By Paul Recer, The Associated Press
and Mike Boyer, The Cincinnati Enquirer

WASHINGTON - Electricity generation stations throughout the United States are interconnected in a system called power grids.

This allows electricity generated in one state to be sent to users in another state.

In the U.S. electrical system, there are more than 6,000 power generating units energized with coal, oil, gas, falling water, wind or nuclear fission.

Power from these stations is moved around the country on almost a half-million miles of bulk transmission lines that carry high voltage electricity.

There are three major regional interconnected transmission grids in the United States. Thursday's failure affected the Eastern Interconnection, which links the Eastern seaboard to the Plains states. It also includes a number of Canadian provinces.

Inside this larger grid are smaller regional transmission organizations. Cinergy is a member of the Midwest Independent Transmission System Operator Inc., which is based in Carmel, Ind.

It covers 100,000 miles of transmission lines in 14 states and one province.

From the high voltage transmission lines, power is stepped down at substations to a current that can be used in homes and offices.

High or unbalanced demands for power that develop suddenly can upset the smooth distribution of electricity in a grid. In some cases, this can cause a blackout in one section of a grid, or even ripple throughout the whole grid, sequentially shutting down one section after another.

Once large sections of a grid are shut down, it may be difficult or impossible to restore power from neighboring grids. In periods of high demand, such as summer, often those nearby grids also are under stress and may not have spare power.

When power is restored, it can cause a sudden surge in demand that the system may not be able to accommodate. When electric motors, transformers and other electrical devices are all energized at once, it can draw many times the normal load and can trip a secondary shutdown.

For this reason, engineers restore power after a massive blackout in stages, one section after another.