Distinguishing Among the Sources of Electric Service Interruptions

More accurate information is needed for utilities, regulators, and policymakers in assessing the reliability of the U.S. electric power system. This work focuses on recent efforts by the Institute of Electric and Electronics Engineers (IEEE) Distribution Reliability Working Group (DRWG) and the U.S. Energy Information Administration (EIA) to assess reliability performance of distribution systems separately from transmission systems. When reliability is measured using the system average interruption duration index (SAIDI), we find the distribution system is responsible for 95% or more of all interruptions. When reliability is measured using the system average interruption frequency index (SAIFI), we find the distribution system is responsible for 91% or more of all interruptions. Major events do not appear to change these results. Based on information collected by IEEE, we find that these trends have been fairly stable in recent years. We hope this work facilitates dialogue among utilities and their regulatory agencies and state or local government that ensures further enhancements or improvements can be informed by more detailed reliability metric information. We believe this work informs discussion regarding how much and in what part of the power system to invest in when assessing improvements in electric reliability.