POWER FACILITIES AND SOURCES OF DAMAGE
The following power scenario is based on panel discussions with Southern California Edison, Los Angeles Department of Water and Power, the Sacramento Municipal Utility District, and California Utilities Emergency Association. In addition to these discussions, one of these entities, provided a detailed but confidential write-up following its internal considerations of the
meteorology, flooding, windspeed, and landslide information provided to the panel. These materials are supplemented by data provided by utilities in follow-up conversations, and with data available in the Homeland Security Infrastructure Program (HSIP) Gold 2007 database, including the
locations of essential facilities such as substations, power transmission routes, and wastewater treatment plants.
Note that representatives from Pacific Gas and Electric (PG&E) stated that they were unable to contribute at that time and that they would inform us later whether they would be able to
participate; as of this writing they have not done so. The following, therefore, does not reflect the opinions or judgment of PG&E personnel. It does reflect our initial interpretation of the statements made during the other meetings, subsequent conversations with representatives of all the other lifeline service providers, and a fairly exhaustive review of newspaper accounts of 1986 and 1997 storm impacts on PG&E facilities, found in the Los Angeles Times, Sacramento Bee, and San Francisco Chronicle.
Several other power utilities were unable to attend panel meetings or were otherwise unable to estimate scenario damage and restoration. To estimate power outage and restoration for these remaining service areas, we make several assumptions, based on the panel discussions and other evidence cited below.
Sources of wind damage. According to panel participants, wooden crossbars and pole-mount transformers on distribution-voltage utility poles can be damaged by wind speeds as low as 60 miles per hour (mph). Moderate winds also can cause lines to sway, touch, and cause cross-phase shorting. Another common cause of wind damage is moderate wind speeds with windborne debris such as palm fronds blown onto lines causing shorts. Where winds are stronger, damage is more severe. Hurricane-force winds (75 mph and higher) can cause transmission lines to sway and cause cross-phase shorting, or cause electrical transmission towers or poles to collapse.
The panels did not postulate damage in high-wind regions (75 mph and higher). In Alpine, Inyo, Mono, and parts of El Dorado, Placer, Riverside, San Bernardino, and Tulare Counties, winds reach 75 to 125 mph. Figure 27 shows where high winds could threaten transmission lines. Note especially how high winds on the eastern side of the Sierra Nevada range coincide with the location of transmission lines.
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Some documentation is available regarding power outage and restoration in south Florida in Hurricane Andrew (Porter and others, 1996). Peak gust velocities in a few Florida locations reached 170 mph, but in many places between Miami and Homestead peak gusts were in the range 100 mph to 125 mph (http://www.nhc.noaa.gov/prelims/1992andfig5.gif), similar to the most strongly affected regions in ARkStorm. About 55 percent of Florida Power and Light transmission lines were out of service because of Hurricane Andrew, (including 80 percent of the 230 kilovolt system and 60 percent of the 138 kilovolt system), along with about 70 percent of the distribution circuit miles. It took approximately 5 days for Florida Power and Light to restore service to 90 percent of its customers, and 30 days to reach 99 percent. Note that electrical facilities in hurricane country may be built to different standards in consideration of higher wind loads.
Figure 27. ARkStorm winds. Yellow indicates maximum windspeed in excess of 70 miles per hour.
Red lines indicate electrical transmission lines.
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Damage to wooden crossbars and pole-mount transformers was one of the more significant causes of system interruption. One panel estimated that 0.2 percent of customers in areas with peak gust velocities generally in the range of 45 to 75 mph could lose power because of wind damage to distribution poles. (Customers, as used here, are counted in electric-service meters, not inhabitants of residences or office buildings. A single-family dwelling, for example, would typically count as one customer.) Damage to poles in moderate windspeed areas is restored within 7 days of the storm.
We assume that 75 percent of customers in counties with higher winds - in the range of 75 to 125 mph - lose service, and that repairs would take 7 days to restore to 90 percent of customers and 4 weeks to restore power to almost all customers. This restoration curve, shown
mathematically in Equation (1), is comparable to the restoration estimated in the confidential utility study for areas with similar, strong winds. The restoration curve is of the form:
1
0exp
f t C r t (1)
where
f(t) is the fraction of customers with power at time t,
C0 is the initial fraction of customers without power (for example, 0.75),
t is time in days after the peak of the storm (January 27, 2011, in southern California, February 9, 2011, in northern California), and
r is a constant reflecting speed of restoration: 0.05 for very slow restoration and 0.30 for fast restoration. A value of 0.25 is used here.
Sources of flooding damage. Power plants, high-voltage substations (also called bulk substations) and control facilities can be sensitive to flooding damage in at least two important ways. Flooding can damage control equipment. High-voltage substations and generating plants have high-voltage transformers (50 to 200 megawatts (MW) at high-voltage substations and 300 to 500 MW at generating plants) that also can be damaged by flooding, for example, by flood-borne debris impacting the transformer and ancillary equipment. A problem is that these transformers are custom made, designed to match impedance at the facility it serves, and each location serves fairly large populations: a high-voltage substation for example can serve in excess of 200,000 people. The transformers are not interchangeable and are too expensive to stockpile backups beyond those available for normal operational redundancy. If one of these large transformers were damaged, it could take 6 months or more to replace. There is typically some redundancy, enough so that at any given high-voltage substation, for example, one of these transformers can be inoperative and the substation can still operate. In addition, agreements between utilities allow for the loan or sale of surplus or idle equipment in an emergency situation. However, flooding is a common-cause failure mode, the implication being that a flood can damage several components simultaneously, potentially damaging two or more of these transformers. Were this to happen, the utility would have to reroute power around the inoperative substation, which could take a few days, and immediately attempt to repair or replace the transformer once dewatering is completed. The reliability of the temporary grid layout would be reduced, meaning greater likelihood of power outages in the affected area.
Flooding also can damage equipment at generation facilities. If demand were high (less likely in the winter months in which the ARkStorm is postulated to occur), temporary emergency generation such as diesel generators—and the necessary fuel supplies—might have to be brought in to serve the affected areas. Figure 28 shows where power plants are located in relation to
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flooded areas in four metropolitan areas of the state. While most power plants are located out of the flooded areas, some are inside, especially in Santa Clara County and Los Angeles.
(a) (b)
(c) (d)
Figure 28. Power plants (green diamonds) overlain with flooding (blue areas) in (a) San Francisco Bay Area, (b) Sacramento and Stockton, (c) Los Angeles and Orange Counties, and (d) San Diego.