Proper functioning of the pitch system is essential to both normal operation and safety critical shut down of modern multi megawatt wind turbines. Several studies on field failure rates for such turbines show that pitch systems are a major contributor to failures which entails an increased risk. Thus, more reliable and safe concepts are needed. A review of patents and patent applications covering fluid power pitch concepts, reveals that many propose closed-type hydraulic systems. This paper proposes a closed-type concept with a bootstrap reservoir. In contrary to a conventional system where a common supply delivers to cylinder drives mounted at each blade, the concept using bootstraps are fully contained in the rotating hub and act as standalone actuators. Clear advantages of such systems are a reduction of high pressure leakage paths, but in turn more components are used for each supply circuit. To allow for comparison, this paper deals with a risk-based analysis of these two types of pitch concepts. The risk-based analysis is conducted according to a qualitative failure analysis method which is verified against recent field failure data for hydraulic pitch systems. The method combines Fault Tree Analysis and Failure Mode and Effect Criticality Analysis in a systematic framework that lowers the bias issues normally encountered for qualitative studies. Under the assumption of similar components, the results indicate an equal risk of the two concepts. A decreased reliability is seen for the bootstrap concept due to additional components in the supply circuit compared to the conventional system. It is noted that careful selection of high reliable pumps and relief valves may significantly reduce risk and increase reliability of the bootstrap concept.

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