Almost wherever you go in the world you will now find wind turbines. Many believe they are the answer to global warming and the reducing reserves of fossil fuels. Wind turbines in the 1980s were in general smaller than modern-day turbines, generating less than 100 kW and with blade diameters of up to 20 meters (60 feet). Modern 7.5 MW wind turbines have rotor diameters of up to 120 meters (400 feet) – that’s the size of a Boeing 747! And now there are plans for 15 MW turbines (1). Whereas once they represented a fraction of a percent of total power generation, they now represent a far more significant proportion, with legislation in place to greatly increase that amount in coming years.

As of 2012, wind power supplied 7 percent of EU electricity demand (2). In Denmark, for example, more than 26 percent of electricity is wind-generated. In Spain the figure is 16 percent and in Germany it is 11 percent (2). There is over 106 GW of installed capacity in Europe.

The United States’ wind energy industry has seen a lot of growth over the past five years. The total installed capacity in the United States is 62 GW (2), over twice what it was in 2008 (2). Over 14 GW more are under construction – an industry record (2). However, wind energy represents only 4.5 percent of electricity demand (3). With Government assistance, a continuing threat of global warming, and growing demand for power, we are sure to see an increase in the number of wind turbines around the world.

A brief guide to the operation of a wind turbine

Wind turbines are remarkable machines. They are designed to operate, unmanned, in very windy locations; typically on remote farmland or at sea. While there are many designs, in many wind turbines the yaw control points the blades into the wind, and the pitch of the blades is constantly varied to control the speed. One or two large bearings support the main shaft driven by the blades. A gearbox increases the speed in order to drive the generator at 1500 RPM, for example.

The blades actually rotate at quite low speed. In the early days of wind turbine design, the speed was 45 to 70 RPM giving a gearbox ratio of between 1:25 and 1:40. However, due to the large diameter of the rotor blades employed in the more powerful wind turbines (>1 MW), the blade RPM had to be reduced in order to keep the blade-tip speed subsonic. Modern wind turbines turn as low as 12 RPM requiring a gearbox with speed ratios of up to 1:150.

Many wind turbine manufacturers utilize planetary gearboxes, often multi-stage planetary gearboxes. These are very complex gearboxes as illustrated below.

Reliability issues

Reliability is important with all rotating machinery. In the case of wind turbines, if the turbine has to stop because of some kind of failure then it is no longer generating electricity, and therefore it is not earning money for the operator. When the turbine is located in a remote location, performing maintenance is very difficult. Replacing bearings on a gearbox can be a very expensive operation. In addition to the significant parts cost, transporting and erecting a crane in order to access the turbine adds to the cost, and extends the downtime period.

 

Reliability has proven to be a huge problem for wind turbine manufacturers and operators. Wind turbines must operate in tough environments. Random wind speeds, and occasional high wind speeds affect the input-side of the gearbox. Changing load conditions on the generator affect the output-side of the gearbox. Wind turbines must potentially operate in corrosive sea air, extremely hot conditions, or in freezing conditions where icing becomes a problem. Resonance of the blades and tower can contribute to reliability issues, and misalignment is a significant issue given the flexibility of the gearbox/generator foundations.

Historically the industry experienced a large number of gearbox failures. The failures occurred across a wide variety of manufacturers, designs and sizes (4). While one manufacturer experienced over 600 gearbox failures that almost sent it bankrupt (5), many of the failures now more commonly relate to bearing failures, not gear wear or tooth failure (4). The problem has been so great that in some wind farms all of the gearboxes have been replaced once or even twice.

Fortunately the industry has survived this period and is learning from the history of failures. New designs, improved lubrication practices, and a greater focus on condition monitoring provide the industry with much greater confidence in going forward.

The end of part 1.

References:

1. European Wind Energy Association

2. American Wind Energy Association

3. U.S. Department of Energy

4. “The Gearbox Reliability Collaborative”, Brian McNiff, McNiff Light Industry

5. “Distributed Generation Drivetrain for Windpower

Application”, by Dehlsen Associates, LLC, for California Energy Commission Public Interest Energy Research Program - CEC-500-2006-018

About the author

Jason Tranter has been involved with condition monitoring since 1984. He is the founder and Managing Director of the Mobius Institute and the Mobius Institute Board of Certification. Mobius Institute is ISO accredited to ISO 17024 and ISO 18436-1. Mobius has training centres in over 50 countries and has trained over 14,000 people in a classroom setting and many thousands more via e-learning. He is the author of the majority of Mobius’ classroom material and the “iLearn” series of products, and is a member of ISO TC108/SC5. Jason holds an engineering degree (BE Hons), is a certified ISO Category IV vibration analyst, Certified Maintenance and Reliability Professional (CMRP) and Certified Reliability Leader (CRL).

Jason Tranter

Managing Director,

Mobius Institute, Australia,

jason@mobiusinstitute.com