Most Utilities Would Agree That a Policy of Run-to-Failure is Not a Prudent Strategy for managing distribution assets, in light of its anticipated impact on system reliability and safety. Yet, many utilities have defaulted to this policy by failing to initiate a program of preemptive infrastructure replacement. In some cases, this has resulted in criticism and even unwanted involvement from regulators.

What has prevented many utilities from making the investment that they feel they should in infrastructure replacement has been their inability to clearly predict the payback. Understandably, a “Let's see what this does” philosophy is hardly a compelling business case for a multimillion-dollar investment. Southern California Edison (SCE; Rosemead, California, U.S.) has developed a method for performing long-term probabilistic forecasts of the reliability of its distribution system as a function of investments in preemptive infrastructure replacement. Simply put, it's a method to calculate the payback.

AGING INFRASTRUCTURE

Despite routine, often regulated equipment-inspection programs, the reliability of most electric distribution systems is dominated by in-service equipment failures. Furthermore, observations of the system average interruption duration index (SAIDI) and system average interruption frequency index (SAIFI) indicate that this trend is increasing for many utilities due to failures of aging equipment rising each year.

This is not surprising. All components of the distribution infrastructure are getting older. The population of underground cable at SCE is aging despite the fact that the utility is adding new cable to the population each year to support customer and load growth. This aging trend is evident in all of SCE's major infrastructure components. When this is coupled with the well-known “bathtub curve,” which tells us that the probability of failure increases with age, the inevitable conclusion is that the challenge posed to system reliability by infrastructure aging is only going to get worse.

UNDERGROUND CABLE

At SCE, underground cable clearly represents the most significant challenge as far as infrastructure aging. Therefore, while data also existed for switches and transformers, SCE chose cable as the subject of its detailed probabilistic analysis of system reliability. (Note that while the balance of this article deals with cable, the methods described here are applicable to all other significant infrastructure components.)

In its analysis, SCE sought to answer three questions:

1. DATA ISSUES

   Where is cable on its approach to its long-term steady-state replacement rate? (See sidebar on page 30.)

2. With no preemptive cable-replacement program, how would future cable failures impact system reliability?

3. FAILURE PROBABILITY

   How would future system reliability be affected by various levels of preemptive cable replacement?

Like most utilities, SCE has an equipment database designed for engineering and maintenance use that contains detailed technical information on most major distribution equipment currently in service. Unfortunately, as at most utilities, cable is not included in the equipment database.

At SCE, the only significant source of data on cable is the capital property accounting record. Unfortunately, this database records changes in capital assets in relatively broad accounts. In the case of cable, inventories of all medium-voltage cable are recorded under one account, with no breakdown by conductor size or insulation rating. However, this database records how much cable was installed in any given year and tracks how much of that original installation was subsequently removed and when. While not everything an engineer might wish for is included in the capital property database, some reasonable assumptions by SCE made this data extremely useful. Since most of the primary cable installed in any given year was purchased under the same specification, the cable insulation type could be inferred. Therefore, the data enabled SCE to assess the current inventory of primary distribution cable by year of installation and by insulation type.

FAILURE PROJECTIONS

While SCE's engineering database includes the reasons for equipment removal (failure, overload, corrosion and idle), the capital property database does not. However, because cable is rarely removed for reasons other than failure, the assumption that all past cable removals were the result of in-service failure was considered reasonable. Nevertheless, with a data system not originally designed to facilitate engineering use, SCE opted to blend industry data with its own in-house data. Using all of that data, SCE then developed cable-reliability models for its various types of cable.

The data allowed SCE to answer the first of its three questions, listed previously, by enabling the calculation of the mean time to failure (MTTF). Assuming an MTTF of 34 years for tree-retardant cross-linked polyethylene (TR-XLPE) cable and an inventory of 46,000 miles (74,030 km), SCE would expect a long-term steady-state replacement rate of 1400 miles (2253 km) per year, a significant increase over the approximately 300 miles (483 km) currently being replaced annually.

THE COST OF DOING NOTHING

SCE is outlining plans to develop an engineering database for cable that would allow it to better correlate cable reliability with characteristics other than age, such as voltage class, cable size, physical environment and cable loading. However, this is a long-term objective.

Considering that failures are a function of inventory, SCE's next step in its analysis was to ascertain how much cable would fail and be replaced in future years. This needed to be done one future year at a time. For instance, to determine how much cable will fail in 2009, the starting point is the inventory in 2008. For each vintage (year of installation) of cable in service in 2008, the number of failures expected in 2009 can be determined by multiplying the volume of cable of that vintage by the probability of failure of that vintage. The sum of all the failures expected from each vintage yields the total number of failures expected in 2009.

PREEMPTIVE Cable REPLACEMENT

However, the inventory in any future year is a function of prior failures. To calculate the number of failures in 2010, for example, the starting point is now the inventory of cable in 2009 (or the year before the future year). The inventory of cable in 2009 must be derived from the inventory of cable in 2008, but adjusted to reflect the fact that all cable segments failing in 2008 resulted in their being replaced with new cable (whose age is now 0 years). The inventory of each vintage will decrease each year by the amount of failed cable segments. However, the total system inventory of cable will remain constant, due to the simplifying assumption of zero customer growth.

SCE had to review its distribution-system historical outage records to translate the number of in-service cable failures into system-level reliability indicators (SAIDI and SAIFI). Based on historical records, SCE estimated the average impact of a single cable failure on SAIDI to be about 0.0208 minutes. This number takes into account the amount of mainline cable versus radial cable and the difference in which their failures affect the circuit.