Outage Impact Index (OII): A New Reliability Metric for Electric Grid Transmission Systems



According to the North American Electric Reliability Corporation (NERC), the definition of a system’s reliability is its ability to withstand disturbances and meet consumer demands consistently. The reliability of the transmission system of the electric power system must be high because a sudden loss can negatively impact millions of people. Furthermore, detailed analysis of system reliability may reveal vulnerable areas in the transmission network by establishing chronological system performance trends. Although a variety of indices have been proposed for assessing reliability of the transmission system, a knowledge gap exists regarding effective ways in which both frequency and duration of an outage can be simultaneously considered for evaluating the system’s reliability.


Invention Description

Researchers at Arizona State University have developed the Outage Impact Index (OII), which establishes chronological outage frequency and duration trends for different voltage levels and outage categories across different years. As such, it can serve as a baseline for other reliability indices such as those already proposed by NERC. System risk evaluation carried out using OII can also help power utilities develop better corrective actions (in relation to operation practices, maintenance strategies, and spare management). The OII compares outage severity in terms of outage percentage per element as well as downtime, where downtime is expressed as a percentage of the service period considered. The service period is set to one year (8,760 hours), which then becomes a common base for all types of outages. For a given voltage level, v, and outage category, a, OII is mathematically defined as:


OIIa,v = { Na,v / Tv }*{ ITa,v / ETv },


where Na,v is the number of outages for a particular category and voltage, Tv is the total number of elements for the given voltage category, ITa,v is the outage duration for a particular category and voltage, and ETv is the exposure time for the study period (8,760 hours).


Potential Applications

•       Electric grid transmission systems


Benefits and Advantages

•       Uniquely emphasizes outage frequency and duration

•       Sufficiently generic for application in any power utility

•       Provides valuable insight for improvement and risk management strategies 


Laboratory Homepage of Professor Anamitra Pal


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