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MEASURİNG ROTOR BLADES WİTH LASERS

Measuring Rotor Blades with Lasers
21 Şubat 2017 Salı 16:16

By Torsten Wohlert, Deutsche Windtechnik Service, Germany

 

 

Deutsche Windtechnik has been measuring wind turbine rotor blades using a laser-based blade angle procedure since 2015. The company has discovered serious shortcomings on some of the 273 Vestas turbines examined to date. Where the relative blade angle deviations were 0.5 of a degree or more, the service technicians corrected the settings. This makes the wind turbines run more quietly, ensures less strain on the material and reduces wear.

 

Deutsche Windtechnik is to carry out measurements on all the wind turbines it maintains as part of a full maintenance contract that includes large components. This year the service provider will be checking the Vestas, NEG Micon and Siemens wind turbines. Similar examination of offshore wind turbines will follow. Badly adjusted rotor blades can easily lead to undesired vibration of the blades and drive-train, which eventually has an impact on the tower and foundations. This is why Deutsche Windtechnik was the first manufacturer-independent service provider to purchase a laser-based rotor blade measuring device at the beginning of 2014.

 

Tests and Evaluation Procedure

After extensive tests, which took nine months in all, and adjustments made in collaboration with the manufacturer, Deutsche Windtechnik began using the procedure on the turbines it maintains in March 2015. The company started with the Vestas type V90/V80 wind turbines, because they are particularly susceptible to rotor blade and tower vibration due to their long rotor blades of up to 125 metres. To verify the new measurement method, Deutsche Windtechnik compared the measurements using the new blade angle procedure with the results of the blade template method that it has been using to increase yield since 2007. Various aspects of both methods are compared in Table 1.

 

Higher Yields with the Blade Template Method

The blade template method involves the service technicians fixing templates to the rotor blades and then turning them to the mechanical limit stop. They then measure the actual deviation to this value and adjust where necessary. Here it is important to always ensure correct calibration of the wind turbine’s pitch sensors regardless of the base settings of the blades. This method is especially used to influence the performance curve of the wind turbine in a controlled manner and to increase the yield, because with the template the blades are optimised to a specific value. Using this approach also allows the aerodynamic imbalance to be reduced, resulting in less blade and tower vibration, and thus protecting the system components. Deutsche Windtechnik has used the blade template method for measuring Nordtank, NEG Micon and Vestas turbines since the company was founded in November 2007. However, the time needed and the downtimes for this measurement method are four times

hose for the laser-based procedure. A further disadvantage is the number of personnel needed: while a person can carry out the laser-based blade measurement procedure on their own, the blade template procedure requires three people.

 

Less Wear with the Blade Angle Method

The laser-based blade angle method is not so much about the yield; this procedure is aimed more at reducing wear and increasing the availability of the wind turbine. To take the measurements, the Deutsche Windtechnik service technicians direct two laser points at the blades and the tower while the turbine is in operation (Figure 1). They create an individual profile for each rotor blade (Figure 2) and determine the relative deviation of the blade angle by means of profile comparison. Because the service team measures the difference between the three rotor blade angles both at the blade root and at the blade tip, in addition to the relative deviations the method can calculate the twist as the difference between the blade root and the blade tip data. The twist should balance different angles of attack along the length of the rotor blade. If the twist of a blade deviates too much it can be an indication of poor production quality or that the surface has been altered as a result of repair work.

 

Corrective Action

For the correction of the blade angle, Deutsche Windtechnik factors in the result from the blade root, because this is where most of the measurements are generated. With hub heights of up to 125 metres and rated capacities of up to 2MW, Deutsche Windtechnik recommends correction where there is a blade angle deviation of half a degree or more. For this class of turbine, the company considers that urgent action is needed if the deviation is 1 degree or more. In the case of larger turbines with larger rated capacities, adjustment is in fact needed when the deviations are smaller. The evaluation is quick and straightforward and can be done directly on site. One person can take the measurements within an hour while the turbine is still in operation, thus minimising any downtime. The subsequent adjustment of wind turbines with either pitch or active stall control also takes only a few hours. More time is required for stall-controlled turbines because the pitching of the blades requires greater mechanical effort (see Table 1). As with the blade template procedure, the correct adjustment of the rotor blades reduces the aerodynamic imbalance, thus minimising blade and tower vibration (Figure 3). Because the loads travel from the rotor blades over the drive-train, mainframe and tower down to the foundation, a proper adjustment will protect all the mechanical components. This ensures less wear and high availability.

 

The Need for Early Measurement

Because great attention is paid to lightweight construction with modern designs, and with an ever-increasing size of wind turbines, it is all the more important to operate a turbine with the correct mechanical settings in order to reduce undesired loads. So that no components fail in the first place, Deutsche Windtechnik recommends control measurements as early as possible using the laser-based procedure, and ideally soon after commissioning. In most cases it is sufficient to measure each turbine just once. If, however, any adjustable components – such as pitch cylinders – are replaced during the operating period, subsequent measurements may be necessary.

 

Deviations on 117 Turbines

While measuring 2MW Vestas V90/V80 turbines since 2015, Deutsche Windtechnik has discovered massive problems with some of them. Of the 273 turbines measured, 68 exhibited blade angle deviations of half a degree or more. Another 49 turbines were, found to have deviations of 1 degree and more. Deutsche Windtechnik sees urgent need for action in such cases.

 

Long Rotor Blades Are Particularly Susceptible

Because Vestas turbines have very long rotor blades and hub heights of up to 125 metres, they are particularly susceptible to strong vibrations in the tower and the rotor blades. Because they are generally smaller, fewer problems are expected with the Siemens turbines measured since the beginning of 2016. Offshore turbines, on the other hand, are subjected to much harsher conditions, which also increases wear. They are to be measured using a similar procedure at a future date.

 

Even if they do not have a full maintenance contract, operators can order this service from Deutsche Windtechnik. The company offers laser-based blade angle measurement for around € 500 per wind turbine throughout Europe.

 

Extended Maintenance Concepts

To further reduce wear, Deutsche Windtechnik also ensures that the vibration absorber in the tower is ideally adjusted. To this end the company has expanded the scope of its annual and semi-annual maintenance. This holistic approach further reduces the ageing of the components.

 

Torsten Wohlert has been at Deutsche Windtechnik Service GmbH & Co. KG in Ostenfeld since 2007. After establishing the service concept for Vestas turbines he headed the E l e c t r i c a l Components and Technical Support departments. The 44-yearold has been head of the Engineering Department at Deutsche Windtechnik Service GmbH since 2012.

 

 

 

 


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