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Vibration Analysis of Wind Turbines(1)

  In Europe, wind power currently supplies 3.7% of EU electricity demand 1. In Denmark, for example, more than 20 percent of electricity is wind-generated. In Spain, the figure is 13 percent and in Germany it is seven percent 2. In 2001, the European Union passed legislation setting a target for 21% of the EU's electricity demand to come from renewable energy by 2010 3.


  The United States wind energy industry is growing at an exceptional pace, and that pace will only accelerate in the coming years. Within days of being elected, President Barack Obama announced a new energy plan which includes measures to "create five million new jobs by strategically investing $150 billion over the next ten years to catalyze private efforts to build a clean energy future"4. The plan also includes "an economy-wide capand- trade system to reduce carbon emissions by the amount scientists say is necessary"4.



  Wind Turbine


  The total installed capacity in the United States is 21,017 MW in 35 states. Over 8,000 MW more are under construction for completion this year or early next year. Over 7,500 MW were installed in 2008, and 5,249 MW were installed in 2007. The American Wind Energy Association (AWEA) stated that in 2008 American wind farms generated "just over 1.5% of U.S. electricity supply, powering the equivalent of over 5.7 million homes". It also states that "to generate the same amount of electricity using the average U.S. power plant fuel mix would cause over 28 million tons of carbon dioxide (CO2) to be emitted annually"5.


  The U.S. is now the world leader in wind electricity generation. While Germany still has more generating capacity installed (about 23,000 megawatts), the U.S. is producing more electricity from wind because of its much stronger winds 5.


  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



  Planetary GearboxWind turbines are remarkable machines. They are designed to operate, unmanned, in very windy locations; typically in remote farmland or at sea. As the wind blows, the yaw control points the blades into the wind, and the pitch of the blades is constantly varied to control the speed. Typically two large bearings support the main shaft driven by the blades. A gearbox increases the speed in order to drive the generator at 1800 RPM, for example.


  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.gearbox


  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 money for the operator.

When the turbine is located in a remote location, performing maintenance is very difficult (Figure 4).



  wind turbines are difficult to work on


  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 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, or in freezing conditions where icing becomes a problem. Resonance of the blades and tower can contribute to reliabilityissues, and misalignment is a significant issue given the flexibility of the foundations.



  Reliability is difficult


  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 onemanufacturer did experience over 600 gearbox failures that almost sent it into bankruptcy, many of the failures now more commonly relate to bearing failures, not gear wear or tooth failure. 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.



  wind turbine


  Now, if you believe recent news reports, the only thing the industry has to worry about is low flying UFO's (Figure 7) .



  UFO hits wind tower


  Vibration Analysis


  If you asked the average vibration analyst what type of situations they least like to deal with, their checklist might contain:


  1.Variable speed and load from one test to the next

  2.Variable speed and load during the actual test

  3.Difficult and limited machine accessibility

  4.Complex gearboxes - planetary gearboxes being the worst


  5.Very low speed shafts


  Well, guess what? You have just accurately described a wind turbine.



  The wind conditions are constantly changing, so each vibration measurement taken could potentially be at a different speed and load condition. And what is worse is that the speed can vary as the blades rotate. Even the nacelle (the house at the top of the tower) will rotate as the wind direction changes. And one more small challenge is that the whole structure can vibrate and resonate due to the construction of the tower and nacelle. Therefore, routine monitoring by vibration analysts visiting the wind turbines on a routine basis is almost out of the question. That's not to say that it is not done - it is simply very, very challenging to acquire data that can be compared to previous readings in order to detect changes in the patterns.





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