When industry experts talk about the ``electric grid'' they're actually referring to three distinct components: power plants, high voltage lines that ferry electricity across vast distances, and a distribution network that moves power from high voltage lines to a home or business.
``But the reality of the system is it's a complex ballet. Everything has to be balanced. If it's not, everything shuts down,'' said Bill Beck, an Indianapolis-based historian who has written extensively about the electric industry.
Following Thursday's collapse of the electric grid that caused the largest blackout in U.S. history, knocking more than 100 power plants offline and leaving 50 million customers in the U.S. and Canada without power, many people have been wondering how such a thing could be possible.
But given the aging state of the transmission system, dozens of new power suppliers boosting power through the grid, and increasing interconnectivity between utilities from the Rocky Mountains to the East Coast, others are surprised the U.S. went 30 years without a major blackout.
Since 1992, when the National Energy Policy Act opened the transmission grid to nonutility energy producers, ``the transmission system is being used in ways it was not designed for,'' said Harry Terhune, an electrical engineer and vice president of operations for American Transmission Co. LLC, of Waukesha, Wis.
Deregulation has allowed new power operators to plug into the grid for the sole purpose of selling power to the highest bidder and the system now is being pressed to its limits, he said. ``It leaves you with a more constant reliability challenge because [power] loads are higher than historical norms,'' Mr. Terhune said.
While each utility, such as FirstEnergy Corp. in Ohio or DTE in Michigan, has a distribution network in its service territory, most of the high voltage lines of the transmission grid are controlled by independent system operators that ignore state and regional boundaries to insure a reliable flow of power.
High voltage transmission lines - which carry up to 750,000 volts - in the eastern U.S. grid that runs from the Rockies to the East Coast (but excludes Texas), are like interstate highways. They have entrance ramps to let power plants feed electricity to the high voltage wires, and exit ramps for cities or other areas to take power from the lines.
``Power comes from the generation plant, gets pumped up in voltage and sent over transmission lines, because at high voltage there's less loss of power, moves a couple of hundred miles, then goes to a substation where the voltage is dropped down again,'' said Richard Hirsch, a Virginia Tech University history professor and electric industry expert.
Substation transformers drop the power down a few hundred volts and send it to people's homes where another transformer drops it down to 110 volts for use, he said.
Technically, it sounds simple, but the reality is just the opposite.
Geographical groups of power users known as “pockets” have different power needs, requiring more or less electricity to be sent to their area. The changes have to be accounted for or a pocket can be overloaded or underloaded. In each case, a blackout would happen if demand and supply aren't balanced, experts said.
The electric grid was built over a 100-year timeframe by privately owned utilities with monopolies over certain regions. But, ``The transmission system we have today is pretty much one we inherited from the early years, and particularly the `60s or `70s,'' Dr. Hirsch said, adding that most high voltage transmission lines, including those in Ohio and Michigan, are about 25 years old.
Regulators have begun focusing their investigation on Cleveland along high voltage lines encircling Lake Erie and connecting Cleveland, Toledo, Detroit, Toronto, and Buffalo.
The so-called Lake Erie Loop appears to be the power conduit that hastened the blackout. It has been called a problem by some experts because the high demand along the loop and the limited number of power plants connected to it.
But the Loop isn't considered a problem by the Midwest ISO, the independent transmission line system operator formed in 1996 to oversee 111,000 miles of high voltage transmission lines in 15 states, including those in Michigan and northern Ohio.
``Calling it a problem is probably overstating the case,'' said James Torgerson, president and chief executive officer of Midwest ISO, which is in Carmel, Ind.
In fact, the Lake Erie Loop is not on the Midwest ISO's $1.8 billion plan to upgrade transmission lines it oversees. Locations in southern Ohio and in other states are felt to be bigger transmission bottlenecks.
Michael Caggiano, a Rutgers University professor of electrical and computer engineering, said numerous power plant and substations on the grid ``have sensors that determine the current and its direction. Once one senses the current is beyond the capability of sustaining the power, it will disconnect to prevent damage,'' he said. ``The decision is usually made in a tenth of a second to shut something down,'' he said.
If a sensor triggers a power plant shutdown, the grid loses power but demand doesn't decrease, Mr. Caggiano said. If one or two power plants shut down in a region, that part of the grid will seek power from elsewhere, creating a power loss ripple effect.
Mr. Terhune said that to prevent that, ``a section of the grid can be separated, almost like an island. In that island there may not be enough generation to support the load. Then you have a blackout area,'' he said.
``What's interesting here is the size of the island that formed was huge,'' he added.
When the cascade began collapsing FirstEnergy's grid territory, automatic circuit breakers flipped open at points that connect adjacent south and west territories owned by American Electric Power Co., stopping the blackout cold.
But many were amazed it could spread so far in just nine seconds.
By comparison, Mr. Beck said the 1965 blackout, caused by a failure at the Sir Adam Beck power plant in upstate New York, took 12 minutes to spread from its origin to New York City.
The increasing interconnectivity of the grid is designed specifically to prevent massive blackouts and let power suppliers fill demand gaps or offset power losses, Mr. Beck said. But in the same way power from Michigan now can be sent to New Jersey, it appears a power outage in Michigan affect New Jersey in the blink of an eye, he added.
Mike Stuart, general counsel and vice president of Wisconsin Public Power, Inc., said it's good to know the safeguards in the grid did eventually work as intended.
``The thing is, we just don't know why it stopped. For all we know, it could have been something that was designed into the system 25 years ago for another purpose,'' Mr. Stuart said.