Power converter reliability: Field measurements illuminate climatic stress

Power converters are among the most frequently failing components of wind turbines. The causes remained unexplained for many years. Seasonal failure patterns point to environmental influences and shift the focus to the climatic conditions to which the power electronics are exposed in the field.

Each year, the converter system of, on average, every second wind turbine suffers damage which requires repairing. These frequent failures result in considerable repair costs and noticeable yield losses, especially offshore. However, more reliable converters can only be developed if the prevailing failure causes and mechanisms are identified. For a long time, it was assumed that the main problem was the fatigue effects in the power electronics caused by the power and temperature cycles. However, field data analyses of an international fleet of wind turbines at Fraunhofer IWES have clearly shown that such damage mechanisms as seen in other applications do not, in fact, play a relevant role in wind turbines (Bartschat et al. (2017), Fischer et al. (2018)). Instead, pronounced seasonal failure patterns indicate climatic environmental influences and, in particular, humidity to be a critical stress factor (Fischer et al. (2019)). Yet the question remains of what levels of humidity and temperature are the converters in the turbines actually exposed to?

Our paper titled “Humidity in Power Converters of Wind Turbines — Field Conditions and Their Relation with Failures” (Fischer et al. (2021)), which was recently published in the open access journal energies, examines this question in detail. It summarizes the results from field measurement campaigns on 31 wind turbines from seven different manufacturers spread across three continents. The measured turbines are extremely diverse: they included both onshore and offshore turbines, turbines with converters in the nacelle or tower base as well as turbines with air- and water-cooled converters.

Figure 1: Sites in North America, Europe, and Asia where the field measurements were carried out; sample time series for seasonal variation of temperature and humidity in the converter cabinet and the environment around a turbine in India.

The results show that the climatic conditions are subject to considerable seasonal fluctuations not just in the environment around the turbines but also inside the converter cabinets (see also Fig. 1). It is noteworthy that the absolute humidity measured in the turbine or directly in the power cabinet barely differs from that of the environment around the turbine, not even in air-conditioned offshore wind turbines in which separate indoor and outdoor air flows are intended to prevent the ingress of sea air.

In light of the heat dissipation from the power electronics, it is not surprising that the air temperature in the converter cabinet is on average 5–30 Kelvin (K) higher than in the ambient air of the wind turbine. Consequently, the relative humidity in the cabinet air remains considerably below that of the turbine ambient air, typically by at least 20% on average over the course of the year. Nevertheless, the further analyses and results in our paper indicate that it is precisely the relative humidity in the cabinet which is key to failure generation. Furthermore, they suggest that even comparably low levels of relative humidity have an impact on reliability.

Figure 2: Frequency distributions for temperature and relative humidity in the converter cabinet of three different wind turbines (based here on the entire specified measurement period, i.e., not broken down by season): an onshore turbine in inland Germany with an air-cooled converter in the tower base (WT1), an offshore turbine in the German North Sea with a liquid-cooled converter in the nacelle (WT2), and an onshore turbine in India with a liquid-cooled converter in the tower base (Source: Fischer et al. (2021)).

The measurement and evaluation results make clear just how much the climatic stresses of the turbine converters differ in the field (see, e.g., Figure 2) depending on the location with its specific climatic ambient conditions, the design and operation of the turbine, and also the cooling principle used for the converters.

The field measurement of temperature and humidity conditions in wind turbine converters on three different continents proved valuable for a number of reasons: For the first time, it has enabled the climatic stresses on the converter to be characterized and has identified which factors these depend on. It has also provided important new insights into the causes of the frequent converter failures. Last but not least, the measurements form the basis for deriving improved test procedures for reliability qualification of converter systems of wind turbines and their components. In the next step, the latest findings on climatic converter stresses will be transferred to a test environment. A laboratory for the multimodal testing of converter systems is currently being set up in Bremen: climatic and electrical loads (up to 10 MW) will then be combined in the HIPE-Lab. Real load cycles, derived from field measurements, will be reproduced there in order to test and measure prototypes and existing converter systems under conditions typical for the application.

Sources:

Bartschat et al. (2018): Bartschat, A.; Broer, C.; Coronado, D.; Fischer, K.; Kucka, J.; Mertens, A.; Meyer, R.; Moriße, M.; Pelka, K.; Tegtmeier, B.; Weber, S.; Wenske, J., „Zuverlässige Leistungselektronik für Windenergieanlagen“, Abschlussbericht zum Fraunhofer-Innovationscluster Leistungselektronik für regenerative Energieversorgung, Fraunhofer-Verlag 2018, ISBN: 978-3-8396-1326-9.

Fischer et al. (2018): Fischer, K.; Pelka, K.; Bartschat, A.; Tegtmeier, B.; Coronado, D.; Broer, C.; Wenske, J., “Reliability of Power Converters in Wind Turbines: Exploratory Analysis of Failure and Operating Data From a Worldwide Turbine Fleet”, IEEE Transactions on Power Electronics, vol. 34, no. 7, pp. 6332-6344, July 2019, DOI 10.1109/TPEL.2018.2875005.

Fischer et al. (2019): Fischer, K.; Pelka, K.; Puls, S.; Poech, M.-H.; Mertens, A.; Bartschat, A.; Tegtmeier, B.; Broer, C.; Wenske, J., “Exploring the Causes of Power-Converter Failure in Wind Turbines based on Comprehensive Field-Data and Damage Analysis”, Energies 2019, 12, 593, DOI 10.3390/en12040593.

Fischer et al. (2021): Fischer, K.; Steffes, M.; Pelka, K.; Tegtmeier, B.; Dörenkämper, M., “Humidity in Power Converters of Wind Turbines-Field Conditions and Their Relation with Failures”, Energies 2021, 14, 1919, DOI 10.3390/en14071919

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