Reliability Indicators

The Bulk Power System Severity Risk Index (SRI) is a daily metric where transmission loss, generation loss, and load loss events aggregate into a single value that indicates performance of the Bulk Electric System (BES). The SRI values range from zero to 1000. Zero is a theoretical condition where no transmission, generation or load loss occurred during the entire day. To the corollary, a value of 1000 would represent a situation where every transmission, generation and load5 was lost during the entire day.

About This Metric

Bulk Power System SRI Calculation

SRI formula shows the aggregation of the three components representing a daily transmission loss, a daily generation loss and a daily load loss into a single daily Severity Risk Index. The load loss component of SRI is based on data collected from the IEEE Power and Energy Society (PES) Distribution Reliability Working Group (DRWG) Benchmark Study7 as described in the 2014 SRI Enhancement whitepaper. The Transmission Component Changes and Generation Component Changes sections are discussed below.

SRIbps  = [(RPL) × wL × (MWL) + wT × (NT) + wG × (NG)] X 1000


SRI = Severity Risk Index for a given day,

wL = 60% - weighting of the load loss component,

MWL = normalized MWL load loss due to transmission or generation sources (MW) for the day;

 SRI Equation.png


MWPeak = daily peak load (MW is aggregated at the NERC level obtained by FERC

TotalC/D = Total Customer served for the day obtained from IEEE benchmark data

CI = Customers Interrupted due to transmission or generation sources for the day obtained from the IEEE PES Distribution Reliability Working Group (DRWG) benchmark data.

RPL = load Restoration Promptness Level:

RPL = 1/4, if TCAIDI < 50,

RPL = 2/4, if 50 <= TCAIDI < 100,

RPL = 3/4, if 100 <= TCAIDI < 200,

RPL = 4/4, if TCAIDI >= 200,

TCAIDI = Transmission (or Generation Source) Customer Average Interruption Duration (in minutes) obtained from IEEE PES DRWG benchmark data.

wT = 30% - weighting of the transmission component,

LossT = daily transmission loss calculated as normalized equivalent MVA of transmission elements lost on a given day, obtained from TADS inventory and outage data ,

wG = 10% - weighting of the generation component,

LossG = daily generation loss calculated as normalized MW lost, obtained from GADS inventory and outage data.


Additional information on formula and weighting factors above is available in the Refinement to Severity Risk Index


​Metric Definition Document

SRI Enhancement Whitepaper