Reliability is an abstract term meaning endurance, dependability, and good performance. Reliability means the ability of a system to perform the function it is designed for under the operating conditions encountered during its projected lifetime.
System reliability is not same as power quality. System reliability pertains to sustained interruptions and momentary interruptions. Whereas power quality involves voltage fluctuations, abnormal waveforms, and harmonic distortions. Greater than five minutes of interruption is generally considered a reliability issue, and less than five minutes are a power quality concern.
Distribution Reliability Indices:
Standard number P1366 of IEEE defines the generally accepted reliability indices and lists several important definitions for reliability. The most common distribution indices include:
System Average Interruption Duration Index (SAIDI)
Customer Average Interruption Duration Index (CAIDI)
System Average Interruption Frequency Index (SAIFI)
Customer Average Interruption Frequency Index (CAIFI)
Momentary Average Interruption Frequency Index (MAIFI)
Customers Interrupted per Interruption Index (CIII)
Average Service Availability Index (ASAI)
A. System Average Interruption Duration Index (SAIDI)
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 a 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. The formula is,
SAIDI = Σ (ri * Ni) / NT
Where,
SAIDI = System Average Interruption Duration Index, minutes.
Σ = Summation function.
ri = Restoration time, minutes.
Ni = Total number of customers interrupted.
NT = Total number of customers served.
B. Customer Average Interruption Duration Index (CAIDI)
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 is,
CAIDI = Σ (ri * Ni) / Σ ( Ni )
Where,
CAIDI = Customer Average Interruption Duration Index, minutes.
Σ = Summation function.
ri = Restoration time, minutes.
Ni = Total number of customers interrupted.
C. System Average Interruption Frequency Index (SAIFI)
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 divided the total number of customers interrupted by the total number of customers served. SAIFI, which is a dimensionless number, is,
SAIFI = Σ(Ni ) / NT
Where,
SAIFI = System Average Interruption Frequency Index.
Σ = Summation function.
Ni = Total number of customers interrupted.
NT = Total number of customers served.
D. Customer Average Interruption Frequency Index (CAIFI)
Similar to SAIFI is CAIFI, which is the Customer Average Interruption Frequency Index. The CAIFI measures the average number of interruptions per customer interrupted per year. It is simply the number of interruptions that occurred divided by the number of customers affected by the interruptions. The CAIFI is,
CAIFI = Σ ( No ) / Σ( Ni )
Where,
CAIFI = Customer Average Interruption Frequency Index.
Σ = Summation function.
No = Number of interruptions.
Ni = Total number of customers interrupted.
E. Customer Interrupted per Interruption Index (CIII)
The Customer Interrupted per Interruption Index (CIII) gives the average number of customers interrupted during an outage. It is the reciprocal of the CAIFI and is,
CIII = Σ( Ni ) / Σ( No )
Where,
CIII = Customer Interruption per Interruption Index.
Σ = Summation function.
No = Number of interruptions.
Ni = Total number of customers interrupted.
F. Momentary Average Interruption Frequency Index (MAIFI)
The MAIFI is the Momentary Average Interruption Frequency Index and measures the average number of momentary interruptions that a customer experiences during a given time period. Most distribution systems only track momentary interruptions at the substation, which does not account for pole-mounted devices that might momentarily interrupt a customer. MAIFI is rarely used in reporting distribution indices because of the difficulty in knowing when a momentary interruption has occurred. MAIFI is calculated by summing the number of device operations (opening and reclosing is counted as one event), multiplying the operations by the number of customers affected, and dividing by the total number of customers served. MAIFI is,
MAIFI = Σ (IDi * Ni ) / NT
Where,
MAIFI = Momentary Average Interruption Frequency Index.
Σ = Summation function.
IDi = Number of interrupting device operations.
Ni = Total number of customers interrupted.
NT = Total number of customers served.
G. Average Service Availability Index (ASAI)
The Average Service Availability Index (ASAI) is the ratio of the total number of customer hours 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 (8,760 hours), but can be calculated for any time period. The ASAI is found as,
ASAI = [1 – (Σ (ri * Ni ) / (NT * T))] * 100
Where,
ASAI = Average System Availability Index, percent.
Σ = Summation function.
T = Time period under study, hours.
ri = Restoration time, hours.
Ni = Total number of customers interrupted.
NT = Total number of customers served.
Understanding how to correctly apply the IEEE standard reliability indices is the first step in measuring the reliability of an electric distribution system. 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.
Keywords: Electric Power System, Reliability, Technical Writing, SAIFI, SAIDI, Distribution System