Logo PTB

Traceable torque measurement for the wind energy sector

06.11.2018

For the first time, torque measurement in nacelle test benches has been traced to the national standard. Common torque calibration standards only provide methods for static torque calibration. However, for torque measurement in nacelle test benches, calibration at varying rotational speeds is required. The indication deviation that has been determined will contribute to a reliable efficiency determination of wind turbines.

The number of wind turbines around the world has risen over the past few years. To ensure the efficiency and reliability of these wind turbines and their individual components, extensive testing is required before they can be installed in the field and put into operation. Several so-called nacelle test benches have been built in recent years that allow such tests to be carried out without being affected by wind. On these nacelle test benches, wind turbines are tested without a rotor (Figure 1, also called a nacelle). Here, the wind is simulated by a motor and hydraulic systems in the form of rotation and additional loads [1].

To determine the efficiency of a wind turbine, not only does the amount of electricity produced at the end have to be determined, but the amount of input power applied has to be measured as well. This input power can be defined as either electrical power via the motor current or mechanical power in the form of the torque occurring at the hub of the wind turbine. Determining the efficiency solely by means of electrical power measurement with a deviation of ± 5 % due to the complex structure of a wind turbine is neither precise nor reliable. With a combination of traceable mechanical power measurement directly at the hub of the wind turbine and electrical output measurement, the efficiency can be determined with improved precision and reliability.

As part of the EMPIR project titled “MN·m Torque”, the torque measurement in such a test bench was traced to the national standard for the first time. For this purpose, a 5 MN·m sensor (Figure 2) was developed and established as a transfer standard. However, the magnitude of the torque was not the only challenge. Since the torque in wind turbines is created under rotation, it must also be measured under rotation. However, currently available torque calibration standards only take static torque measurement into account. For this reason, a calibration procedure has been developed especially for nacelle test benches with varying rotational speeds [2]. This calibration procedure was tested on the test bench of the Center for Wind Power Drives at RWTH Aachen University. This presents the first time that torque measurement in the measuring range up to 1.5 MN·m under rotation has been traced and that the associated measurement uncertainty has been determined [3].

One result of the project is a “Good Practice Guide” that describes torque calibration under rotation in nacelle test benches using torque transfer standards in the MN·m range. The transfer standard that has been developed will remain available for measurements even after completion of the project.

Figure 1, left: Wind turbine on the nacelle test bench of the Center for Wind Power Drives at RWTH Aachen University with PTB torque transfer standard. right: 5 MN·m torque transfer standard including data acquisition and telemetry system for use under rotation.

Literature:

[1] G. Foyer, S. Kock 2017 Measurement uncertainty evaluation of torque measurements in nacelle test benches in Proc. of TC3 IMEKO, Helsinki, Finland, Opens external link in new windowLink
[2] P. Weidinger, G. Foyer, S. Kock, J. Gnauert, R. Kumme 2018 Development of a torque calibration procedure under rotation for nacelle test benches in IOP Conf. Series 1037, Opens external link in new windowLink
[3] P. Weidinger, G. Foyer, S. Kock, J. Gnauert, R. Kumme 2018 Procedure for torque calibration under constant rotation investigated on a nacelle test bench in Proc. of Sensoren und Messsysteme 2018, Nürnberg, Germany, Opens external link in new windowLink

Contact:

Paula Weidinger, FB 1.2, AG 1.22, E-Mail: Opens window for sending emailpaula.weidinger(at)ptb.de