Failure rates of main components. In section A1 the technical availability was introduced and it was stated that this availability is limited mainly because of wind turbine failures. But it was also declared that the technical availability does not give any hints on the reliability of the wind turbine components because there is no direct relation between breakdown time and resulting costs. A very good overview about availability of wind turbine components is given by the Royal Institute of Technology (KTH) Stockholm [49] in a survey of failures in wind turbines during 1997 - 2005. This reference presents results from an investigation of failure statistics from sources from Sweden, Finland and Germany. Gearbox Figure A.2: Distribution of numbers of failures [49] Figure A.2 shows the percentage distribution of failures that occurred during the years 2000 - 2004. First of all one can see that most failures are linked to the electrical system followed by sensors, and blade / pitch components. The larger main components e.g. gearbox and generator have a minor percentage on the failures. However, Figure A.2 does not give hints about the importance of the component failures. Therefore, information about the breakdown time that is caused by each component failure is needed. This kind of information is included in Figure A.4. One can see that especially the components that have a larger failure rate, like the electric system, do cause smaller down times for each failure. In contrast, the main components that have a smaller amount of failures like the gearbox, or especially the drive train do cause a long wind turbine downtime for each failure. The different downtimes are on the one hand caused by the needed repair effort for the components but also caused by availability of replacement parts and the capacity of personnel. average downtime per failure hours 350 300 250 200 150 100 50 0 Drive Train Yaw system Gears Generator Control system Mechanical brakes Electric system Structure Blades / Pitch Entire Unit Sensors Hydraulics Hub Figure A.3: Average downtime in case of component damage [49] Comparing the failure rates of the components, see Figure A.2, with the average downtime per case of damage, see Figure A.3, the reliability of the wind turbine components can be evaluated in another way. Therefore Figure A.4 shows the distribution of downtime per component failure for the years 2000 - 2004. One can see that the gearbox that have a minor percentage of failures, do cause the longest wind turbine...
