LONDON — Wind turbine downtime is particularly costly during winter, for two reasons. First, November to April is the period over which most such plants around the world produce around two thirds of their electricity yield. The wind farms depend on the availability of wind, which in turn depends on the season. Second, logistical costs for maintenance during this time of year are high. So turbine failure at this time must be avoided at all costs. Condition monitoring, perhaps integrated with programmable logic control, can help a plant maintain its energy availability over this crucial period.
As such a system enables the detection of impending mechanical damage in advance and during operation, a plant can use a condition monitoring system (CMS) to avoid the sudden failure of a bearing on the gear shaft.
Here replacement of the component requires a crane to be used, the cost of which is large, particularly in the case of offshore installations. The turbine also has to be shut down for around three to four days, but in the case of a sudden failure caused after wear damage has failed to be detected in good time, the downtime can easily be 10 times longer since material and personnel first have to be organized and brought to the plant.
Sudden failures are also often due to higher wind loads in times of strong wind and therefore high yields, and if consequential damage occurs to components whose dimensions mean they can only be partially repaired at the plant, additional expenditure is incurred.
A CMS supplies the technical operational management team with a continuous stream of data, mostly based on vibration monitoring. ‘For this, vibration or body-sound sensors are fitted at critical locations along the drive train of a wind turbine,’ explains Holger Fritsch, CEO of Bachmann Monitoring. Other meaningful, physical variables such as temperatures or lubricant consistency can also be measured.
Changes in the vibration behavior of the monitored components enable the detection of impending mechanical damage, and a comparison with reference measurements, such as those taken directly after erection of the installation allows conclusions to be drawn about the actual condition of gears, generators, roller bearings, rotors and other elements.
The key advantage of a CMS is the permanent automatic monitoring and evaluation of trends under comparable operating conditions. These are therefore superior to vibration measurements taken at specific points, which are not referenced to any defined operating conditions, and thus involve a considerably greater degree of variance, and are hardly suitable for comparison.
“With a CMS plant shutdowns required for repair and maintenance can thus be planned and prepared systematically, plants can be kept longer on the grid, and consequential damage prevented,” Fritsch emphasizes. He adds: “The average difference between planned and unplanned repair times can be up to 30 percent and more – depending on the type of wind turbine.”
Another benefit of condition monitoring is that it allows condition-based maintenance. Instead of replacing components after fixed time intervals based on empirical values, they can be replaced only when required, such as when signs of wear are detected. A CMS also supports the technical plant manager in controlling the maintenance intervals and in setting priorities systematically. By taking seasonal conditions into account, maintenance schedules, personnel and materials can be planned cost-effectively. “The results also help to improve medium-term maintenance strategies and forecast the expected operating costs and yield of a plant more clearly,” Axel Ringhandt, wind sector manager at Bachmann Electronic, says.
The key benefit of an integrated solution is the linking of the CMS’s measured values with other operating parameters of the wind turbine. This increases the diagnostic reliability of the condition monitoring. Fault patterns can be compared with the current operating situation and interpreted with greater accuracy.
Selective control of the plant even enables mechanical loads to be reduced. In this way, adjusted operating conditions can extend the lifespan of partly damaged parts up to the next plannable maintenance date. “Nowadays a plant would be shut down for safety reasons if there was any damage to the gears,” explains Fritsch. “If the controller is provided with additional sensor data that enables the damage to be restricted, it would be possible to keep the wind turbine plant connected to the grid at a suitably reduced power output – with further continuous monitoring and the agreement of all involved, right through to the insurers. This makes it possible to considerably reduce a loss in yield.”
The additional data available enables the monitoring centre to perform more precise diagnostics. The plant manager not only receives a verified fault message, but also an assessment of the operational relevance and specific action recommendations.
Any reserves available can be identified in conjunction with technical operational management by combining the information from the automation system and the currently prevailing environmental conditions, such as wind speed or ambient temperature, with the evaluation of the load on relevant components. This enables the plant manager to further exploit the potential of the wind turbine and run it safely and at optimum yield for the operator or investor.
Wind turbines are complex systems that are continuously exposed to changing operating conditions. Therefore it makes sense to tailor the CMS to the special characteristics of the turbine type and parameterise accordingly. Monitoring via remote access is possible through the use of state-of-the-art communications media, so that competent and fast support is ensured when required. There are, for example, condition monitoring centers which specialize in internet-based remote service.
The monitoring centre consolidates and analyses the data coming from the plants it oversees. In the event of a fault, the staff of the remote monitoring service more closely examines the automatically formed characteristic values and compare them with other characteristic values and trends until a consistent fault pattern is formed. “In our experience it is already possible at this stage to give the plant operator indications about the cause of the fault,” says Fritsch. “This then enables the use of additional examination techniques such as video endoscopy to be carried out selectively and efficiently.”
Certification Guidelines for CMS Systems
Guidelines from Germanischer Lloyd and the Allianz Center for Technology for the certification of CMSs that monitor wind turbines require that at least the main bearings, main gears, generator and nacelle including the tower should be covered by condition monitoring. Monitoring of the main bearings, main gears and the generator requires at least six acceleration sensors. For the nacelle and tower one such sensor is required in each axial wind direction as well as perpendicular to it.
In all cases, Germanischer Lloyd’s guidelines for wind incorporate the state-of-the-art in technology. They therefore stipulate the most important basic conditions for the development, installation and operation of these systems. They also represent a basis for the testing of CMSs.
Monitoring centers must, for example, explain how limit values are determined and why they are selected in this form. This ensures that the complex CMS data is evaluated and interpreted in a sufficiently qualified way.
In drawing up the guidelines, Germanischer Lloyd Wind contacted wind farm operators that operate different systems, the manufacturers of wind turbines and CMSs, and the insurance sector. This ensured the greatest possible neutrality and acceptance of the guidelines across the industry.
The guidelines also form the basis for the development and installation of CMSs and regulate the use of measured values, such as how they are evaluated, interpreted and stored. They also describe the operating procedures when specified limit values are exceeded.
Gabriel Schwanzer is director of the wind business unit at Bachmann Electronic.
Lead image: Wind turbine via Shutterstock