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Comment: Alignment with target values reduces vibrations in wind turbine drive trains


Dr. Edwin Becker and Ole Holstein, PRÜFTECHNIK

Misalignment in the drive trains of wind turbines can cause vibrations that reduce the service life and availability of gears and generators. These errors can be identified by means of vibration measurements or using condition monitoring systems.

If the simple rotational frequency (1st order) and/or the double rotational frequency (2nd order) strongly dominates the vibration velocity spectra of the wind turbine generator or gearbox, the most recent alignment report should be checked and the shaft alignment between the generator and gearbox should corrected, if necessary.

Aligning drive trains in wind turbines

In the drive trains of wind turbines, the alignment of the generator shaft to the gearbox output shaft should be corrected by taking into consideration the flexible generator and gearbox setup and the coupling type. The objective of aligning the system is to reduce the horizontal and vertical parallel offset and the horizontal and vertical angular offset under operating conditions to the point where the coupling is able to compensate any remaining forces without a reaction force and with low wear.

The alignment tolerances depend on the type of coupling in use and the rotational speed. Figure 2 shows a joint coupling being aligned.

While this type of coupling is able to compensate a relatively high angular offset, this will increase the restoring forces that the coupling and bearings in the wind turbine need to absorb. The offset values contained in coupling catalogues are operating offset values that should never be exceeded. The alignment offset is already contained in this operating offset.

In the interest of a long service life of the couplings and the connected wind turbine machine train, the coupling manufacturers, too, recommend keeping the alignment offset as low as possible to compensate for the additional displacements that occur. When misalignments do occur, this raises the strain on the coupling, thereby increasing vibration, temperatures and wear.

Wind turbine machines can be aligned using dial gauges or laser-optical alignment technology. Laser-optical alignment is fast and reliable, and the results are clearly recorded in the laser alignment device. Operators of wind turbines are recommended to ensure that the current condition of the machine alignment be "readable, identified with a plus/minus notation and interpretable", even when aligning with dial gauges. Figure 3 shows examples of results taken from the Alignment Center software that were measured using a dial gauge and a laser.

Alignment corrections

Before correcting the alignment, the current position of the wind turbine generator relative to the gearbox should be examined. The generator can be too high or too low and/or shifted to the left or right. In all of its alignment systems and in the software, PRÜFTECHNIK uses the plus/minus notation shown in Figure 4.

A feature of most wind turbines is that, due to the flexibly mounted gearbox and generators, shaft positions vary greatly relative to each other as a function of load and rotational speed. Therefore, the wind turbines must be aligned with target values, or the drive train must be "incorrectly aligned" while at a standstill so that the ideal shaft alignment is reached under operating conditions.

For example, it is common for "back-heavy" wind power generators to be erected several millimetres higher when they are first aligned to compensate for 'fatigue' of the flexible rubber-bonded metals. Of course, the exact target values also depend on the extent to which the base frame yields.

Wind turbine operators are encouraged to ask the system manufacturer to provide the alignment target values specific to the system or to have the target values determined. These target values can then be entered as shown in Figure 4 and are automatically taken into account during the alignment procedure.

Target values for alignment

Target values can be calculated and/or determined by measurements. The influences on wind turbine gearboxes that need to be taken into account are clearly described in the VDI Guideline 2726 [1], published by the Association of German Engineers (VDI).

The guideline differentiates between the following influences on the alignment condition:

  • Heat expansion of the wind turbine drive foundation, housing and shafts due to their temperature difference during mounting and operation;
  • Flexible deformations of the wind turbine drive foundation, housing and shafts during operation;
  • Shaft displacement due to differences between the installation and operating condition in bearing play, gear tooth forces and hydrodynamic lubrication;
  • Skewed position of the wind turbine's gear shafts due to masses applied on the outside (couplings, brake discs);
  • Radial and axial run-out of the flange;
  • Radial and axial rigidity of the connecting coupling.

Apart from the influences listed above, wind turbine drive trains are also subject to considerable displacements due to the 'soft' mounting of generators and gearboxes. Some wind turbine types combine different versions of base frames, drive train components, rubber-bonded metals and rubber buffers from different manufacturers. This results in varying shaft displacements, which means that different alignment targets need to be used every time.

Determining target values by measurement

Alignment targets can be determined by measurement using CMS and two PERMALIGN sets. This involves mounting laser components and reflectors in a vertical and horizontal direction to continuously measure the current horizontal and vertical radial and angular positions of the wind turbine generator relative to the gearbox at a rate of once per second.

If there is a spatial displacement, the precision laser beam is diverted to a new position on the receiver surface and the parallel and angular surface positions are measured.

Figure 1 shows the laser/receiver unit mounted on the gearbox. The radial and angular xy positions can be recorded, and can also be converted to coupling offset and shim values online. For customers who already use a Condition Monitoring system of the WinTControl, VIBROWEB XP or VIBGUARD brand as an online system, the installed CMS can be used to determine the target values by measurement.

The two PERMALIGN sets are simply connected to the internal RS232 interface and the programme used for the alignment measurements is loaded into the existing CMS and started. PERMALIGN measurements should then be made under conditions of no wind (with the system at a standstill), low wind (with the system turning slowly), medium wind and strong wind.

Figure 6 shows examples of results in xy displays that have been scaled for comparison.

PRÜFTECHNIK experience has shown that the temporary measurement sessions take from one to four weeks, depending on the location of the wind turbines and wind conditions. This period may be shorter after measurements have been made under calm and high wind conditions.

If condition parameters and vibrations are recorded as well, it will be easier to evaluate the data later on, and it becomes possible to determine when the additional vibrations occur. If there is a possibility of mailing the measurement results to the Monitoring Center, an evaluation can be performed online. PRÜFTECHNIK determines alignment targets as a service and for a rental fee. If a PRÜFTECHNIK CMS is not in use in the wind turbine, a CMS can be provided temporarily.

Final remarks

It is still a common belief that alignment is not an issue because catalogues specify such large misalignments for wind turbine couplings. Please note that the coupling working ranges specified in the catalogues are not alignment tolerances [2].

The rule of thumb is that the alignment tolerance should be assumed to be at least 1/3 of the coupling working range. Thus, only 2/3 of the coupling working range is available for operational displacements over the entire operating duration. For competitive reasons, coupling manufactures usually only specify the working range of their couplings without noting that the reaction forces increase with increasing parallel and angular offsets within the working range of their couplings. The restoring forces are zero only if there is no misalignment.

Literature

  • [1] VDI Guideline 2726: Ausrichten von Getrieben, 1982
  • [2] Edwin Pernleitner: Ausrichten von Maschinensatz- Wellen, VDI-Verlag 1995

About the Authors:

Dr. Edwin Becker PRÜFTECHNIK Condition Monitoring GmbH, Oskar-Messter-Straße 19-21 85737 Ismaning www.pruftechnik.com Tel. 089/99616-340 edwin.becker@pruftechnik.com ... is Director of the PRÜFTECHNIK Service & Diagnose Center and is an expert for the wind sector and beyond.

Ole Holstein PRÜFTECHNIK Alignment Systems GmbH Technisches Büro, Nord Kremper Weg 97 25524 Breitenburg / Nordoe Tel. 04821-89100 ole.holstein@pruftechnik.com ... is at home in northern Germany and has been working in the field for PRÜFTECHNIK for 16 years.

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