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T&D Reliability: The Next Battleground in Re-Regulation

Fortnightly Magazine - March 1 1999

can expect to be fined whenever their reliability frustrates customers' expectations, especially in a public way.

Dan O'Neill is vice president of Metzler & Associates, a management consulting firm headquartered in Deerfield, Ill. Research assistance was provided by his associate, Howard Friedman.

Performance Measures

For interruptions, excluding momentaries and certain sustained outages.

SAIDI (em System Average Interruption Duration Index. The average number of minutes in a year that the typical customer is interrupted. The ratio of total minutes divided by the average number of customers.

SAIFI (em System Average Interruption Frequency Index. The average number of times per year that the typical customer is interrupted. The ratio of total customers interrupted divided by the average number of customers.

CAIDI (em Customer Average Interruption Duration Index. The average duration of a customer interruption. The ratio of total minutes divided by the total number of customers interrupted.

ASAI (em Average System Availability Index. The number of SAIDI minutes divided by the total number of minutes in a year, subtracted from 100 percent, to yield a positive measure. A typical transmission system accounts for less than ten minutes of interruption per year, so ASAIs for transmission are typically greater than 99.998 percent.

Example: SAIDI is the product of SAIFI and CAIDI, so if the average customer sees 1.5 interruptions per year and the average duration of each interruption is 90 minutes, then the average number of minutes of interruption experienced is 135 minutes. In this case ASAI would be 99.974 percent. (em Author

The California Case PG&E Outage Highlights Rule-Tightening

In its Jan. 25 report to the California Public Utilities Commission, Pacific Gas & Electric Co. attributes its Dec. 8 power outage to human error. But the incident (em and the PUC's response in ordering a full investigation (em highlight increasing state-level regulation of electric reliability.

Incident. PG&E says the outage, which left more than 1 million of the utility's customers in San Francisco and San Mateo counties without power, occurred when a construction crew at the San Mateo Substation improperly removed temporary protective grounds. Separately, a transmission operator at the substation then energized the lines, but failed to engage protective relays. With the local protective system disengaged, electric current was sent to ground. The system took a half-second to isolate the fault instead of the one-tenth of a second that would be required normally.

Reaction. "The reliability of the transmission and distribution systems are crucial elements to our efforts in electric restructuring," noted assigned commissioner Richard A. Bilas in his initial ruling. He described the CPUC's work aimed at setting up a performance-based ratemaking mechanism for each investor owned utility, with standards for maintenance and repair and emergency response procedures and stricter tree-trimming rules.

Mark Ziering, the commission's Reliability Project Team manager, agrees that California's T&D systems are receiving more critical regulatory attention since deregulation. "The commission has full jurisdiction over distribution-level outages, which account for the vast majority of outages and outage-hours," he says. "The CPUC also is concerned about transmission outages because those can be the most catastrophic."

Key Policies. Bilas also