Wind Turbine Reliability

June 15, 2017
Introduction

During the last twenty years, global wind power capacity has grown at a rapid pace, with an estimated cumulative global capacity of approximately 433 gigawatts in 20151  (see Figure 1). Wind power supplied more new power generation globally than any other technology in 2015 and accounted for almost half of global electricity growth in that year. The United States produced 190 terawatt-hours of wind power in 2015, 4.7% of the total amount of electricity generated in the US that year.

The cost of wind power can be reduced by improving wind turbine reliability. This article will review recent research in this area and will present best practices for improving wind turbine reliability.

Wind Turbine Reliability Fig 1
Figure 1. Growth of global wind power capacity 2000-20151

Wind Turbine Design Trends

A wind turbine is an electromechanical machine that converts the kinetic energy of the wind into electrical energy. The components of a typical geared wind turbine are illustrated in Figure 2. The wind spins the turbine blades, causing the low-speed shaft to turn. A gear box transforms the low-speed rotation into a high-speed rotation that is suitable for driving the generator. The generator then converts the rotational energy of the high-speed shaft into electrical energy. The wind turbine components are housed in a nacelle located at hub height, often 60 meters or higher above the ground.

Wind Turbine Reliability Fig2
Figure 2.
Wind turbine components and subsystems2

Wind turbine design is evolving rapidly, with a trend toward increasing tower heights and rotor diameters. This allows them to capture more wind energy, leading to increased electrical output (see Figure 33). The increase in size also increases the loads on the turbine components, especially the rotors and drivetrain components. This increased load has led to the use of new rotor materials and innovative rotor designs and manufacturing techniques. Gear box and generator designs are also evolving to improve reliability and to reduce downtime. Direct-drive wind turbines are now being built, coupling the low-speed rotor shaft directly to a specially-designed low-speed generator. This eliminates the gear box, and hence gear box failures, entirely.

Wind Turbine Reliability Fig3
Figure 3. Capacity, hub height and rotor diameter trends 1998-20153

Wind Turbine Operating and Maintenance Costs

There has been an increasing focus on reducing the cost of wind power, particularly operating and maintenance (O&M) costs, which include day-to-day operations, preventive (scheduled) maintenance, and unscheduled maintenance due to unexpected failures of wind turbine components.

Cumulative O&M costs of wind turbines are significant and can equal 65%-95% of a turbine’s investment cost over the assumed 20 year lifetime of the turbine4. The cost of unscheduled maintenance is difficult to measure industry-wide, but some studies suggest that it accounts for 30%-60% of the total O&M cost, and that it generally increases over time as a wind turbine ages4. Unscheduled maintenance due to failures is especially troublesome in the wind power industry. The cost of energy in the wind power industry is typically expressed in dollars per kilowatt-hour, with the ratio of the annual cost of producing the energy divided by the annual energy output4. When a wind turbine component fails unexpectedly, the cost of operating the turbine goes up, and the energy output of the turbine goes down, driving up the $/kilowatt-hour for operating the turbine.

A number of factors unique to wind turbines tend to increase the downtime and cost of unscheduled maintenance. Wind turbines are often situated in remote locations, making them difficult to access. Wind turbine components must often be shipped from overseas and then transported to these remote locations, a problem depicted in Figure 45. Moreover, the major turbine systems are often located at heights upwards of 65-80 m above the ground. In order to make repairs, crews must either climb the tower and work at those elevations in conditions that are windy or have special cranes transported to the remote location to enable components to be removed and replaced from the turbine.

Wind Turbine Reliability Fig4
Figure 4. The difficulty of transporting wind turbine blades5

Wind Turbine Reliability Challenges

Reliability is defined as the probability that a product will perform its intended function under stated conditions for a specified period of time. Reliability engineers and researchers use field data, experiments, and analytical techniques to determine the failure rates of products over time under specific conditions, and then work with design engineers to make products more robust.

Determining the reliability of currently installed wind turbines is an active and challenging area of research. There are a number of databases globally that track wind turbine failures and downtimes, but there is no uniform method for deciding what data to collect, how to collect it, and how to record it. Researchers7,8 have identified other problems as well:

  • Necessary data may not be available because it is considered proprietary by wind turbine operators.
  • It is difficult to compare data from one wind turbine to the next due to differences in component technologies and construction. 
  • It is difficult to compare data from similar wind turbines operating in different environments (dry vs. wet, hot vs. cold, etc.).
  • Wind turbine designs and technologies are evolving rapidly, making it difficult to compare data from newer wind turbines to data from older, smaller wind turbines. 
  • Wind turbines are typically designed for a 20-year lifetime, but most of the turbines in the field were installed less than 20 years ago. Complete lifecycle field data does not exist in most cases, and the oldest wind turbines with the most field data available are not representative of the latest designs and technologies.
  • There is also a limited amount of published work regarding failure analyses of wind turbine components, and much of the data that does exist is from older wind turbines. This makes it difficult to compare failures of similar components in different turbines. For example, there are many ways a gear box could fail. Without knowledge of exactly how the gear boxes failed in the field, it can be challenging to analyze gear box failures on similar wind turbines.

Despite these difficulties, researchers have made estimates of wind turbine reliability.7,8,9,10 In general, about half of wind turbine failures are due to electric components and to the control system, but these failures have low downtimes.Generator and gear box failures are less frequent but have longer downtimes.One study found that 25% of wind turbine faults caused 95% of the downtime. Reliability of wind turbines has improved with time and has achieved an availability of 98%, but wind turbines fail at least once per year, on average, with larger wind turbines failing relatively more frequently.7 A recent study of US wind turbines found that when all sources of downtime are accounted for, the average wind turbine actively generates power for 1.5 days between downtime events and that the average downtime is 1.6 hours.10

Best Practices for Improving Wind Turbine Reliability

Researchers, operators, manufacturers, and others within the wind turbine industry agree that increased reliability and maintainability is desired, especially given the trend towards offshore wind farms. Offshore wind turbines operate in a harsh environment and must be accessed by ship or helicopter, increasing the expected probability of failures, the downtimes associated with them, and the associated costs of repair and maintenance. This has spurred the wind turbine industry to investigate ways to improve reliability and maintainability by implementing best practices found in other industries.

The best practices discussed include4,8,11

  • Design: Improve the designed-in reliability and robustness of wind turbine components and subsystems, as well as the wind turbine as a whole. 
  • Testing: Use accelerated life testing and stress testing to determine the failure modes of the wind turbine components and subsystems.
  • Failure Analysis: Determine the root causes of each failure mode and eliminate them where possible by revising and improving the component designs.
  • Manufacturing Quality: Use best practices such as Six Sigma to produce high quality components with minimal variation.
  • Maintenance: Use preventive maintenance techniques to minimize the downtime of wind turbine components, extend their useful lifetimes, and replace them before they fail.
  • Condition Monitoring: Instrument wind turbine components with sensors to monitor temperatures, vibration signatures, structural integrity, and other factors to gauge the health of components and sub-systems in real time and anticipate impending component failures.
  • Benchmarking: Collect and analyze industry-wide data on failures, downtimes, and other operational metrics. Standardize data collection formats and techniques to allow cross-industry analysis.
  • Modeling: Develop better statistical reliability models of wind turbine components to manage risk and improve maintenance planning.
  • Standards: Improve industry standards to incorporate best practices and to keep pace with rapidly evolving wind turbine technologies.

How Exponent Can Help


Exponent is involved in a variety of wind turbine projects, and we are well-suited to assist companies in performing best practices toward improved reliability. The trend in increasing global wind power capacity is expected to continue, as well as the trend toward challenging off-shore wind turbine installations. We foresee an increasing need for wind turbine reliability analysis and improvement using the best practices outlined above. Given our experience leading investigations of turbine failures, we are uniquely positioned to make valuable contributions in this area.

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  1. Global Wind Report Annual Market Update 2015. Global Wind Energy Council.
  2. (Link) accessed 4/10/2017
  3. 2015 Wind Technologies Market Report U.S. Department of Energy. August, 2016.
  4. Walford, C. Wind Turbine Reliability: Understanding and Minimizing Wind Turbine Operation and Maintenance Costs. Sandia Report Sandia, 2006-110 (2006).
  5. Wind Vision: A New Era for Wind Power in the United States. U.S. Department of Energy, DOE/GO-102015-4557. March 2015.
  6. Hill et. al. Wind Turbine Reliability: A Database and Analysis Approach. Sandia Report SAND2008-0983 (2008).
  7. Echivarria, et. al. Reliability of Wind Turbine Technology Through Time. Journal of Solar Engineering, vol. 130. August, 2008.
  8. Reder et. al. 2016. Wind Turbine Failures – Tackling Current Problems in Failure Data Analysis. J. of Physics: Conference Series 753 (2016) 072027.
  9. Sheng, S. Report on Wind Turbine Subsystem Reliability – A Survey of Various Databases. National Renewable Energy Laboratory, NREL/PR-5000-59111. June, 2013.
  10. Peters, et. al. Wind Plant Reliability Benchmark September 2012. Sandia Report #2012-7329P (2012).
  11. Wenske, J and Wefer, M. Testing, validation, an opportunity for offshore wind power. PES Wind Issue 4 (link).

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