7. CONCLUSIONS
A design tool for risk evaluation was proposed to facilitate qualitative failure analysis of fluid power pitch systems. Based on reliability field studies of multi-megawatt wind turbines, it was shown that the pitch system is a large contributor to turbine down time and failure rate. The field data also showed a large number of failures for valves and accumulators. Additionally, leakage faults where seen to be significant. However, the field data did not reveal details of the root causes for failures, and thus, the need for additional failure analysis was established. Past research was shown to focus on quantitative reliability calculation for pitch systems, and only few failure modes were previously considered. During the literature review for this work, no past papers were encountered on creating the qualitative basis. This emphasized the need for the design tool. Thus, the objective of the paper was to provide industry development with a systematic framework of known methods to aid in selecting proper system architecture in the early design phase.
The presented design tool was based on the industry standard failure analysis methods FTA and FMECA. FTA was chosen as it enables systematic discovery of relevant failure modes and shows fault propagation through the system. A procedure for performing FTA for fluid power systems was presented, which improves its usefulness when comparing different concepts. FMECA was used to construct a detailed overview of correlation between root cause, failure mode and system effect where the latter two were given by the FTA. The FMECA build upon prior research concerning wind turbine concepts and allowed risk evaluation via the RPN. Guidelines for determining RPN was adapted to pitch systems and standards were established in selecting the severity, occurrence and detection scores.
