developed countries, although much of this difference is due to major differences in population density and power system confgurations
As indicated in Figure 1.3, large outages in the United States between 1984 and 2000 were more frequent than might have been anticipated on the basis of a simple exponential distribution. Although in recent years in the United States there has been no signifcant change in the frequency of outages (Figure 1.4), there has been a very signifcant increase in the frequency of transmission loading relief events (Figure 1.5).3
Most problems occurring in the transmission of electric power can easily be corrected by automatic controls and actions taken by system operators. However, occasionally these actions are not suffcient to keep power fowing. Problems or failures originating in one part of the system may give rise to problems (such as overloads) in other parts of the system, which in turn cause additional problems that may ultimately result in a cascading power failure. The fact that the power system uses alternating current (AC) means that the system behavior is sometimes further complicated by oscillatory or other complex dynamic behavior, as illustrated in Figure 1.6. Although they are rare, such events sometimes cause a loss of power to many customers (Table 1.1 and Figure 1.3).
Because electricity is so essential to modern industrialized societies, the power system has frequently been identifed as a potential terrorist target. For example, more than 15 years ago, in a report titled Physical Vulnerability of the Electric System to Natural Disasters and Sabotage (OTA, 1990. p. 14), the Office of Technology Assessment concluded:
Some terrorist groups hostile to the United States clearly have the capability of causing massive damage the loss of so many generating or transmission facilities that major metropolitan areas or even multi-state regions suffer severe, long-term, power shortages. The absence of such attacks has as much to do with how terrorists view their opportunities as with their ability. U.S. electric power systems are only one target out of many ways of striking at America, and not necessarily the most attractive.
More recently, the National Research Council report Making the Nation Safer (NRC, 2002. p. 178) noted that:
[a]nalysis of possible targets, weapons, and delivery systems and of direct and indirect consequences reveals several very dangerous scenarios. The scenarios of greatest concern involve the electrical system. When service is lost, there are immediate consequences to every person, home, and business. An extended outage of electricity would have profound consequences.
The same report emphasized (p. 180):
[t]he impact of a prolonged interruption in the electric power supply to any region of the country would be much larger than the economic loss to the energy sector alone. The nationl electric power systems must clearly be made more resilient to terrorist attack.
Potential attackers, as shown in Figure 1.7, include the following.
Most problematic are terrorist groups with signifcant technical capabilities and resources who want to kill large numbers of people or cause widespread societal or economic damage. Although not very likely, as noted above, such terrorists might view the power system as a primary target. As discussed later in this chapter, a sophisticated attack could cause a lengthy blackout over an extensive region. An attack during a period of extreme weather, such as a heat wave, might lead to the deaths of many people, albeit in a far less spectacular way than in a large explosion or a chemical or biological attack. However, the drawn-out agony produced by such an attack would clearly create great public anxiety and outrage, especially if government and private responses were seen as inadequate, and perhaps, too, if the frst attack were followed by other similar attacks. Public confdence could also be eroded, and anger heightened, if terrorists were able to hold the grid hostage by mounting limited demonstration attacks with promises of worse to come if demands were not met.
Although international terrorist groups such as al-Qaeda have been more interested in killing people that in causing economic damage, different groups with different motivations could emerge. An attack that brought a power system
3A transmission loading relief event occurs when congestion on the transmission system prevents the transmission of electricity for which a transaction has been contracted.