In this paper, the efficiency of a 2.75 MW nacelle drivetrain on a test bench was determined traceably at various load points. For this purpose, so-called transfer standards for mechanical and electrical power measurement were additionally installed and integrated into the nacelle drivetrain. As torque measurement contributes greatly to the overall uncertainty, the static torque calibration was expanded to include additional influences present under rotation. An overall system efficiency of 89% was measured in the high torque and high speed range. The relative expanded measurement uncertainty for efficiency determination was between 0.30% and 0.72% over the entire operating range. Both the efficiency and the relative expanded measurement uncertainty were calculated for each operating point.

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.

In this paper, the efficiency of a 2.75 MW nacelle is compared using calibrated and uncalibrated measurement devices. The results show that especially calibration of mechanical input is important, as the torque measurement is off by about 5 % and results in significantly lower system efficiencies.

All around the world, new plans and strategies are being implemented for installing energy-efficient wind turbines in order to increase renewable energy capacities and meet the demand for clean energy. Advances in wind turbine technologies and their extremely sophisticated systems have led to an increasing need for proper efficiency determination methods, both in the field and on test benches, to keep up with the accelerated deployment of wind turbines around the globe. This paper examines the feasibility and practicability of new wind turbine efficiency determination methods on test benches that are based on international standards for electrical machines. Test methods and the static calibration certificates of the employed specialised instruments and sensors were used to evaluate the efficiency and associated measurement uncertainties in ambient conditions. Furthermore, the results were compared with those obtained through the innovative iso-efficiency method, which comprehensively maps the wide operating range of wind turbines.