Prepping your wind farm for condition monitoring
May 7, 2011 by Windpower Engineering
Filed under Condition Monitoring, Editorial, Maintenance
David Clark
Wind Consultant
El Dorado Hills, Calif.
Here’s a little secret: You can probably use anyone’s condition monitoring on a single turbine and get data good enough to predict basic failures. But expand the monitoring scope to several turbines or multiple sites and everything changes.
To cost justify outfitting a 100-turbine farm with condition monitoring equipment, an O&M crew needs a heads-up on only two to three gearbox failures over the next 18 years. A design life of 20 years means 18 of those will be spent out of warranty. And no turbine runs better with age. Regardless of supplier, you will have to get vibration data from nacelles into a server, and then analyze all the data. These few key considerations can assist with the integration of condition monitoring into your wind farm and organization.
The vibration sensor will gather 2,100 measurements or more in a year. There are usually 6 to 8 sensors per turbine, each taking three vibration measurements (demodulation, velocity, and acceleration at a minimum). This potentially adds up to more than a potentially million readings annually which all must be transmitted out of the nacelle, stored, and analyzed
Sending data from the nacelle
First, know what is available in the turbine for transmitting data. Condition monitoring equipment will need a network connection to communicate with data storage on the ground. If it is not possible to run wire, then wireless is an option, with the right radio. More owners specify installing additiona runs of fiber at the time of commissioning. Other turbine models will have an Ethernet provision available in the nacelle or at the tower base. Connecting point A to point B seems basic. Nonetheless, consult with someone who knows from experience what works. There is a long list of what might work or should work. The list of what actually works is quite short.
Powering the system
Know what’s available and what’s required for your system. It will usually require a 100, 110, or 120 Vac, or 24 Vdc connection up-tower. Both are common. An existing power supply may also suffice. As with communications, power available in the varies greatly from site to site and manufacturer to manufacturer. A turbine a few years old will need assessments as to what’s available up-tower and whether or not it will work with your system. If you are specifying a new turbine’s requirements, foresight allows anticipating power requirements.
Managing the data
Depending on the number of turbines, wind-farm sites, and analysis location, determine a data-storage requirement for the condition-monitoring system. Consider these points to size the server:
- 8 measurement locations on a turbine x 3 vibration measurements each, plus one tachometer reading = 25 measurements per turbine.
- Each of the 25 measurements averages 2 kB in size or 50 kB of data per turbine each time readings are captured.
- 50 kB x 100 turbines x 2 daily recordings = 5,000 vibration measurements, or 10 MBs daily.
- This equates to 1,625,000 readings and 3.65 GB annually.
An actual site similar to the example size produced just over 10.5 GB in a year. This was after a year that included measurement interruptions due to lack of wind, service outages, and other events. This amount of data was accumulated with just two series of measurements a day. Increase this frequency to 12 times a day and you will quickly consume a terabyte of server space. If you have purchased a well-thought out system, it will have data thinning features to eliminate unneeded data over the course of time.
The table tells of the percentage of network bandwidth use for a typical condition monitoring system. Between SCADA and condition monitoring, does your network have enough capacity? You may have to work with what is installed.
Data frequency
Most people want to know why you cannot take more data more often. This perspective may stem from examining SCADA data which is so dynamic and ever changing, therefore, you need lots of it. Vibration condition monitoring must be the same, right? Wrong. Vibration is not ever changing. Either you have a bad bearing or you don’t. A failing main bearing it will take several months to fail. Taking readings every 6 sec on something that will take months to fail is quantity monitoring not quality monitoring. In addition, physical limitations choke the number of potential readings. There are two reasons for limitations:
One limitation to a high frequency of measurements is the time it takes to gather the readings. On a typical 18 rpm main bearing, readings taken in a common velocity vibration measurement might take up to three minutes for a single reading. There are 24 more measurements after the first three minutes so the average turbine will take upwards of 30 to 45 minutes for accurate and meaningful measurements, for each turbine on the site, and each time data is taken.
The second limitation comes from the available bandwidth. Some controllers take 90% and more of the available bandwidth, leaving only a small portion for condition monitoring. So even if you wanted to take more data, there is no way to transmit it.
Is there really a problem with fewer readings? Reading twice a day for a year equates to just over 18,000 vibration measurements per turbine, over 2,100 per point. If you can’t detect a bad generator bearing with 2,100 measurements, change analysts or condition monitoring systems. Reach David Clark at (530) 677-9785.
Analyzing the data
Suppose you take 1.65 million quality measurements annually for the 100-turbine example. Who will make sense of the measurements? It’s not as daunting a task as it sounds, but it could be worse if the measurements are not qualified. So who will monitor the fleet? What seems to work is a blend of temporary outsourced expertise and internal resources. This is highly dependent upon the number of turbines and internal capabilities. Usually the wind farm is set-up with the software and monitored for several months until the wind-farm owner gets someone trained as a vibration analyst at using the software and performing analysis. Most software platforms have features to streamline the analysis so it focuses only on areas that need attention.
This grossly over simplifies what is required for expertise. But suffice it to say, you will have to dedicate personnel on the task while partnering with a reputable vendor that has wind-specific vibration experience. Most farms and fleets are internally managed, assuming you can guarantee stability in the position and dedicate the time required today and a year from now.
WPE
Misalignment, looseness, and imbalance are all correctable problems
June 30, 2010 by KRemington
Filed under Wind Power News
While condition monitoring technologies can track many signals, its purpose boils down to detecting wear and preventing the eventual failure of monitored components. Not all megawatt-class wind turbine drivetrains are monitored with the intent to catch degrading components. A few frequently encountered conditions are not caused by wear, making them correctable.
These conditions, when left uncorrected, manifest themselves in damaged components along with associated collateral damage. This is the typical pay-a-little-now-or-a-lot-later scenario. The task is certainly one for condition monitoring, but this topic could be called “correctible detection”. Here’s an overview of three common correctible conditions along with how they can be detected and corrected.
Misalignment
It’s common between the gearbox output (or high-speed shaft) and the generator input shaft. Causes include a flexible bedplate, large temperature variations, cantilevered mounting, thermal growth, and others. It is prevalent enough that some manufacturers recommend shaft alignment at prescribed intervals while others specify laser-based tools for aligning this portion of the turbine. “Because of the dynamic movement and flexibility of the turbine, alignment tolerances are much broader than what we see in standard industrial applications,” says alignment expert Paul Berberian of Alignment Supplies Inc. “Sometimes acceptable wind-turbine-alignment specifications are 2 to 4 times higher than those found elsewhere.” When left uncorrected, high-speed-shaft bearings and generator input-shaft bearings suffer and fail at a faster rate that when corrected. In some cases the coupling also bears the brunt of neglect and fails.
Detection
Vibration analysis detects misalignment remotely and over a progression indicated by a specific vibration signature detailing the fault. Misalignment saps production and performance. The graphs in Before and after balancing shows a vibration reading or FFT spectrum between a misaligned gearbox and generator coupling. The vibration signals come from remote monitoring. The lower signal is partly due to aligned shafts.
Correction
An alignment using lasers has largely replaced older, more time consuming methods. The alignment tool mounts across the coupling with two lasers pointing at laser detectors. The shaft is rotated 180° or less and three points are taken to define a circle. The lasers represent the shaft centerlines so the difference in the two lasers provide a measure of misalignment which is corrected at least two ways:
• The generator’s mount-ing pads can be shimmed up or down to correct mis-alignment in those directions.
• The generator’s “jack bolts” can be moved side to side to correct misalignment in those directions.
Looseness
If you have ever been in a wind turbine, you have noticed things move around up there. Because of the nacelle’s yawing and pitching, things may come loose. When parts break, it is usually due to being under repetitive or fatigue loads they were never intended to handle. Such failures can be real photo opportunities.
Detection
Vibration analysis detects looseness as multiple frequency peaks. One can remotely isolate the source of vibration without an up-tower climb. This is a decent alternative to climbing and checking everything that could be loose.
Correction
This one is easy – tighten what’s loose to the specified torque.
When loose things rotate, they generate many noticeable vibration peaks.
Imbalance
The image Before and after balancing shows vibration signals from those periods. Several components are susceptible to imbalance in a typical wind turbine. These include generator fans, generator rotors, and couplings to name a few. The causes of an increase in imbalance can range from simple things such as debris build-up on blades to material degradation, and possibly damage in the field.
Blade imbalance was once a major issue on smaller and much older wind turbines, mostly kilowatt-class machines. A common solution was to add balance weights of up to a few pounds. As the industry has grown, so have blade manufacturing methods. Much improved quality control allows creating matched blade sets weighing within close tolerances.
Misalignment comes in several variations, each of which is detectable and correctable. Parallel misalignment, for example, can produce vibration signals of this sort.
Detection
Using vibration analysis, you can see imbalance clearly and its degree. There are a few different types of imbalance, each having a unique vibration signature. There are also a few different types of balancing dependent upon the machine and type of imbalance detected.
Correction:There is a three step procedure for balancing in a single plane. Getting started requires a vibration analyzer, tachometer (or strobe light), vibration accelerometer, and a constant running speed. It also requires eliminating any looseness and misalignment prior to attempting to balance a component. The three-step method first calls for a:
• Baseline run. This is where vibration from the imbalance is initially measured, creating a baseline.
• A trial run then attaches a weight as a trial to induce a 30-30 rule. This means you are looking for a 30% increase or reduction in vibration, or a 30% shift in phase. This also means the trial weight has had some measurable affect on the imbalance. A calculated correction weight is placed on the machine, completing the initial balance job.
Angular misalignment can produce signals of this sort.
• Lastly, trim runs are performed to finalize balancing when an acceptable imbalance is not yet within tolerance. Several trim runs may be needed before the balance signal is within specs. ISO standards provide acceptable balancing specs.
Remember two simple things: Detect and correct. Tools and technology can detect these correctible conditions, then choose the appropriate corrective action for each of the three conditions.
The previously mentioned faults are avoidable and detectible using basic technologies readily available to wind farm owners, O&M providers, and manufacturers. Of course this discussion is meant to be a cursory overview of the process, not a detailed description. The details should be part of critical asset management tasks. Correcting the problems described will improve a fleet’s overall reliability and wring maximum performance from it. WPE
David Clark/Director/Turningpoint Inc., El Dorado Hills, Calif./turningpointwind.com
