Reliability and availability comparison of energy chain architectures

Reliability and availability calculation is a useful method used for energy chain architectures comparisons. In this work, a VRLA battery reliability taking into account age, working temperature, floating voltage regulation, and autonomy of the battery strings, is proposed. Others components failures rates are gathered from the field. Then an availability calculation program for architectures, with and without generator, is developed. This program takes into account the used battery autonomy and the maintenance conditions of the energy station. Finally, reliability data with high level of inaccuracy are adjusted to fit observed unavailability results, during a five year period on France telecom energy station population. Based on this program, a sensibility study is done to determine the influence of the battery autonomy and the maintenance conditions. Finally, battery autonomy rules are derived from this study, to fulfill a very common unavailability target of 5 mn/year with the minimum of maintenance conditions. The necessity for a back-up generator with a switchover system is also discussed depending on site power AC grid quality.