Up Time Magazine - February/March 2009 - (Page 56) 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. Figure 7 - Today’s wind industry has learned much from past mistakes, but there are always new challenges. Vibration Analysis Challenges Let’s explore some of these challenges in a little more detail. Location and Environment — Although we will concentrate on discussing the technical issues, you cannot skip the challenge associated with accessing the wind turbines. At best they are on land not too far from civilization. At worst they could be out at sea. And once you get to the wind turbine, you then you have to climb up the tower. Believe me, it is a long way up – you had better be fit (see Figure 9). Variable Speed and Load — One of the key requirements for successful vibration analysis is Figure 9 - Once on location, there is still a lot of work to do in accessing the turbine. 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 to be able to compare the current readings to either a previously collected set of readings, or to a set of alarm limits. We want to see how the vibration patterns have changed. In a standard power station, the majority of the machines will run at the same speed and load from one test to the next. Comparisons with older data are easy, and alarm limits can be generated based on experience with the machine, or based on statistical analysis of the history of data. But it is not that easy with a wind turbine. As the wind speed varies, the load on the blades, shaft, bearings, gears and generator will change. The speed of the machine will also change. The result is that the peaks in the spectrum will not line-up with peaks in previous spectra, and the amplitudes of peaks are no longer comparable. Not only does the load affect the amplitude of the peaks in the spectrum, natural frequencies will either cause the measured vibration amplitudes to be higher or lower than when the machine was running under a different speed or load. It is certainly possible to “order normalize” the spectrum, so that the speed-related peaks in the spectrum will be aligned, but that does not address the changes in amplitude. The solution is to define one or more bands of operation where spectra (and time waveforms) collected within that band can be deemed “comparable”. The “band of operation” may be specified by the RPM of the input shaft, or the power generated by the turbine, february/march 2009 Figure 8 - Remote locations pose a major problem in wind turbine maintenance. Planned and scheduled maintenance is hard enough, and unplanned maintenance can potentially jeopardize the viability of the wind power operation. 56
For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.