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Power Grid of the Future
The United States has about 5000 power plants with a total generating capacity of more than 800,000 megawatts (MW) of electricity. Our country also has 254,000 kilometers (158,000 miles) of transmission lines to carry electric power to millions of customers. Over the next 10 years, demand for power is expected to rise by about 25%. Under current plans, electric transmission capacity will increase by only 4%, according to the Report of the President’s National Energy Policy Development (NEPD) Group, published in May 2001. “This shortage could lead to serious transmission congestion and reliability problems,” the report says. In addition, the report adds, regional shortages of generating capacity and transmission constraints, such as occurred in California in the spring of 2001, combine to reduce supply reliability and quality of power delivered to end users. Uninterrupted power, which is essential to computer users, is becoming an increasingly important issue as our digital economy expands. The power grid and power plants make up a significant part of the nation’s energy infrastructure. After the terrorist attacks of September 11, 2001, the Department of Energy began developing a National Energy Infrastructure Assurance Plan, to identify vulnerabilities in the infrastructure and ways to improve its protection against natural, accidental, and deliberate threats. TRANSMISSION GRID STUDY The NEPD report, noting that the United States has three regional power grids that are not synchronously interconnected, directed the Secretary of Energy to examine the benefits of establishing a national electrical grid and to identify major transmission bottlenecks and remedies to remove them. The Department of Energy responded by conducting a “National Transmission Grid Study,” to examine both the technical and economic issues resulting from transmission constraints and reduced power system reliability and to provide innovative solutions to reverse these trends.
ORNL researchers whose work on transmission-grid reliability issues has been nationally recognized also contributed to this study. The grid study report consists of six issue papers published and released in March 2002. Brendan Kirby of ORNL’s Engineering Science and Technology Division (ESTD) is the lead author of one issue paper, entitled “Reliability Management and Oversight,” and co-author of a second paper, “Transmission Planning and the Need for New Capacity.” “The study clearly identifies a nationwide decline in transmission system capacity and finds that this decline can be explained by a combination of technology, business structure, and regulatory reasons,” says Kirby. “By addressing each of these issues, DOE can help develop the robust 21st century national transmission system that our economy needs. To ensure greater reliability and efficiency of electricity transmission, DOE should continue to encourage the development and demonstration of new technologies, such as advanced cables, including superconducting transmission lines in available rights-of-way. These approaches will help the power industry meet growing demands for energy while reducing the cost of electricity for customers all over the nation.” The grid study report recognizes the increased concerns about providing security to protect the U.S. power system from deliberate attack. It recommends more study of technical and administrative methods of enhancing security. ADVANCED CABLES
ORNL researchers are helping industry develop and evaluate new technologies that could improve the efficiency and reliability of existing transmission lines. For example, 3M is developing and ORNL is testing designs of advanced overhead cables with cores made of a metal-matrix composite instead of steel. Such cables could carry up to three times the current of today’s transmission lines without the need for tower modifications or additional rights-of-way. “Using available technology to build expensive new rights-of-way and new tower systems will cost well over $1 million per mile,” says Kirby. “Upgrading existing transmission lines with composite conductors will significantly improve the overloaded electrical infrastructure at a fraction of the cost.” Today’s overhead transmission lines consist of aluminum conductor strands wrapped around a steel core. Because of the weight and properties of the steel, these cables will stretch and sag if they are heated up too much by carrying too much current. Sagging lines caused by excessive current and hot weather triggered a major power outage in 1996 in the northwestern United States. To overcome this limitation, 3M developed a composite consisting of Nextel ceramic fibers and an aluminum-zirconium alloy to make an advanced cable that can carry more current than steel-aluminum lines without sagging at higher temperatures. Vinod Sikka of ORNL’s Metals and Ceramics Division and ESTD’s Roger Kisner worked with 3M to improve the quality and production of this composite material. Tom Rizy and John Stovall, both of ESTD, are responsible for testing 3M’s advanced cable material to determine how well it holds up over time as it is run through many cycles of current flow. It will be heated by increased current loads to temperatures around 210°C and, every tenth cycle, to 240°C (simulating an emergency load). “We have developed instrumentation for detecting the extent of sag as temperature changes in planned tests at ORNL,” says Rizy. “We will use this information to write algorithms. We will later collect and analyze data and prepare a report after a planned field test in which a mile of the 3M cable will be strung between transmission towers in North Dakota where icing has caused sagging of conventional lines.” All this work by ORNL researchers is being done as part of a cooperative research and development agreement with 3M.
“Another advantage of the advanced composite conductor lines,” Kirby says, “is that they can be kept in inventory and put up faster than conventional aluminum-conductor, steel-reinforced lines to replace, for example, a 300-mile line that has been knocked down. Because they are lighter than conventional conductors, composite conductor lines can be quickly strung between temporary towers that are farther apart than the regular towers. After this emergency restoration of power, new lines can be put up on the permanent transmission towers.” SUPERCONDUCTING TECHNOLOGIES The power grid of the future will include high-temperature superconducting (HTS) cables, which offer much less resistance to the flow of electricity than do copper lines. “A superconducting cable will conduct up to 5 times as much current as a copper cable of the same size,” says Bob Hawsey, manager of ORNL’s Superconductivity Program. “Because an HTS cable loses little energy as heat, it will cut electrical transmission losses in half, from 8% to 4%.” An HTS cable is more environmentally friendly than a copper cable also because it is cooled with safe, inexpensive liquid nitrogen rather than oil-impregnated paper insulation, which may leak oil. Also, it can be installed in an existing underground duct, where it is better protected against natural, accidental, or deliberate threats than are overhead lines.
ORNL researchers led by Mike Gouge of ORNL’s Fusion Energy Division (FED) have helped Southwire Company develop an HTS cable 30 meters (100 feet) long for the company’s facility in Carrollton, Georgia. The cable is made of bismuth-strontium-calcium-copper-oxide (BSCCO) first-generation wires that are chilled using liquid nitrogen. The HTS cable has provided power to the Southwire industrial complex for more than 14,000 hours. A new innovation is a “triaxial” cable design first tested at ORNL in 2001. In this concept, all three alternating-current phases of the cable system are concentric and contained in one cryostat (as shown in the illustration), resulting in the most compact superconducting cable configuration possible. ORNL’s patented technology called RABITS™ (rolling assisted, biaxially textured substrates) is being developed for a number of electrical power applications, including cables being made by Southwire Company. ORNL developers of the RABITS™ technology showed that texture introduced to metal (e.g., nickel alloyed with tungsten) by rolling and annealing it into tapes can be transferred to a superconductive oxide coating through buffer layers deposited on the metal substrate. The resulting orientation of crystals in the superconductive oxide allows it to conduct large electrical currents without resistance at liquid nitrogen temperature (77K). Industrial collaborators have made wires of this material that are longer than a meter.
ORNL researchers Bill Schwenterly, Jonathan Demko, and others also have contributed to the development of a superconducting transformer. Unlike a conventional transformer, a superconducting transformer has no oil, greatly reducing the potential for fire damage to a substation if a transformer were to fail. These researchers in FED developed an innovative, compact cryogenic cooling system for an electric transformer made by Waukesha Electric Systems and IGC-SuperPower. Waukesha’s HTS transformer will be tested on the Wisconsin electric grid in the fall of 2002. ORNL has DOE funding to work with General Electric, to develop a 100-million-volt-ampere (MVA) superconducting generator; with Southwire and American Electric Power (AEP), to develop a 300-meter- (1000-foot-) long super-conducting cable to be installed at an AEP substation in Columbus, Ohio; and with IGC-Waukesha, to build a prototype, utility-sized superconducting transformer. The goal of these and other DOE-funded partnerships is to advance the introduction of HTS devices into the American marketplace. IMPROVING THE GRID'S RELIABILITY, SAFETY, AND EFFICIENCY Today more and more people are purchasing their own sources of electricity for their homes or businesses. These gas-driven microturbines, fuel cells, diesel generators, solar cells, and wind turbines, called distributed energy resources (DER), can meet owners’ electricity needs.
“Providing locally based electricity generation or electricity plus heating and cooling can reduce the reliance on a centralized grid, improving security,” says Mike Karnitz of ORNL’s Energy Efficiency and Renewable Energy Program. “The grid becomes a less-tempting target to attack. A shift in the electrical grid to more-responsive loads with self-generation improves the resilience of the system. As a result, the vulnerability of the grid to a widespread outage is reduced.” Because DER can supply owners with even more electricity than they need, they might someday sell their excess electricity to other customers through the distribution system and power grid. But DER presents a potential safety and reliability problem. Electric utility distribution systems are not designed and protected for reverse power flow (i.e., power coming back out of a house or business, potentially causing voltage and frequency problems). Even worse, if an unexpected voltage were introduced to a transmission line being repaired, the line maintenance personnel might face a hazard. To address these problems, ORNL researchers are examining some potential technological solutions. ORNL researchers led by ESTD’s Don Adams and Leon Tolbert are proposing the development of a multilevel inverter to control power distribution and replace the transformer. This system would improve control and reliability of power flow. If excess electrical power from a microturbine could be stored in a battery until needed, the direct current from the battery could be converted by an inverter to alternating current for distribution to other users. “Most large transformers today are designed for a specific location,” Kirby says. “If a large transformer is destroyed, it might take two years to build one to replace it. If the nation were to have a stockpile of multilevel inverters, it would be easy to replace each large, disabled transformer with an inverter. The multilevel inverter would keep the transmission system functioning while a new replacement transformer is being built.” Another technology being studied is the flexible alternating-current transmission system (FACTS), a combination of large-scale power electronics devices that can control the flow of power through transmission and distribution lines.
“FACTS can control the voltage magnitude and phase angle at both ends of the line, as well as the amount of real and reactive power that is passed through the line,” says Kirby. “FACTS devices could greatly increase the power-flow capacity and stability of our existing transmission lines. “A FACTS device can block the flow of power to a line that is about to be dangerously overloaded. It reroutes the electricity to a line that has the capacity to carry the additional current. It could also direct power to another line if a transmission line is brought down as a result of a natural, accidental, or deliberate action.” The problem is that FACTS devices are too expensive to be used widely. So, ORNL researchers are looking at ways to reduce their costs. For example, ORNL researchers Orin Holland, Tony Haynes, and Darrell Thomas, all of the Solid State Division (SSD); Fang Peng, Tim Theiss, Syed Islam, and Leon Tolbert, all of ESTD, and Len Feldman of Vanderbilt University are developing and testing highly efficient power electronics modules based on silicon carbide (SiC), to replace today’s silicon-based modules (including FACTS devices). The SSD researchers have made advances in materials processing and synthesis that allow fabrication of SiC devices that will take full advantage of the intrinsic properties of the material without being limited by process-induced defects found in silicon-based counterparts. Compared with silicon modules, the SiC-based electronic modules should operate more efficiently at higher temperatures, voltages, and switching speeds. Another way to make existing transmission lines more reliable is to control power flow so that it is available to users when and where they need it—even during a fault, such as a power line brought down by a falling tree during a storm. “Electricity transmission today is done by command and control through a centralized system,” says Karnitz. “A more effective approach is a layered control system in which distributed, intelligent agents in the form of computers and sensors monitor power lines and other equipment. Like captains and lieutenants reporting the latest news to a commanding general, these inexpensive agents can quickly report faults to a main computer, which could reroute power through another transmission line to avoid an outage.” SHEDDING ELECTRICAL LOADS DURING POWER SHORTAGES In the winter of 2001, Californians were outraged by spikes in the price of electricity as their bills jumped 50% or more. They were also upset by rolling blackouts that shut down computers, caused traffic snarls, and left some people stuck in elevators. On August 10, 1996, a major power outage was experienced in the electrical grid in the West, leaving 12 million Americans in nine states without electricity for up to 8 hours and costing an estimated $2 billion. August 10 was a very hot day. The demand for air conditioning and other electrical services was unusually high. The searing temperatures caused transmission lines to sag into trees, forcing the power to shut off. Fewer transmission lines were available to carry the extremely high load of electricity. As a result, the Pacific Northwest’s grid was weakened, knocking out four main power highways that send electricity to other states. On that day, lights and air conditioners flicked off in homes and businesses; movie screens and traffic lights went black; factories shut down; and amusement park rides came to a stop. Crises like these have convinced the Department of Energy that new technologies are needed to decrease price volatility and increase reliability in the U.S. electric power system. So DOE has organized a new Consortium for Electric Reliability Technology Solutions (CERTS), of which ORNL is one of four national laboratory partners. According to a CERTS brochure, “The U.S. electric power system is in the midst of a fundamental transition from a centrally planned and utility-controlled structure to one that will depend on competitive market forces for investment, operations, and reliability management. Electricity system operators are being challenged to maintain the reliability levels needed for the digital economy in the context of a changing industry structure and evolving market rules. The economic growth of the nation is tied ever closer to the availability of reliable electricity service. CERTS was organized to conduct needed public interest research on electricity reliability technologies.” Current CERTS research focuses on prototyping and demonstrating real-time reliability management tools, developing new system security management tools, and conducting basic research and outreach related to advanced measurement technologies and controls. How has ORNL contributed to CERTS? ORNL’s John Kueck (ESTD), Kirby, and Bob Staunton (ESTD) have written a white paper for CERTS on “demonstrating load as a reliability resource.” “The idea is to make the power grid 99.99% reliable with no fluctuations, to avoid causing interruptions in the digital economy,” says Kirby. “By improving the reliability of the electrical grid, we are hardening it against natural, accidental, or deliberate threats.” Kirby, Kueck, and Staunton are investigating the benefits of shedding loads when there’s a bottleneck. According to Kueck, “Intelligent control technologies and an ‘on-line’ power market could be used to enable users to voluntarily control their own load in response to market conditions. The residential, commercial, or industrial users would make the decision to curtail their load, based on their own needs and the market-price incentive for reducing load. “For example, in the summer of 2000 and the early winter of 2001, aluminum shelters shut down several afternoons a month and avoided using 100 megawatts of power in exchange for significantly reduced electric bills,” Kueck says. “Aluminum companies contracted to voluntarily shut down their operations whenever the power they used was critically needed elsewhere. The ‘reward’ for curtailment was sufficiently attractive during times of power shortages to convince them to make this decision for themselves. This approach is much more palatable for industry than having Big Brother come along and flip the switch. The technology for such a real-time, market-and-control system exists today.” HOW VULNERABLE IS A NETWORK TO AN OUTAGE? CERTS is also supporting research by FED’s Ben Carreras and Vickie Lynch of ORNL’s Computational Sciences and Engineering Division, who have been analyzing the degree to which a network is vulnerable to an outage. They are also conducting research sponsored by the National Science Foundation in collaboration with Power Systems Engineering Research Center and Alaska University. “From our analysis of data on blackouts in the United States over the past 15 years, we have found that there is on average a blackout every 13 days,” Carreras says. “We have concluded that the probability of a blackout of a given size decreases slowly as its size increases. In other words, the probability of a large blackout affecting millions of people, lasting eight hours, and representing a large loss of power is smaller than the probability of a small blackout affecting thousands of people for an hour and representing a small loss of power. However, the probability is not as small as expected. The situation is different, for instance, in South America, where blackouts are frequent but only very small. In the United States, we have some blackouts that are huge but don’t happen very often.” The problem with the U.S. electric system is that it is overstressed, as is most of its infrastructure, Carreras says. “There is a general tendency for the demand to increase up to the limits of supply,” he adds. Because the electric system is pushed to its limits, a small event such as a tree falling on a line or a few lines sagging because of the heat can trigger large cascading phenomena—the domino effect—as was the case in the August 10, 1996, power outage. Carreras and Lynch developed a dynamic computer model that simulates the management of a U.S. electrical network that is operated close to the breaking point. The model predicts the probability that the network will have a catastrophic blackout. “The probability is lower than you might expect,” he says. He suggests that a network’s vulnerability to blackouts may be reduced by monitoring demand fluctuations to determine what the peak demands are and how often they occur. “This information will help determine how much generation power must be held in reserve so that peak demands for electricity can be met without bringing the system down,” he says. “We are using our model to predict the need for generator reserve on a network. The information we get could suggest a need for changes in operating policy when the electrical system approaches its limits.”
The software tool Carreras and Lynch developed is based on ideas from their computer model of a sand pile, originally conceived to describe the loss of energy from unstable fusion plasmas in research tokamak devices. Imagine a child dumping a bucketful of sand on top of a sand pile at the beach. When the pile gets too high, avalanches occur, and the sand particles in the center are carried to the edge. However, the slope of the sand pile remains the same. The shifting sand pile is a paradigm for “self-organized criticality,” in which complex systems tend to rearrange themselves to be close to their limits, living at the edge of chaos. Methods of self-organized criticality can also provide insight into the vulnerability of complex systems such as power grids. This approach can allow researchers to identify the electrical networks that are the most vulnerable to failure in the event of a natural, accidental, or deliberate action. For large electrical grids, a blown transformer circuit could cause the rerouting of power, possibly overloading an alternate transmission line. To improve the reliability of power distribution and prevent major blackouts, it might be necessary to perturb the system a little, say, by occasionally manipulating circuit breakers to see if the system responds in a healthy or unhealthy way. But Carreras still has more work to do with his model to determine if periodic testing of an electrical network by slightly perturbing it is a good idea. “Simple solutions to a small problem can cause a much bigger problem that is not anticipated,” he says. “In an overstressed network, the probability increases that a small event—like adding a control system, cables, or relays—can make large, cascading events more likely. A small change to a complex system can backfire on you. It also can give you more confidence that you are in control so you are less vigilant and more likely to overlook other, emerging problems that could bring the system down.” ORNL is helping the U.S. electrical industry transform today’s transmission system into the power grid of the future. Related Web sites
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