Vibration Analysis of Wind Turbines(2)
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 climbup 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 to be able to compare the current readings to either a previously collected set of readings, vor 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, or perhaps another parameter. You will then need to wait until the required conditions are met before the vibration measurements are acquired. Alarm limits can also be defined for that "band of operation".
Variable Speed During the Measurement
When the analyzer (or monitoring system) acquires the "time record" that is used to compute the spectrum (via the FFT calculation), it is assumed that the machine being monitored operates at a constant speed during that test.
For example, if you acquire a 1600 line spectrum with an Fmax of 1000 Hz, the analyzer will acquire 1.6 seconds of vibration data in order to compute the FFT (for just one average). An 1800 RPM generator will rotate 48 times during the test, but the 15 RPM input shaft will rotate just 40% of one rotation... In order to capture 10 rotations, we need an Fmax of 40 Hz (with resolution set to 1600 lines), and the measurement will take 40 seconds!
If the speed of the wind turbine varies during the test, the peaks in the spectrum can blur - the peaks will be wider than they should be, and the amplitude of each peak will be reduced. And this blurring effect may not be consistent from one test to the next. (Note: The blurring effect will be more noticeable at higher frequencies.)
variable speed and variable loads make vibration testing difficult
There is one more challenge when monitoring gearboxes; especially planetary gearboxes. In an ideal world the vibration sensor (accelerometer) would be placed close to the bearing and/or gear of interest. However not only do these gearboxes have a large number of bearings and gears, it is difficult to get an accelerometer close to certain bearings; the planet bearings for example. When analyzing spectra, either conventional spectra or demodulated spectra (or Peak Vue, SPM, etc.), it is necessary to resolve three issues:
1.Computing the speed of each shaft, and the gearmesh frequencies, can be quite a challenge with planetary gearboxes.
2.Computing the bearing frequencies will be very complicated due to the large number of bearings and different shaft speeds. Both jobs are made even more difficult if the manufacturer is not willing to provide the details of the bearings used and gear ratios.
3.The amplitude of the vibration measured when a planet bearing begins to fail, for example, will be lower than the vibration from a bearing in contact with the gearbox case due to the transmission path involved.
Almost all of the vendors of portable data collectors and analyzers now manufacture online monitoring systems designed specifically for the wind turbine application. There are an awful lot of wind turbines, and each one requires its own monitoring system. These vendors all recognize both the challenge and the opportunity.
Systems are designed to monitor the speed of the turbines, and other process parameters, so that they can correctly determine when the turbine is operating in the pre-defined "band of operation".
In fact, many of these systems can define multiple "bands of operation". Each band will have its own set of alarm limits, and all readings are tagged with their band of operation so that graphical comparisons can be performed. It is important to have multiple bands for two reasons
1.Unless the weather conditions are reasonably constant, the turbine will not be operating in any one band for a large proportion of time. By defining multiple bands, the system will monitor and check the turbine far more frequently.
2.The bearings, gearbox, and generator will react differently under
different speed and load conditions. It is, therefore, very helpful to monitor the machine-train during the majority of operating conditions. For example, a problem with the support structure may only be detected when the turbine is operating at highest load.
how many sensors are required for effective wind turbine condition monitoring
The Challenge With All On-Line Monitoring Systems
All on-line monitoring systems face a number of challenges that can limit their effectiveness, but these challenges are compounded when applied to wind turbines. I have already discussed the issue related to varying speed and load, but let's take a look at some of the other challenges:
The Number of Monitoring Points -- One of the most critical decisions is selecting the number of sensors that should be installed on the gearbox, generator and bearings, and selecting their location. Every sensor costs money, and it requires another channel in the monitoring system. And when you multiply these additional costs with the number of wind turbines (see Figure 11), you can see that it is a very sensitive issue.
As with all vibration monitoring applications, it is essential that the monitoring system can at least acquire enough data to warn when the vibration levels are increasing - even if there is not enough data to actually diagnose the problem remotely. But, as discussed previously, when monitoring large planetary gearboxes, the spectral data can be very complex.
Knowing the failure modes of the turbine can help immeasurably. If you know which gearsand bearings are most likely to fail, then you can position the accelerometers accordingly.
The Central Monitoring Service
The "central monitoring service" is the group of people who will respond to the alarms, analyze the data and make final recommendations. It is essential that this group has access to the required data and has the experience to make recommendations. Obviously a communication link must be established with the wind turbine monitoring systems.
Centralized or De-Centralized
The monitoring system must not only acquire data when the turbine is operating within pre-defined bands, but it must compare the data to alarm limits and take the appropriate action. There are at least two approaches: perform all of these operations within the system that is installed in the nacelle and communicate directly with a central monitoring service, or install a more sophisticated system centrally within the wind park and use it to communicate with both the wind turbine monitoring systems, and with the central monitoring service. Many wind farms have a wired or wireless network, and the monitoring system may be allowed to tap into that network.
The Effectiveness of Alarm Checking Software
Many vibration analysts running 'normal' vibration monitoring programs do not have an effective set of alarm limits set up for their machines which allows them to run an exception report that provides useful, actionable information. The solution is to manually analyze each and every measurement. This is not possible when performing on-line monitoring.
It is therefore very important that the alarm limits are set up carefully, and they need to be refined frequently. Too many on-line monitoring systems generate "thousands of alarm exceptions" - as a result faith in the system is lost. There are methods that can be used to set up effective alarm limits, such as statistical alarm generation, but that will need to be covered in a separate article.
Wind turbines are being installed at an amazing pace, and while some of the earlier reliability problems have been resolved, there is no doubt that reliability will be an on-going issue. Condition monitoring technologies such as vibration monitoring, oil analysis, and performance monitoring will play a very important role in the viability of wind farm operation. As long as monitoring system vendors and wind turbine manufacturers continue to improve their designs, focus on reliability, and share information, renewable energy from wind power will continue to grow as a source of affordable and clean energy around the world.