Saifi system average interruption frequency index

Electric System Reliability Indices

Introduction

include measures of outage duration, frequency of

outages, system availability, and respon se time.

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 concer n.

T his course 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.

Sustained Interruption –
Any inter ruption not classified as a momentary event.

T he most common distribution indices include the System Average Interruption Duration Index (SAIDI), Customer Average Interruption Duration Index (CAIDI), System

SAIDI = Σ(ri * Ni ) / NT

Where,
S AIDI = System Average Interruption Duration Index, minutes. Σ = Sum mation function.

As you can see from the table, the first outage was at 9:53 in the morning and 10
c ustomers where out of service for 90 minutes (1.5 hours). Therefore, the customer hours are 10 * 1.5 or 15 hours.

///Table 1

* 60 = 21,408 customer-minutes.

The SAIDI is,

daily values.

B. Customer Average Interruption Duration Index (CAIDI)

CAIDI = Σ(ri * Ni ) / Σ( Ni )

CAIDI = Customer Average In terruption Duration Index, minutes.

The customer-minutes are 21,408 and 1,104 customers were i nterrupted on the 28th (See

T able 1). Therefore, the CAIDI is,

C. System Av erage 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 un der 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,

F rom our previous examples, on the 28th there were 1,104 customers interrupted during 5 separate events and the total number of customers served by the utility is 50,000 so the SAIFI is,

S AIFI = 1,014 / 50,000

S AIFI = 0.428 / 21.1 = 0.020

D . Customer Average Interruption Frequency Index (CAIFI)

E . Customer Interrupted per Interruption Index (CIII)

T he 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,

A total of 1,104 customers were interrupted during five separate events and the total number of customers served by the utility is 50,000, so the CIII is,

T he 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 momentar ily 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 affec ted, and dividing by the total number of customers served. MAIFI is,

MAIFI = Σ( IDi * Ni ) / NT

Assume the system had six momentary su bstation breaker operations on the 28th. One b reaker operated twice affecting 1,015 customers and four other breakers operated once affecting 867, 2,005, 1,500, and 1,330 customers. The utility serves 50,000 customers. What is the MAIFI?

The sum of the break er operations times the number of customers affected is,

MAIFI = 0.128

On average, the customers experienced 0.128 momentary interruptions on the 28th.

T = Time period under study, hours.

ri = Restoration time, hours .

From Table 1, the customer-hours are 356.80. Since only one day, the 28th, is unde r

st udy the study period is 24 hours. So, the ASAI is,

From the ASAI, we see that the system ha s an average availability of 99.97% for the 28th.

S ome utilities have set an ASAI goal of “four-nines” or 99.99% reliability. A “four-

public utility commissions are mandating reliability standards based on the indices and

An IEEE working group has proposed a statistical approach to the problem to define Major Event Days or MEDs. Their recommendation, known as the Beta Method, works like this,

• A Major Event Day (MED) is any day that exceeds a daily SAIDI threshold called TMED.

Where,
TMED = Major Event Threshold, minutes.
e = Exponential function, 2.718.

α = Log-average of the data.

From the data in table 2, we see that the total of the natural logarithms of the data is –99.348 and with 29 days the log average is -3.4258. Using an Excel spreadsheet and the standard deviation function (STDEV) the log-standard deviation is 2.4413. With this data, TMED is,

TMED = e (-.3.4258 +2.5 * 2.4413)

With the advent of performance-based rates, utilities are taking a closer look at their reliability data and working to improve their indices. Studies have shown that reliability is greatly affected by lightning, circuit length, circuit density, and system voltage. There is an almost direct correlation between lightning and reliability (the more lightning flashes, the lower the reliability), as well as circuit length, with longer circuits have more interruptions. Some data also suggests that utilities with higher system voltages tend to have more outages, but this may be related to the length of the circuit more than the voltage.

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.

© 2004 Lee Layton. All Rights Reserved.