Elevating operations practices to be best in class
An example - Identifying the improvement areas
One of the primary benefits of an outage management system mentioned above is reducing
service unavailability (known as the System Average Interruption Duration Index - SAIDI).
SADI = Total Customer Minutes Interrupted / Total Customer Served
SAIDI is a standard in the United States for measuring the ability of electric utilities to minimize
the effects of service interruptions on their customers. It is typically used as a summary of the
utility's entire restoration process over a period of time and is calculated as follows:
When you look at the components of SAIDI you will see the following:
SAIDI = Sum(# Customers Affected X Length of Interruption / Total Customers Served
While the SAIDI index itself is a standard, the basic definitions of each of the factors in the
numerator of the equation are not. The numerator represents many different types of interruption
and restoration processes. The processes reflected by the numerator are numerous and unique to
the types of interruptions that occur, as well as the characteristics and types of information
available to the utility at the time. Figure 1 reflects some of the inconsistencies typically found in
the numerator and denominator of SAIDI.
Figure 1 - Defining Standards for SAIDI
The first challenge is to define the components of the index so that reporting is consistent within
and across electric utilities. A utility's ability to define the components listed above is often
based on its level of automation and the results can be advantageous or disadvantageous. In some
cases, the utility that has better reporting capability will look "worse" than the utility that has
little or no automation. For example, a utility that does not have SCADA will use the first
customer call as an outage start time instead of using the breaker operation time. Likewise,
feeder outages appear longer in duration for a utility that has SCADA and that uses breaker
operation time in automated outage management systems (OMS).
Targeting improvement areas
'If you can't measure it, you don't understand it'
It is not always necessary to wait for the application of technology to address many of these
issues and to start measuring and defining the potential improvement areas. Once the criteria for
calculating service unavailability has been determined, the steps to identifying improvement
areas are as follows:
- Stratify the types of outages by their contribution to service unavailability (see Figure 2)
- Target areas for improvement that can be addressed immediately and easily (pick the low
hanging fruit)
- Define the components of the current processes and measure them. Identify improvement
areas.
- Change the processes and measure again (this step may still not involve technology yet)
- Apply technology where appropriate and measure again.
Steps 1 & 2 - Stratify the Interruptions & Target Improvement Areas
Figure 2 shows a typical distribution of the types of outages utilities experience over a period of
time (several months or a year).
Figure 2 - Typical Stratification of Outages
Figure 2 categorizes outages by the protective device (feeder breaker, lateral fuse, etc.) that
opened, and organizes them by how much they contributed to service unavailability. The outages
are measured against total customer minutes interrupted (CMI) on the left and the percent of the
total CMI on the right. The line with the dots reflects the cumulative percentage for each
category of interruption. Not surprisingly, the infrequent feeder and recloser outages are typically
the biggest contributors to service unavailability because of the number of customers they affect.
Lateral, transformer, and other outages, while more frequent, have a lesser impact on overall
unavailability. This paper focuses on feeder and recloser outages because they offer the biggest
opportunities for improvement in most utilities. Reducing the duration of an outage on a feeder
with 1000 customers by one minute will have a more dramatic impact on service unavailability
than reducing a lateral or transformer outage by one minute (assuming non-feeder interruptions
remain somewhat level).
