Reliably Determining Forces and Moments on Wind Turbines

Offshore wind turbines, in particular, are subject to extreme loading, and undergo several billion alternating load cycles over time. For a new wind turbine to gain type approval, it must comply with the wide-ranging requirements of the international standard for wind turbines IEC 61400, which describes a wind turbine as an "electric component, machine or building – or a wind turbine". The forces and moments occurring on wind turbines are particularly high – protection must be provided against forces up to 5 MN and moments up to 5 MNm and more. The forces and moments occur between the components as well as on the component surfaces in the form of tension – i.e., as very low-frequency oscillation.

Bolt connections as critical factor

The components in the gondola and on the drive train, the electrical components, as well as the rotor blades, towers and foundations are normally joined together by bolts, as they are made of steel. The bolts are subject to major loading. They secure key subfunctions, and as such assure the overall functionality of the installation. Relative forces and moments occur between the components as a result of the bolt connections. The tightening torque must also be assured, so that the bolt connection is able to optimally withstand the many alternating load cycles.

The sizes of bolts fitted on wind turbines vary. Size M12 bolts are used, though most are larger, such as M27, M36 or M48. In some cases even M64 or M72 bolts are used, on pipe and lattice towers for example. The bolts are mostly specified to property class 10.9 as per ISO 898-1, and the nuts to property class 10 as per ISO 898-2.

Monitoring the bolt connection during operation

HBM KMR series force washers measure static and dynamic compressive forces, and are particularly well suited to the monitoring of bolt fastening forces. With its KMR force washers, HBM provides a solution for effective monitoring of the many flange joints, such as those between tower segments, or between the hub and the rotor blades. These durable components, specified with IP67 protection, act as "smart washers", using strain gauge technology. The signals from the force washers can be supplemented by a suitable measuring amplifier. For even larger bolts, type C6A force transducers can be used in the same way as the KMR series.

Measuring technology is also employed when fitting the bolts, particularly to measure the tightening torque and installation pre-stress force. Bolt tensioning cylinders operating at very high pressures of up to 2000 bar are frequently used when tightening the bolts in ring flanges. For this pressure measuring range HBM offers the P2V, a highly precise and robust pressure transmitter capable of accurately assuring the correct applied pressure, which is the key parameter in terms of the tightening torque.

Additionally, bolt tensioning cylinder settings must be tracked in order to ensure that their tightening torques are also correct. A so-called transfer torque wrench type TTS from HBM can be used for tracking the actual torque of a torque tool such as a bolt tensioning cylinder or torque wrench. The details on this are set out in the German guideline DKD-R 3-7.

How can these torques be traced back to the national normal at the German National Metrology Institute PTB? HBM works closely with the PTB in this field, providing reference torque transducers such as the types TN, TB1 and TB2. HBM also offers high-precision measuring amplifiers such as the DMP41, MGCplus ML38B or QuantumX MX238B.

Sensors to measure the mechanical load on components

Wind turbine components are subject to heavy mechanical loading. Strain gauges and strain transducers are used to measure those loads.

Strain gauges

Strain gauges, or strain transducers, are excellent means of measuring very low-frequency oscillations on wind turbines. Major oscillations occur on wind turbines especially in the range between 0 …0.5 Hz. These can be much more effectively measured by a strain gauge than by piezoelectric sensors. HBM offers both electric and optical strain gauges, as well as hybrid systems.

There are also custom strain gauges for a wide range of special applications in the wind power sector, such as for the very wide wind turbine gear tooth flanks. Customized strain gauges measure the load distribution across the full width of the wind turbine gear tooth flanks, enabling the gearbox to be optimized accordingly.

Strain transducers

Encapsulated strain transducers are even more durable. HBM even offers type SLB700 strain transducers in a variant with integrated electronics. The strain gauge is sealed, as well as being mechanically protected by the strain transducer's housing. Moreover, type SLB700 strain transducers are very easy to install, and permit very high forces and moments to be measured. This is termed force shunt measurement. That is to say, the strain transducer is force shunted, and only measures the strain condition of the component, which in turn is dependent on the material – in this case steel. A typical application of SLB700 series strain transducers is in the yaw brakes of wind turbines, for example, enabling the brake shoe force to be recorded by means of a strain measurement on the back of the brake.

IEA Wind Task 35

HBM is a member of the “IEA Wind” international forum’s working group on “Ground Based Testing of Wind Turbines and their Components”. This working group not only focuses on large-scale testing facilities for ground-based gear units and generators but also on the evaluation of rotor blade loads. It mainly seeks to develop guidelines and offer recommendations for testing facilities that simulate the loads observed in wind turbine components in a controlled environment on site, with the objective of verifying system or component functionality, reliability, and durability.

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