STEP 3. DEVELOPING THE PROGRAM PLAN 13 undergrounding strategy selected will dictate if these efforts are readily visible in the overall SAIFI averages and potentially mask the impact of these efforts. SAIFI is normally calculated system-wide and, if possible, targeting the SAIFI calculation on the geography where undergrounding strategies are taking place is a better way to measure performance impact. If this targeted measure is not possible, another will likely have to be used to demonstrate impact. CAIDI (Customer Average Interruption Duration Index) – Measures reliability as the average time to restore service to an interrupted customer; calculated as Customer Minutes Interrupted (CMI) divided by Customers Interrupted (CI) or SAIDI divided by SAIFI. o Undergrounding Applicability: Strategies for improving SAIFI and SAIDI can sometimes adversely affect CAIDI. Because the measure focuses just on customers experiencing an outage, effective undergrounding strategies that reduce the number of minutes or customers experiencing a future outage are more likely to be visible with this measure when it is compared to performance before the start of an undergrounding program. CAIFI (Customer Average Interruption Frequency Index) – Measures reliability as the average frequency of sustained interruptions for those customers experiencing sustained interruptions; calculated as Customers Interrupted (CI) divided by Total number of distinct customers interrupted. (Note: The customer is counted once, regardless of the number of times interrupted for this calculation.) Improvements in CAIFI do not necessarily correspond to improvements in reliability. o Undergrounding Applicability: Because the measure focuses just on customers experiencing an outage, effective undergrounding strategies that reduce the number of customers experiencing a future outage are more likely to be visible with this measure when it is compared to performance before the start of an undergrounding program. Listed below is a non-traditional measure with feedback on its applicability to observe or make visible undergrounding efforts. TLR (Total Line Restoration Time) – Measures reliability as the accumulated time to restore line segments in outage; calculated as the accumulated restoration time measured in hours and normalized by the number of customers remaining in an outage. The normalization is necessary to compare accumulated restoration time in historic or future outages of different sizes and durations. (Author Note: The development and use of TLR is described in the MID-ATLANTIC UTILITIES UNDERGROUNDING PROGRAM CASE STUDY.) o Undergrounding Applicability: Because the measure focuses just on customers experiencing an outage, it can more effectively reflect improvements. Customers in outage-focused reliability metrics, such as CAIFI, CTAIDI, CEMI-5, CELID, and TLR are more relevant in assessing the impact of targeted resiliency programs that use undergrounding strategies. In the case of TLR, it is a non-traditional measure and used by Dominion Energy to better weigh the cost-effectiveness and resiliency improvement of their Strategic Undergrounding Program (SUP) (#33 & #240). This approach also better considers certain areas where numerous, extended overhead outages have occurred, and how strategic undergrounding would help. Cost Versus Benefit Assessment Due to the potential expense associated with undergrounding strategies, a robust cost versus benefit assessment is required. As an example, undergrounding is designed to eliminate exposure to certain types of extreme weather risk. The frequency and severity of risk exposure will dictate if undergrounding is an appropriate strategy. Undergrounding installations prior to 1990 did experience faults and failures at a higher rate than expected. There is now accumulating evidence that by eliminating extreme operational duty cycle stressors (overvoltages and overcurrents), underground cables have demonstrated the ability to survive beyond the 40-year mark (#255). Modest and targeted efforts can typically have significant benefits versus the cost when outage-based measures of
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