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.

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

Wind 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.

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).

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.

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.

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

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.

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.)

Therefore, depending upon the nature of the turbine, and the wind conditions, this effect can either be tolerated, or the "order tracking" technique must be employed. Either the once-per-rev tachometer signal must be fed into the analyzer (with an internal "tracking ratio synthesizer") such that the analyzer varies its sample rate in proportion to the RPM, or a shaft-encoder must be used to generate a "pulse train" that contains, for example, 360 pulses per rotation of the shaft which is used to control the analyzer's sample rate.

Gearbox Measurements

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.

The Solution

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.

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 gears
and 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.

Conclusion

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.

References

1. European Wind Energy Association: http://www.ewea.org/index.php?id=58
2. U.S. Department of Energy, Energy Efficiency and Renewable Energy, Annual Report on U.S. Wind Power Installation, Cost, and Performance, Trends: 2007 (May 2008)
3. European Wind Energy Association: http://www.ewea.org/fileadmin/ewea_documents/documents/publications/WD/2008_november/Viewpoint.pdf
4. "Barack Obama and Joe Biden: New Energy For America" http://www.barackobama.com/pdf/factsheet_energy_speech_080308.pdf
5. American Wind Energy Association: http://www.awea.org/pubs/factsheets/Market_Update.pdf
6. "The Gearbox Reliability Collaborative", Brian McNiff, McNiff Light Industry: http://www.sandia.gov/wind/2007ReliabilityWorkshopPDFs/Tues-1-A-BrianMcNiff.pdf
7. "Distributed Generation Drivetrain for Windpower Application", by Dehlsen Associates, LLC, for California Energy Commission Public Interest Energy ResearchProgram - CEC-500-2006-018
8. http://au.video.yahoo.com/watch/4262445/11458525

Acknowledgements

I would like to acknowledge the assistance given by the following helpful people: Pedro Cortez, SCM, Costa Rica; John van Bynen, Commotion Systems, Australia; Shaw Makaremi, Clipper Windpower, USA; David Clarke, Turning Point, USA; Steve Barber, Windrisk, USA. Thanks guys!