Up Time Magazine - February/March 2009 - (Page 55) money for the operator. When the turbine is located in a remote location, performing maintenance is very difficult (Figure 4). Replacing bearings or 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 environFigure 3 – Inside a Planetary Gearbox. 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 foundations. Historically the industry has experienced a large number of gearbox failures. The failures have occurred across a wide variety of manufacturers, designs and sizes6. While one manufacturer did experience over 600 gearbox failures that almost sent it into bankruptcy7, many of the failures now more commonly relate to bearing failures, not gear wear or tooth failure6. 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 (see Figure 6), improved lubrication, and a greater focus on condition monitoring provide the industry with much greater confidence going forward. The blades actually rotate at quite low speeds. In the early days of wind turbine design, the speed was 45 to 70 RPM; therefore the gearbox ratio was 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 15 RPM requiring a gearbox with speed ratios of up to 1:100. Many wind turbine manufacturers utilize a planetary gearbox; often multi-stage planetary gearboxes. These are very complex gearboxes as illustrated in Figures 2 and 3. Figure 5 – Reliability has not been easy to establish in wind turbines. Now, if you believe recent news reports8, the only thing the industry has to worry about is low flying UFO’s (Figure 7) . Reliability issues Reliability is important with all rotating machinery. In the case of wind turbines, if the turbine has to stop then it is no longer generating electricity, and therefore it is not earning ments. 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, or in freezing conditions Vibration Analysis If you asked the average vibration analyst what type of situations they least like to deal with, their checklist might contain: Figure 4 - Reliability is extremely important in wind turbines because of the machinery’s usual remote location. Photo courtesy of Tom Obdan, Energy Research Centre of the Netherlands Figure 6 – Innovative designs, like the Liberty 2.5 MW turbine from Clipper Windpower, are paving the way for wind turbines to be more and more reliable. www.uptimemagazine.com 55 http://www.uptimemagazine.com
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