In view of the frequent and costly failures of power converters in wind turbines, a large consortium of research institutes and companies has joined forces to investigate the underlying causes and key driving factors of the failures. This paper presents an exploratory statistical analysis of the comprehensive field data provided by the project partners. The evaluated dataset covers converter failures recorded from 2003-2017 during almost 7400 operating years of variable-speed wind turbines of different manufacturers and types, operating at onshore and offshore sites in 23 countries. The results include the distribution of failures within the converter system and the comparison of converter failure rates among turbines with different generator-converter concepts, from different manufacturers as well as from different turbine generations. By means of combined analyses of converter-failure data with operating and climate data, conditions promoting failure are identified. In line with the results of a previous, much smaller study of the authors, the present analysis provides further indications against the wide-spread assumption that thermal-cycling induced fatigue is the lifetime-limiting mechanism in the power converters of wind turbines. Instead, the results suggest that humidity and condensation play an important role in the emergence of converter failures in this application.

Power converters are among the most frequently failing components of wind turbines. Despite their massive economic impact, the actual causes and mechanisms underlying these failures have remained in the dark for many years. In view of this situation, a large consortium of three research institutes and 16 companies, including wind-turbine and component manufacturers, operators and maintenance-service providers has joined forces to identify the main causes and driving factors of the power-converter failures in wind turbines to create a basis for effective remedial measures. The present paper summarizes and discusses the results of this research initiative, which have been achieved through the evaluation of converter-specific failure and operating data of a large and diverse worldwide wind-turbine fleet, field measurements as well as post-mortem investigation of returned converter components. A key conclusion of the work is that the thermal-cycling induced fatigue of bond-chip contacts and die-attach solder, which is a known issue in other fields of power-electronics applications and which has been widely assumed to be the principle damage mechanisms also in wind turbines, is no relevant contributor to the observed converter failures in this application. Instead, the results indicate that environmental factors such as humidity and contamination but also design and quality issues as well as human errors play an important part in the incidence of these failures.

Rotor blade bearings enable rotor blades to pivot about their longitudinal axis and thus control the power output and reduce the loads acting on the wind turbine. Over a design period of 20 years, rolling bearings are exposed to frequent oscillation movements with amplitude ratios of $x/2b$ > 1, especially due to new control concepts such as Individual Pitch Control, which can lead to wear and a reduction in service life. The objective of this paper was to identify the dominant wear mechanisms and their consequences for the operation of oscillating bearings. Oscillating experiments with an increasing number of cycles on the angular contact ball bearings of two different sizes (types 7208 and 7220) show that the damage initiation starts with adhesive and corrosive wear mechanisms, which result in a sharp increase in the torque as well as the wear volume on the bearing raceway. As the number of cycles increases, an abrasive mechanism occurs, resulting in a lower slope of the wear curve and a smoothing of the resulting wear depressions. The wear and torque curves were evaluated and classified using an energy-wear approach according to Fouvry.

Grease-lubricated rolling element bearings can suffer from wear due to lubricant starvation under certain oscillating operating conditions. Especially for large-scale slewing bearings, such as blade bearings in wind turbines, experimental investigations are complex compared to small-scale reference testing. For an easier manner of testing, it is desirable to know whether the results of small-scale testing are applicable to larger-sized bearings. In this work, three different bearing types were tested and compared to already published results from a small-scale ACBB with a pitch diameter of 60 mm. The newly tested bearing types comprise a downscaled blade bearing (4-point contact double row ball bearing) with a pitch diameter of 673 mm, a small-scale CRTB with a pitch diameter of 77.5 mm and another ACBB with a pitch diameter of 95 mm. Qualitatively, all tested bearings show similar wear behaviour in terms of friction energy when operation parameters are varied. With higher oscillation frequency, damage becomes more severe. The oscillation amplitude shows three distinctive regimes. Within the range of small amplitudes, an increase in amplitude leads to more pronounced damage. We observe a threshold amplitude where this is no longer the case; a further increase in amplitude counteracts wear initiation until a final threshold is reached, beyond which no more wear is observed. These findings are in accordance with the reference results of the small-scale ACBB. Direct comparison between point and line contact shows that the latter is more prone to wear initiation under grease-lubricated, oscillating operating conditions. Furthermore, a previously introduced empirical number shows good performance in assessing critical operating parameters of the different bearing types. Specifically, harmful operating conditions can be classified for all studied bearing types with an accuracy of 78%. This method can be useful to assess operating conditions of greased, oscillating, rolling element bearings, e.g., to assess different pitch controllers or designs of slewing bearings.

The characteristics of a pitch controller determine how the wind turbine reacts to different wind conditions. Control strategies like individual pitch control are known for their ability to reduce the amplitudes of load cycles of the structures of the wind turbine while influencing the operation conditions of the blade bearings in a challenging way. However, the control strategy is not the only influencing factor with respect to failure modes of blade bearings like wear and raceway fatigue. The site-specific and stochastic wind conditions can cause wear-critical operating conditions, which are usually not reflected in the rather short time frames of aeroelastic simulations. This work analyses exemplary wind and operating conditions of one specific site regarding their influence on wear in blade bearings. It is based on measured wind conditions and the modeled behavior of the individual pitch-controlled IWT-7.5-164 reference wind turbine with respect to its pitch activity. The simulation data are used to determine the longest period of uninterrupted wear-critical operation and create a test program based on it for scaled and real-size blade bearings. Experimental results based on this test program show that wear-critical operation conditions can occur during normal operation of a wind turbine and cause mild wear damage to the bearing raceways.

In this paper, a detailed model of a 2 MW wind turbine system with a permanent magnet synchronous generator (PMSG), including the aerodynamics, the mechanics, and the electrical domain is presented. It is then used to examine the influences of the level of detail in the model of a wind turbine system on the calculated lifetime consumption of the converter's power modules. This is done by comparing the lifetime calculations of the detailed model with the ones of simplified models. While the results show that the dynamic effects of the wind turbine components do not essentially affect the life expectancy of the generator-side converter, it has to be considered that additional control measures like the damping of drive train oscillations might do.