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The indices presented herein are used by utilities to measure their present performance against past history and to compare their performance to other utilities.
For the indices to have meaning there must be a standardized method of recording data. Applying the indices to routine outages is straight forward. The issue is how do utilities separate major events from routine occurrences? Almost all regulators allow utilities to separate major events from routine outages. What is a major event? An IEEE working group proposes a statistical approach to define Major Event Days or MEDs. It works like this:
TMED = e (a+2.5*B) e = Exponential function, 2.718 a = Log-average of the data B = Log-standard deviation of the data Major events must be removed from the base data so that reliability measures are not distorted and to help the utility track improvements to the electric system. The most difficult part of using reliability indices is knowing how to interpret the data and understanding what the performance indices are really saying about the system performance. The Average Service Availability Index (ASAI) is the ratio of the total number of customer hous that service was available during a given time period to the total customer hours demanded. This is sometimes called the service reliability index. The ASAI is usually calculated on either a monthly basis (730 hours) or a yearly basis (8760 hours), but can be calculated for any time period. ASAI formula:
ASAI = [1 - (E(ri * Ni) / (Nt * T))] * 100 ASAI = Average System Availability Index, percent E = Summation function T = Time period under study, hours ri = Restoration time, hours Ni = Total number of customers interrupted Nt = Total number of customers served In this calculation, the restoration time ri, is in hours instead of minutes The System Average Interruption Frequency Index (SAIFI) is the average number of times that a system customer experiences an outage during the year (or time period under study). The SAIFI is found by dividing the total number of customers interrupted by the total number of customers served. SAIFI formula:
SAIFI = E(Ni) / Nt SAIFI = System Average Interruption Frequency Index E = Summation function Ni = Total number of customers interrupted Nt = Total number of customers served Once an outage occurs the average time to restore service is found from the Customer Average Interruption Duration Index (CAIDI). CAIDI is calculated similar to SAIDI except that the denominator is the number of customers interrupted versus the total number of utility customers. CAIDI formula:
CAIDI = E(ri * Ni) / E(Ni) CAIDI = Customer Average Interruption Duration Index, minutes. E = Summation funtion ri = Restoration time, minutes Ni = Total number of customers interrupted The most often used performance measurement for a sustained interruption is the System Average Interruption Duration Index (SAIDI). This index measures the total duration of an interruption for the average customer during a given time period. SAIDI is normally calculated on either monthly or yearly basis; however, it can also be calculated daily, or for any other time period.
To calculate SAIDI, each interruption during the time period is multiplied by the duration of the interruption to find the customer-minutes of interruption. The customer-minutes of all interruptions are then summed to determine the total customer-minutes. To find the SAIDI value, the customer minutes are divided by the total customers. SAIDI formula: SAIDI = E(ri * Ni) / Nt SAIDI = System Average Interruption Duration Index, minutes. E = Summation Function ri = Restoration time, minutes Ni = Total number of customers interrupted Nt = Total number of customers served The Institute of Electrical and Electronic Engineers (IEEE) defines the generally accepted reliability indices in its' standard number P1366, "Guide for Electric Distribution Reliability Indices". IEEE-P1366 lists several important definitions for reliability including what are momentary interruptions, momentary interruption events, and sustained interruptions.
MOMENTARY INTERRUPTION - A single operation of an interrupting device that results in zero voltage. MOMENTARY INTERRUPTION EVENT - An interruption of duration limited to the period required to restore service by an interrupting device. This must be completed within five minutes. SUSTAINED INTERRUPTION - Any interruption not classified as a momentary event. The most common distribution indices include the System Average Interruption Duration Index (SAIDI), Customer Average Interruption Duration Index (CAIDI), System Average Interruption Frequency Index (SAIFI), Momentary Average Interruption Frequency Index (MAIFI), Customer Average Interruption Frequency Index (CAIFI), Customer Interrupted per Interruption Index (CIII), and the Average Service Availability Index (ASAI). We will review each of these indices with and examples of how to use them. The electric utility industry is moving toward a deregulated, competitive environment where utilities must have accurate information about system performance to ensure that maintenance dollars are spent wisely and that customer expectations are met.
To measure system performance, the electric utility industry has developed several measures of reliability. These reliability indices include measures of outage duration, frequency of outages, system availability, and response time. System reliability is not the same as power quality. System reliability pertains to sustained interruptions and momentary interruptions. Power quality involves voltage fluctuations, abnormal waveforms, and harmonic distortions. An interruption of greater than five minutes is generally considered a reliability issue, and interruptions of less than five minutes are a power quality concern. This page explains the reliability indices used to measure distribution system reliability, how to calculate the indices, and discusses some of the factors that influence the indices. |
