Reliability and Availability Modeling of Uninterruptible Power Supply (UPS) Systems using Monte-Carlo Simulation

Inhalt:
This paper has introduced the Monte-Carlo simulation concept to estimate the Reliability and Availability values of the Uninterruptible Power Supply (UPS) systems. Using this method, the failure rates (λ), and availability (A) of the major components in the UPS are simulated by the use of random number generators that produces deterministic reliability parameters that try to mimic results that would be obtained from field trials. Reliability parameters such as the system’s Failure Rates (λ), Mean Time Between Failures (MTBF), System Availability (A), and System Unavailability (W), can be achieved. The obtained parameters can determine the reliability level of the UPS systems. In this paper, the method are applied to five UPS configurations (online with and without bypass, offline, line-interactive with buck/boost transformer and line-interactive with ferroresonant transformer), and comparisons on the results obtained from this method were made to the results from other reliability estimation method such as the Reliability Block Diagram (RBD), Boolean Truth Table and Field Data estimation method. All the four methods agreed that there was a slight improvement in connecting a bypass line to the Online UPS. The results also confirmed that for the Line-Interactive UPS, configuration with ferroresonant transformer gives far better results in system’s reliability terms compared to buck/boost transformer scheme. The numerical results obtained from the proposed method have shown that in order to increase the UPS system’s reliability, the bypass line should be taken from other source rather than the primary utility line that supplying the system. If this bypass supply is taken from the same source as the utility, the system reliability will be unchanged. However, bypass line provides huge improvements in terms of system’s availability, whether it is connected from the same or different source from the utility. The results from this study also suggested that the bi-directional converter modules in the Line-Interactive UPS that acts as a rectifier (charging the batteries) in normal operation and as an inverter (supplying ac power to the loads) during utility power failure, can increase the systems’ reliability.