Stefan Kock

Development of a 4 MW full-size wind-turbine test bench

The in-field validation of wind turbines behavior is very time consuming and cost intensive, especially when fault ride-through (FRT) tests are conducted. Full-size wind-turbine test benches allow a realistic operation of wind turbines in an artificial environment. Due to the independency of wind and grid conditions, the cost and duration of the test program and certification can be reduced. This paper presents the development of 4 MW full-size wind-turbine test bench following a multi-physics hardware in-the-loop (HiL) concept. With the currently installed test bench setup, a synchronization of the device-under-test converters is possible. Through the measurement results of the test programs conducted on the test bench, the capability of the test bench in replicating the field conditions is demonstrated. In addition, a time consuming efficiency measurement can be performed with the reduced duration on the test bench. This shows another main benefit of the test bench compared to the conventional test method for wind turbines.

Robustness Test for Wind Turbine Gearbox Bearings

This paper introduces an innovative approach for the creation of the robustness test against specific failures of the planetary and HSS bearings (e.g. classical fatigue, smearing, micro-pitting, and lip fractures) in the wind turbine gearboxes. The introduced Bearing Robustness Test (BRT) considers the system-dependent characteristics (e.g. drive train design, interaction between components, assembly process, lubricant aging) and real dynamic load conditions, frequencies and sequence. The creation of the BRT is based on field and simulation data. The core element is the simulative approach for the determination of the relation between external wind and grid loads on the one side and local loads of the bearing on the other side. BRT aims the mapping of the most critical, but real, field load situations in the bearing test rig. By means of the BRT it is possible to evaluate the robustness of bearing against specific field conditions in the early stage of the product cycle and consequently to enhance the quality and to reduce the failure rate of the bearing. 1. Motivation and Objectives The cost-efficiency of the wind turbines is reduced by the frequent failures of the main gearboxes. The main gearbox is responsible for nearly 60 % of the downtime of the wind turbine, see Figure 1. This fact is caused by a fluctuating and dynamic wind and grid loads, dynamic interaction between drive train components as well as the demand for high power density of the gearbox components [1]. Therefore, the rolling contact bearings are the most critical component of the main gearbox. They contribute to over 67 % of main gearbox failures according to the current research, see Figure 1. The planetary and high-speed shaft (HSS) bearings failures cause high reparation, replacement and service costs [3]. Bearing testing methods within the product development process could contribute to more reliable and robust gearbox bearings and thereby increase the availability of the wind turbines. Nowadays, the existing testing methods cannot completely simulate the real elastic surroundings and reproduce the complex load situation on the bearing as well as the complex interaction between the gearbox components. Consequently, it is difficult to reproduce specific wind turbine bearing failures in a realistic way [4]. To solve this challenge, a diversified consortium of a complete product value chain (bearing, gearbox and wind turbine manufacturer)1 under the lead management of Chair for Wind Power Drives (CWD) got together to develop a new bearing test rig designs (a planetary 1 Schaeffler, SKF, Timken, NTN, Siemens Winergy, ZF Wind Power, Eickhoff, Vestas, Nordex, Senvion Figure 1. Causes for wind turbine downtime. 3% 3% 8% 2% 25%

Friction as a major uncertainty factor on torque measurement in wind turbine test benches

Most of the existing multi-MW nacelle test benches (NTB) measure the MNm torque before load application system (LAS) to reduce the cross-talk effect of the multi-component forces and bending moments on torque measurement. This means that the friction torque of the LAS reduces the applied torque and consequently directly determines the input torque on the device under test (DUT). Therefore, the knowledge of the friction torque is necessary for the precise experimental investigations (e.g. efficiency measurement). At the beginning of this paper, a Computational Fluid Dynamics (CFD) simulation method for the determination of the friction torque of the LAS, which is suspended by hydrostatical plain bearings, is introduced. Subsequently, this method is validated with experimental results. Afterwards the friction torque of the LAS is quantified under different operation conditions (e.g. variation of rotational speed, multi-component load and temperature). Finally, the influence of the quantified friction torque on the uncertainty of the MNm measurement is compiled.