FEATURE ARTICLE
Reengineering the Electric Grid
Deregulation places new demands on one of the world's largest engineered structures—and presents new opportunities for educated consumers
Thomas Overbye
The Rules of the Market
The electricity market has some features in common with just about any other market. First, there is a product: electric power supplied at the desired voltage level and frequency. Second, there are sellers. Reality is complex, but for simplicity we can think of them as independent generators, each capable of making at least some of our electric energy product. Third, there are buyers. Sweeping a lot of individual variations under the rug, engineers refer to consumers or groups of consumers as electric loads, each requiring a specified amount of the electric power product. Fourth, there is the transportation system that moves the product from the buyers to the sellers, the electrical transmission grid.
But the nature of the product imposes some very unusual rules on this market. For starters, there is no way to realistically store our electric power product. As a parent of young children I am quite familiar with the ability of AA batteries to make toys squeak and talk for hours on end, but power storage is rarely feasible on a larger scale. For all practical purposes, the total amount of our electricity product produced by the generators must at all times be equal to the amount consumed by the loads, plus any losses incurred in the transmission system. Thus the electric grid represents the ultimate in "just in time" manufacturing. Capable of moving at almost the speed of light, the product is always delivered to the customers fresh, within milliseconds of being "manufactured" by the generators.
Another unique rule is that the customers are in complete control of the amount of the product they use. After all, they control the light switch. They never get placed on hold, are never told the product is on order and are never told there is no room at the inn. So the generators must continually match the load, even though daily fluctuations in demand of more than 100 percent are not uncommon.
In addition, with few exceptions, there are no mechanisms to control how the product flows through the transmission system from the generators to the load. No valves can change the flow of electricity down a particular line (short of cutting it off entirely with a circuit breaker). Our electricity product does not check a map to determine the shortest route. Rather, electric power flows through the grid as dictated by the impedances of the transmission lines and the locations where electric power is injected by the generators and removed by the loads. This effect is called "loop flow." Because of loop flow a single shipment of power may actually spread throughout a large portion of the grid, changing the flows on a large number of lines. Figure 5 shows how a transmission of power from Wisconsin to Florida would affect power lines as far away as Minnesota (even though that is in the opposite direction!) and Louisiana.
Figure 6 illustrates a simple hypothetical transfer between two utilities. In the left part of the figure, two utilities called Left and Right are operating in a mode that involves no net transfer of power. The two lines at the top are conveying 47 megawatts and 12 megawatts of power from Right to Left, while a line at the bottom is conveying 59 megawatts from Left to Right.
Now suppose that Left and Right sign a contract for Left to deliver 150 megawatts of power to Right. This is not a matter of "sending" more electricity flow along one of the lines. The only thing that the utilities control directly are their generators. The right-hand part of the figure shows how Left and Right can accomplish the transfer. Note that the transfer affects every transmission line in the system. If one of the lines had been pushed past its capacity (shown by the pie charts), then the transfer might not even have been possible—with consequences we shall see below.
A final peculiarity of the electric transmission grid is that its available transfer capacity (ATC) is often difficult to quantify. As the Left-Right example suggests, ATC is directly dependent on the current or assumed system operating conditions. But since the electric load is constantly changing, and some transmission lines and generators are always out of service for some reason, the ATC constantly varies. Most of the transmission network consists of wires hanging on big tall towers, taking their share of hits from the weather. Thus the determination of ATC requires considering not just one particular operating condition but rather a large set of plausible contingent operating conditions. A typical ATC study requires studying the set of all contingent states with one line or generator out of service. Finally, transfer-capacity values are dependent on a number of constraints, including the need to avoid exceeding transmission or transformer thermal limits and voltage limits. Any element that exceeds its limit is said to be congested. The ATC value is determined when the first element hits its limit in either the base case or one of the contingent cases.
Once a set of ATC values has been determined, it must be updated any time a new transfer takes place in any direction. One of the consequences of loop flow is that a new power transfer in one direction affects the remaining capacity for making transfers in other directions. For an analogy, imagine that making a reservation on airline A's flight from Chicago to Atlanta changes the number of available seats on every other flight by every airline in the eastern part of North America. Furthermore, determining the new number of available seats requires calculations that can easily generate 100 gigabytes of data!
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