tions, and circuit breakers that open and close connections while also acting as giant fuses to protect expensive equipment from damage, as well as a variety of other devices. Most substations sit out in the open protected only by a simple chain-link fence. All but a few high voltage lines are also in the open. Thus, both substations and the lines that connect them are vulnerable to damage from storms and to terrorist attack.

When power reaches an area where it will be used, the voltage is reduced and power is distributed to customers over lower-voltage distribution lines. Unlike the transmission system, which is a large interconnected network, many distribution systems branch out radially to deliver power to customers, although some older, dense urban areas, such as New York City, use network configurations for distribution. All the elements of the transmission system, and increasingly those of the distribution system, are monitored and controlled by information and communication systems.

Although problems in any part of the system can disrupt the supply of electric power, this report focuses on the transmission and distribution (T&D) system, substations, and other associated parts, discussing generation only as it relates to issues involving transmission and distribution. Details on how the T&D system is controlled, operated, managed, and regulated are given in Chapter 2.

Non-malicious Threats to the Electricity Delivery System

By its very nature, the T&D system is not perfectly reliable. Even without terrorist activity, the power sometimes goes out, usually for just a few seconds, minutes, or hours, but sometimes for a few days. On very rare occasions, and in limited locations, outages may stretch on for weeks. As the duration and geographic extent of an outage increase, people become seriously inconvenienced, and economic and other costs rise, but people generally do not experience “terror.”

Keeping power flowing to customers is a continuous process of control, recovery, and repair. Most outages are local, brief in duration, and caused by problems at the level of the distribution system—such as lightning strikes, wind storms and tree falls, short circuits caused by wild animals such as squirrels, vehicles that crash into power poles, and similar events. Line crews can usually fix these outages in a matter of hours. Distribution systems that incorporate automation can often isolate a problem and restore service for many affected customers in a matter of seconds or minutes.

Outages caused by disruptions in the high-voltage transmission system are less common. When they do occur, because of faulty equipment, weather, or for other reasons, many such outages are never noticed by customers, because automatic controls and system operators can limit their impact and maintain the supply of power to the distribution system. But, of course, the transmission system does occasionally experience problems that result in loss of service to customers. Weather events, such as hurricanes and ice storms, earthquakes, and similar natural events, can bring down many transmission lines, and, less frequently, can damage transformers, circuit breakers, and other equipment such as the terminal facilities for direct-current (DC) lines. Inadequate attention to maintenance can also contribute to blackouts—as in the recent case of an improperly sized circuit breaker in London, or several instances of arcing to vegetation that have resulted from inadequate tree trimming in the United States.


FIGURE 1.1a System Average Interruption Duration Index (SAIDI) indicators for U.S. utilities for the period 1992 to 2001 (excluding major events). SOURCE: EPRI (2003).

As explained in greater detail in Chapter 2, the transmission system is much more stressed, and thus more vulnerable, than it was a few decades ago, principally as a result of two factors: (1) years of underinvestment in system upgrades stemming from ambiguities and altered incentives that resulted from electric power restructuring and associated changes in the regulatory environment and (2) demands on the system to move power between sellers and buyers in new competitive power markets in greater volume and in ways in which the system was not designed to operate.2

Figures 1.1a and 1.1b show the trend in two common measures of power supply disruption in the United States over the decade from 1992 to—2001 the System Average Interruption Duration Index (SAIDI), which indicates the average time that customers are without power during the period analyzed (Figure 1.1a), and the System Average Interruption Frequency Index (SAIFI), which indicates the average number of interruptions per customer served per year (Figure 1.1b). Both reflect principally the effects of distribution system disturbances and exclude outages caused by major events. Figures 1.2a and 1.2b show SAIDI and SAIFI measures of reliability internationally. Reliability in the United States appears to be poorer, on average, than that experienced by customers for electric power in some other


2Much of the transmission system was originally designed to serve the needs of vertically integrated regulated utilities. Following deregulation of the power industry and the introduction of competition among generators, the transmission system is now being expected to move power in ways that have resulted in patterns of power flow that did not exist previously under regulation.

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement