The signal of the drive train mechanical torque can be a very informative input for new technologies on the wind turbine, helping further reducing the cost of energy. However, measuring the mechanical torque is not an easy task even during a test campaign on a prototype. In case of the long‐term measurement for operational purpose of use on the turbine, it is often considered as technically and economically not feasible. This paper discusses possible ways of the torque measurement and presents a test campaign where two different methods of torque measurement are conducted. One of the methods measures the shear strain signal using strain gauges at one section of the turbine main shaft. The other method measures the torsional deformation between two positions of the drive train, with an incremental encoder measuring the angular position of the drive train at each position. The tests are conducted on an 8‐MW wind turbine drive train under tests on the nacelle test bench “DyNaLab” of Fraunhofer IWES. During the tests, different load steps of torque and other load components are applied to the drive train. The measurement results from both measuring methods are presented and analysed. In the end, technical and economical feasibilities are discussed for both methods of torque measurement; some suggestions are also given respectively.
The mechanical torque input into the wind turbine drive train is a very useful measurement for tests performed on a test bench. To ensure the accuracy and the reliability, an accurate calibration of the torque measurement must be carried out and repeated within a certain period of time. However, owing to the high torque level and large structure size, such a calibration is both expensive and time consuming. To overcome this challenge, a new calibration method is proposed here. The method is based on the electrical power measurement, where a high level of accuracy is much easier to achieve. With the help of a special test process, a relationship between the torque‐measuring signal and the electrical power can be established. The process comprises two tests with the drive train running in different operating modes. The calibration is possible by carrying out the same test process on several different torque levels. Detailed uncertainty analysis of the method is presented, whereby the uncertainty can be calculated by means of matrix operation and also numerically. As a demonstration, the implementation of the method on a test bench drive train that contains two 5‐MW motors in tandem with the motors operating in a back‐to‐back configuration is also presented. Finally, some variations on the method and possible ways of achieving better accuracy are discussed.
Decreasing the levelized cost of energy is a major design objective for wind turbines. Accordingly, the control is generally optimized to achieve a high energy production and a high-power coefficient. In partial load range, speed and torque are controlled via the generator torque but the rotor torque determines the power coefficient of the turbine. High uncertainties for the uncalibrated low-speed shaft torque measurement and varying drivetrain efficiencies which depend on the speed, load and temperature lead to a torque control error that reduces the power coefficient of the wind turbine. In this paper the rotor torque control error and the impact on the power coefficient of wind turbines is quantified. For this purpose, the variation of drivetrain efficiency is analyzed. An efficiency model for the wind turbine drivetrain is build and validated on the test bench. Then, the influence of the drivetrain speed, torque loads, non-torque loads, and temperature on the efficiency is quantified. Finally, the influence of the rotor torque control error on the power coefficient was simulated with an aerodynamic model. The results show that of all examined influences only torque and temperature significantly impacting the efficiency leading to rotor torque control errors that reduce the power coefficient and consequently increase the levelized cost of energy. Improved efficiency measurement on WT test benches or drivetrain efficiency modelling can reduce the rotor torque control error and therefore decrease the LCOE.
Modern wind turbines have some of the highest levels of torque and non-torque loads of all industrial sectors. These high loads present a great challenge for the design of wind turbines. On a nacelle test bench, the wind turbine drivetrain can be tested and validated against the design. It is therefore important to measure the correct level of each load and all its dynamic behaviors during the test. The best way to achieve this is to measure the loads directly in front of the drivetrain. This paper presents a method of direct load measurement on the shaft adapter which connects the drivetrain to the test bench. The technical solution and some important details about the instrumentation on the adapter are also presented. Methods of signal nulling as well as signal conversion from raw signals to the loads are compared as well. The measurements obtained are then compared with the applied loads from the test bench which show good agreement. As a special case, the torque measurement is validated and calibrated up to 5 MN m by means of a state-of-the-art torque transducer.
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