UPS reliability analysis with non-exponential duration distribution

In this paper, the reliability performance of uninterruptible power systems is studied. After the description of the protection system against anomalous conditions, a brief failure mode analysis is performed in order to define the fault tree referring to compensator output voltage. An analytical model able to deal with non-exponential life and repair time distributions is developed, using semi-Markov processes and assuming stochastic independence for the components of the system under study. The mean time between failure (MTBF) and the mean time to restoration of the compensator output voltage are then calculated exactly. Finally a mechanical bypass switch, which connects the load to the mains directly during long failures, is taken into account in the MTBF calculation via a simple approximation. Monte Carlo simulations are performed to validate the results of the analytical model and of the approximation.