Reliability modelling of uninterruptible power supply systems using fault tree analysis method

The aim of this paper is to investigate the reliability parameters estimation method for the uninterruptible power supply (UPS) systems using the fault tree analysis (FTA) technique. FTA is a top-down approach to identify all potential causes leading to system failure. The computation of the system's failure probability is the main goal of this analysis, as this value can be used to calculate other important system reliability parameters such as failure rates, mean time between failures (MTBFs) and reliability. In this paper, the FTA method was applied to five different UPS topologies and the results obtained were compared and discussed in detail. By comparing the critical fault path of the system, it was found that the inverter failures contributed most significantly to the system failure. It was also found that the probability of failure of a UPS system can be reduced by the inclusion of bypass supply, given that the failure rate of the events that causing the failure of the bypass supply should be lower compared to the main utility supply. Finally, to validate the results obtained from this method, comparisons were made to the results from other methods such as the reliability block diagram (RBD), Boolean Truth Table, probability tree, Monte-Carlo simulation and field data reliability estimation methods.