The latest trends in wind turbine maintenance and operation strategies incorporate the use of condition monitoring systems. Condition monitoring involves the measurement of physical operating parameters and analyzing time based trends and specific characteristics to predict failure of individual components.
Data can be obtained from any or all of the following sources:
• Vibration analysis
• Oil analysis
• Infrared thermography
Currently, only the first two are commonly used for condition monitoring of wind turbine plant.
The most common form of condition monitoring is the measurement of vibration for moving parts. In the case of a wind turbine this is the mechanical drive train, particularly main shaft, gearbox, coupling and generator. Changes in amplitude of the vibration, both for broadband frequency as well as at specific frequencies tells a story which experienced analysts can use to determine what is happening to the machine.
Use of technology to sample particle levels in oil and thermographic temperature sensing provide further sources of condition monitoring data.
Recent developments in technology specific to wind turbines also include the use of strain gauges (including optic fibre) embedded into structural elements such as blades and towers that are able to monitor and observe changes in the dynamic structural responses. They also allow records to be maintained of load amplitude and frequency changes that can be used to determine whether a structural element has exceeded its design parameters.
Many turbine manufacturers and component manufacturers such as LM Glassfiber are now starting to incorporate condition monitoring technologies into their newer multi MW designs; this includes both building in sensors and integration into SCADA.
Vibration condition monitoring systems are available from traditional suppliers as retrofit systems and are being marketed to wind farm owners. Though becoming common on mainland Europe, the first trial systems are being installed within wind turbines in Australia.
Benefits of Condition Monitoring
The benefits of continuous condition monitoring include:
• Increased availability as unplanned shutdowns are avoided.
• More control over timing of service and repair works. Prediction of failures provides the capability to pre-plan maintenance activities and ordering of Repairs can also be scheduled during periods of low wind.
• Reduced maintenance costs as catastrophic failures are avoided. This includes savings in manpower, parts and downtime.
• Diagnosis is supported by obtained data. In some cases, it may be possible to diagnose the fault purely by analysing the data.
Insurance companies are also providing premium benefits where condition monitoring is used.
Condition monitoring has however been slow to be implemented, in a large part due to high cost of the technology and because each individual wind turbine needs to be fitted with sensors and recorders. This is starting to change, as technology improves, cost of technology reduces and the scale of the wind turbines increase. The development of offshore and more remote windfarms also increases viability of condition monitoring as site access costs are significantly higher.
Implementation of Condition Monitoring
One of the misconceptions of condition monitoring is that it is assumed that if sensors and recorders are installed, then you will have a condition monitoring system and reap the benefits from it. This is not true. Condition monitoring requires also putting in place the systems that can use and respond to the data collected.
Analysis of Data
Volumes of data will be collected. Modern Condition Monitoring software has the ability to process the data, and isolate specific characteristics. This is not a replacement to a human interface but rather a supplemental tool. It is important to have a knowledgeable operator who is actually looking at the data, interpreting it in relation to experience with that machine. Over reliance on the software can either miss problems or result in an overreaction.
Setting alarm levels
Individual machines have individual characteristics, what is right for one may not be right for another. There needs to be a feedback loop that ensures regular review and adjustment of alarm levels for individual machines.
Remote Monitoring Services
Many of the wind turbine manufacturers offer centralised remote monitoring centres that are able to monitor the wind turbines, reset remotely and notify local personnel if on site attendance is required. These services are used during the warranty periods but often extend beyond warranty.
Utilities who invest in windfarms generally have 24/7 facilities for their major plant and often elect to monitor the windfarm themselves.
It has been observed that these large remote monitoring centres do not always translate to higher levels of performance. It is not clear why this would be but it is suggested that there are several causes like which are mentioned below
• Scale and location – why would an operator in Europe pay any more attention to one wind turbine in , say, Australia over the hundreds that they look at every day?
• Knowledge – Ability to diagnose and correct problems remotely, training, knowledge of specific machine characteristics. Particularly applicable to operators of steam plant dealing with wind plant.
• Communication – Timing of, message transfer and translation of machine outages to local service personnel.
There is an argument for using smaller, local Operations Managers who have a greater focus on individual performance of the facilities that they monitor. The cost to implement is higher, but the gains in generation income are generally found to offset this.