National Energy Grid
The electricity powering your computer as you read this may well have been generated at some power plant several hundred miles away, less than one second ago. How did it get from there to you? You can thank some very interesting properties of electricity itself - and "The Grid," which is the largest machine ever built by humans. Yes, the interconnected electric transmission grid in North America is one of the largest and most complex man-made systems ever assembled. "The grid consists of billions of components, millions of miles of wire, and thousands of generators," stated Thomas Overbye, a University of Illinois professor of electrical and computer engineering. "Failures in one location can quickly propagate through the system and affect millions of customers with losses reaching billions of dollars."
The whole idea of electricity flowing on "The Grid" wouldn't work at all without the unique property of electricity being able to travel down whatever path provides the least resistance, kind of like water running downhill. One can think of the grid as if it were a huge network of hoses that carry water (similar to a giant sprinkler system), which is connected at many points so that the water in one hose can flow into another. When water is pumped into any of the interconnected hoses, it becomes available to the entire system. This imaginary giant network of hoses holds a cumulative reservoir of water.
Well, the electric power grid is a giant network of thick aluminum wires (transmission lines) that holds a reservoir of electricity. The individual watts in any one of those wires might have come from any of the generating stations pumping power into the grid. Types of generation stations today include those that use coal, natural gas, oil, nuclear, hydropower, and non-hydro renewables. All of which pump the power they generate into the grid.
Substations are the crossroads on the grid, which take in the high-voltage juice from these transmission lines, redirect some of it back out onto other high-power lines, and transform some of it into lower-voltage power that goes into a distribution network that usually runs along the streets and finally into your house. Yes, every time you turn on anything plugged into the wall, you are drawing electrical power from the Grid, at the speed of light - 186,000 miles per second.
Because there is no mechanism to efficiently store electrical energy, total electrical generation must equal total load plus losses at all times. This continual matching must occur even as the load on the transmission grid constantly varies. This along with the finite capacity of components on the grid, and the difficulties to quantify the capabilities to transfer power, can easily create problems that may affect larger areas of the grid. Efficient management of electricity markets, where congestion on a single element can affect thousands of other elements and/or power transfers, continues to be a significant challenge.
Dispatchers at the approximately 150 grid control stations spread across the United States and Canada are constantly monitoring the grid for tiny variations in the frequencies of the electrical energy in the transmission system. By monitoring the variations, they know how to adjust things so that the demand for electricity from consumers, along with the supply from the generators, always matches. Balance of the electrical forces in the grid is vital to protect it from failure.
Too much electricity flowing over a transmission line can cause it to heat up and sag. This condition can permanently diminish the wire's capacity to carry electricity. If a line gets too hot - sags too low, it can cause reliability and safety problems, and the line may have to be taken out of service. When that happens, the electricity from that line suddenly flows into all the other interconnected paths. They in turn, can get hotter and sag. Hence, if conditions exist that makes even one line on the Grid too hot, a sudden imbalance between supply and demand occurs, which can have a domino effect on the whole system. A sudden loss of one major transmission line or a substation due to a storm or other unforeseeable event, can trigger chain reactions on the Grid. The resulting imbalance will typically trigger switching and safety equipment to protect the system from overheating.
Too much demand and not enough supply can also lead to an imbalance. This condition may cause a voltage collapse, which can also result in downing a major section of a regional grid. Voltage is like the pressure that pushes the electricity through the wire. If the voltage is strong at the power plant where energy is being generated and pumped on the grid, but weak at the other end where too many customers are all drawing power from the grid, an imbalance in voltage between one end and the other occurs, which interrupts the smooth electromagnetic field in a transmission system for efficient flow of electricity.
Since the balance between supply and demand is so important, bringing the Grid back online after a big shutdown is more than a matter of just flipping a switch. Dispatchers have to restore the system one transmission line at a time, so they don't suddenly shock things with more supply than there is demand.
As the demand for electric power continues to increase, and the power industry becomes increasingly more competitive towards deregulation, knowledge concerning the capacity and constraints of the electrical generation and distribution system will become a commodity of great value. This, along with the new tools being developed for the grid, and the way that the power industry as a whole is changing to accommodate Open Access business practices and security concerns, is certainly providing many new challenges for the largest and most complex man-made system ever assembled - The Grid.