, other monitoring points may be added, including tower/structure vibration, blade vibration, oil temperature, oil pressure, oil quality, and generator temperature.

This system integrates built-in hardware auto-diagnostics, which continuously checks all sensors, cabling, and electronics for any faults, signal interruption, shorts, or power failures. Any malfunctions trigger an alarm.
Such systems can even allow operators to use the information to control or postpone repairs.

Case in point: A WindCon condition monitoring unit was installed on a wind turbine at a U.K.-based wind turbine that had already experienced damage to the low-speed part of the gearbox. 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.

By tracking and evaluating component health, 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, data can prompt repairs when most timely without risking additional damage or failure. Ultimately, a Condition Monitoring System can assist wind farm operators in performing appropriate inspection, maintenance activities, and fixes at the right time when and where needed, regardless 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 THE FUTURE
Looking ahead, significant research is under way on condition monitoring techniques, fault analysis, and optimized maintenance procedures that may hold O&M costs to modest levels and contribute overall toward the cost-effectiveness of wind turbine technology. Most leading industry authorities expect that operating costs, both onshore and offshore, will continue to fall as the industry gains more experience. The Danish Energy Agency, for example, anticipates that onshore costs will fall about 22 percent by 2020. Offshore costs are predicted to fall more rapidly, by as much as 40 percent in the same timeframe, according to the agency.

The emphasis for Condition Monitoring Systems will focus on ergonomics and improved diagnostics and prognostics. And technician requirements will likely change consistent with advances in technology. Today, according to at least one OEM, one technician is required for every 8-10 turbines. Soon, one technician will be able to oversee double that number. Business opportunities can be expected to flow for third-party CMS providers as tangible rewards accrue from Condition-Based Maintenance programs. And, with maintenance conducted in a more timely, efficient, and targeted manner, repair and maintenance providers will become vital team players in responding to particular component problems. In fact, wind turbine service is well on its way to becoming a considerable source for renewable business profits.

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Gregory J. Ziegler is the business development manager of On Line Systems at SKF USA Inc., 890 Forty Foot Road, Lansdale, PA 19446, 610-220-2667, Greg.Ziegler@skf.com, www.skfusa.com.