On modelling of grouped reliability data for wind turbines

Energy generation by wind turbines (WTs) has increased enormously during the last decade, and plans for future expansion, in particular large-scale offshore developments, emphasize the need to study reliability of WTs. Substantial data sets on occurrences of failures in WTs are available, but unfortunately they only provide grouped data with little information on individual turbines or maintenance activities. In this paper, we consider the use of basic non-homogeneous Poisson process models, in particular the power law process, to try to deduce from these data the change, through the years, in reliability of WTs and some critical subsystems. Unfortunately, it turns out that there are major problems that avoid clear conclusions to be drawn, as such the main contribution of this paper is a discussion on choice of the use of either calendar time or total time on test as the key time variable in reliability models, and some advice on future data collection, which will be equally important for a range of industrial activities.