Most power system reliability studies assume a constant failure rate and an exponentially-distributed repair time. For redundant (parallel) components, using the average component failure rates gives erroneous results. The effect of time-varying failure rates is discussed for adverse weather and for component aging. A primary-selective supply is used to introduce the phenomena. Especially adverse weather is shown to significantly affect the interruption frequency. Both adverse weather and component aging require additional data collection efforts. Including component aging also requires a serious modelling effort.
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