“The electric power grid and its controls are now much more complex…and this directly increases the vulnerability of the grid to catastrophic loss.” 
The vast majority of the electricity consumed in the United States, as in all countries, is produced by centralized power plants. Typically these are large generators fueled by coal, natural gas, nuclear, hydro power and, to a small but growing extent, solar and wind energy.
Power plants are often located far from where the electricity they produce will be used, so the electricity is delivered across long-distance transmission lines. These typically end near population centers, at facilities known as substations. There the electricity is sent across local distribution lines to consumers.
The nature of electricity is such that in order to transmit it efficiently over long distances, the voltage must be very high. At the power plant, an electrical device known as a transformer increases the voltage before it goes into the transmission line. At the far end of the transmission line, where it terminates at a substation, another transformer reduces the voltage before it is sent across local distribution lines.
Electricity is typically conducted over transmission lines at 110,000 volts or higher. This is also referred to as 110 kilovolts, abbreviated as 110kV. The largest transmission lines, known as extra high voltage lines (often abbreviated as EHV), conduct electricity at 230,000 volts (230kV) or higher. There are approximately 80,000 miles of extra high voltage transmission lines in the United States and they form the backbone of our electrical transmission system.
Technology does not yet permit the practical storage of electricity in large quantities. Instead, a complex, dynamic and fragile balancing act is continually required between consumption and generation.
To illustrate this process with a simplified example, consider the chain of events that occurs when you turn a light switch on. First, the electrical load created by the light bulb is sensed at a load control center. The control center then signals a power plant to instantly generate a bit more power. That additional power is stepped up to a very high voltage and conducted over transmission lines, often for long distances. At the other end of the lines, the electricity is stepped down to a lower, but still high voltage, and is then conducted across local distribution lines. Somewhere near your home, another transformer steps the electrical voltage down to the 120 volts we normally use. The electricity then enters your house and the light bulb comes on.
The continuous balancing act needed to respond to many sudden increases and decreases in loads is inherently fragile. Even a minor imbalance in this continuous and complex system can trigger a chain of events resulting in large-scale power outages.
“A transmission line outage acts like a dam, forcing the electricity around the blockage onto other lines. If adjacent transmission lines cannot handle the extra power flow, safety devices may switch them off to prevent damage. Further overloads can lead to cascading outages and system-wide failure, i.e., a blackout. This is one of the disadvantages of the interconnectedness of the transmission grid. Multiple failures in one location can quickly affect the entire system, producing a large scale blackout.” 
In each of the following two chapters, one component of the electric power transmission system (commonly known as “the grid”) will be examined. Both components are highly vulnerable to remarkably simple methods of attack. One of these components is relatively straightforward and inexpensive to secure against attack. Curiously, the required security measure has not generally been adopted. Securing the other component is far more problematic and would require significant changes in the way electricity is transmitted. The potentially catastrophic consequences of an attack on one or both components is described in Chapter 6.
 R. Narasimha, Science and Technology to Counter Terrorism: Proceedings of an Indo-U.S Workshop (National Academy of Sciences, 2007) P. 95
 Wikipedia, “Electric power transmission” http://en.wikipedia.org/wiki/Electric_power_transmission
 Homeland Security, “Power Hungry: Prototyping Replacement EVH Transformers” March, 2012 http://www.dhs.gov/files/programs/st-snapshots-prototyping-replacement-ehv-transformers.shtm
 Public Service Commission of Wisconsin, “Electric Transmission Lines” May, 2011 P. 2 http://psc.wi.gov/thelibrary/publications/electric/electric09.pdf
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