Service Restoration Impacts

As electric utilities confront the challenge of reducing the duration of service interruptions (measured as “Customer Minutes of Interruptions” or CMI), there are a limited number of factors that are common among poor and marginally performing utilities:

• Weather is the primary differentiator in electric utilities meeting their reliability targets, particularly when they are in the midst of a comprehensive circuit protection program spanning a number of years. As the system infrastructure becomes “storm hardened”, the frequency of non-excludable storms is likely to increase, leading to an increase in number of outage cases, which, in turn, directly impacts outage duration. The figure to the right typifies the relationship between outage duration (measured as CAIDI minutes per day) and the number of outages experienced per day, indicating a critical point where CAIDI starts to increase exponentially as the number of outage cases increases (in this instance at approximately 35 outages per district.
• Inadequate circuit protection (combination of reclosers/sectionalizers on the distribution feeder backbone and fuses at the taps) increases the number of customer interruptions and makes it more difficult to isolate the location of the fault-producing event.
• The use of subtransmission lines for distribution poses a number of challenges with respect to service restoration:
  • Typically these circuits are not well-sectionalized,
  • Due to higher voltages, they require additional precautions and measures in effecting repairs,
  • Large number of customers over a large geographic area have the combined impact of an increased number of customer interruptions and a large area to patrol for the cause and subsequent repair/restoration
• Limited switching capability or capacity precludes electric utilities from fully deploying backfeed load switching (either automatically or manually).
• Geographic and demographic variables can lead to lower customer densities across larger coverage areas resulting in extended travel times over greater distances. Fewer customers within a coverage area generally result in lower priorities in terms of directing investments that would otherwise mitigate or eliminate outages.

The issues around inadequate circuit protection, the use of subtransmission for distribution and limited switching capability/capacity can all be addressed through a well-targeted investment strategy. Similarly, although an electric utility can do little about the occurrence of poor weather, there are a number of actions that can be instituted to improve service restoration (in addition to improving circuit protection and switching capability/capacity). Specifically, they can

• Systematize storm planning and pre-mobilization,
• Employ adaptive relaying, and
• Adopt an enhanced and/or optimized tree trimming program.

With respect to the challenge of reducing the outage durations in areas of lower customer densities, to the extent that some discretionary investment can be committed to these districts to better isolate the impact of outages and provide for a degree of automation, there will be improvement in reliability. In the interim, many utilities have experienced success with the cost-effective measure of installing Directional Fault Indicators throughout the system. This limits the patrol time necessary to locate the fault-causing condition.