America's Energy Future Technology and Transformation (2009) / Chapter Skim
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9 Electricity Transmission and Distribution
9 Electricity Transmission and Distribution
Pages 563-638
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This chapter reviews the status of current T&D systems and discusses the potential for modernizing them (thus creating the "modern grid")
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"Transmission substations" connect two or more transmission lines.
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are responsible for operating the transmission system reliably, includ ing constantly dispatching power to balance demand with supply and monitoring the power flows over transmission lines owned by other public or private entities. The ISO/RTOs, with oversight by FERC and NERC, monitor their systems' capac
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Source: Courtesy of NETL Modern Grid Team. ities and conduct the wholesale market to clear short-term transactions.2 There are nine ISO/RTOs in North America, as shown in Figure 9.3.
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In the white regions, where the industry has not been restructured, vertically integrated power utilities continue to operate the transmission system. Source: North American Electric Reliability Corporation.
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Source: North American Electric Reliability Corporation. The Western Interconnection, which extends from the Pacific coast to the Rockies; The ERCOT Interconnection, which encompasses most of Texas; The Quebec Interconnection, which is shown in Figure 9.4 as part of the Eastern Interconnection because they are operated jointly.
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Ironically, new transmission lines also are the object of considerable public opposition even while the need for them is increased by opposition to generating stations. 4Reserve margin is the amount of transmission capacity available above the maximum power expected to be delivered over the system.
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Data were adjusted as necessary using the Handy-Whitman index of Public Utility Construction Costs. Sources: 1975–2003 from EEI, 2005; 2000–2007 from Owens, 2008.
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The 2003 blackouts in the world's two largest grids -- the North American Eastern Interconnection and the West European Interconnection -- resulted from such cascading failures (see Box 9.1)
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Modernization is progressing much more rapidly abroad. For example, China and India are building 800 kV HVDC and 1000 kV AC transmission lines, along with the underlying high-power infrastructure.
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. The system could also have been restored much more rapidly if a modern grid had been in place.
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Modern communications and controls can move much faster to diag nose problems and bypass or isolate them. The same technology can provide cost benefits by maximizing power flows and integrating power from renewable energy sources.
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New technologies such as power electronics, real-time thermal rating of transmission lines, and composite conductors can allow an increase in power flow on the existing T&D system, but new lines also will be needed. Modern T&D systems are intended to provide effective operation, asset optimization, and systems planning capabilities under routine conditions and emergency response and fast restoration after a system failure.
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A modern trans mission infrastructure would include emerging technologies such as large-scale variable power sources and advanced energy storage devices. For example, it could smooth the variability of power from remotely located intermittent renewable resources11 and maintain reac tive power12 on the system.
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15"Power quality" refers to the voltage, frequency, and harmonic content (frequencies that are integer multiples of the fundamental 60 Hz frequency) of the electricity supply.
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KEY TECHNOLOGIES FOR A MODERN ELECTRIC T&D SYSTEM Many of the technologies needed for a modern T&D system already exist, and some, to a limited extent, are already deployed in parts of the T&D systems. However, many technologies will need to be deployed in a systematic and inte grated way to realize maximum benefits from a modernized T&D system.
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Power electronics on the AC transmission system are referred to as flexible alternating current transmission system (FACTS) devices.18 FACTS devices can control both real and reactive power flows along transmission corridors, thereby maintaining the stability of transmission voltage.
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High power qual ity is needed for many modern applications, especially in industries with auto mated production, which could benefit from more economical local solutions to improved power quality. Power electronics also plays an important role in smart metering with two-way power flow (to encourage local power generation)
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Today, this type of storage is largely limited to pumped hydro storage, where water is pumped uphill into a reservoir and released to power turbines when needed. Another technology that has been demonstrated and is currently available for commercial deployment is compressed air energy storage (CAES)
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Other longer-term possibilities for energy storage in the grid include superca pacitors, superconducting energy storage, and flywheels. None of these technolo gies is currently suitable for grid use because of high costs and low energy-storage density.
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The market for amorphous steel transformers has been small, however, primarily because of their higher cost. This material may become more competitive economically as a result of new DOE standards regarding distribution-transformer efficiency for new equipment.26 Potential for Future Deployment Many of the technologies needed to implement a modern T&D system, such as FACTS and custom power devices, are presently available for commercial deployment.
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At larger scales that may be needed to support large quantities of intermittent renewable energy sources, pumped hydroelectric power and CAES will be the only viable options before 2020. Batteries may also be used for large-scale storage in the T&D systems but are unlikely to be available for deployment at the hundreds-of-MW scales until after 2020.
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, polled by the SCADA, and sent to the control center over relatively slow communications channels -- usually microwave. In modern substations, some of which are already in place, the substation control and protection system is digital and the connectivity is through a local area network (LAN)
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must be developed, along with real time data handling software that can collect and move the data to where they are needed. If the measurement technologies described above are fully implemented, each control center will need to process approximately one million data points per second.33 The existing communication channels between the control centers and the substations, many dating from the 1960s, cannot handle these data rates.
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and more efficient operation and optimization of assets. If the T&D system is equipped with new measurement sensors, a high-speed communication network, and power electronics, fast wide-area controllers can be designed and installed with software only.
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can control voltages and power flows with response times measured in milliseconds. Moreover, fast wide-area controls, com bining rapid communications with remotely controlled FACTS devices, are becom ing feasible.
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Improved visualization interfaces and decision-support technologies will increase reliability, decrease outages due to natural causes and human error, and enhance asset management. IDST covers three general systems-operations categories: Grid visualization.
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Operations-planning decisions set the schedules of how the T&D system will be operated over the next day. Decisions include forecasting the load, scheduling dispatchable generation and long-term contracts to meet the load, conducting auc tion markets, using power contracts to check on possible congestion on the trans mission system, and modifying the power contracts if congestion is indicated.
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Integrating Technologies to Create a Modern Electric T&D System The key technologies discussed above are in various stages of development, with many already having been deployed in a limited way. However, the primary challenge will be the integrated deployment of these technologies to achieve the desired characteristics and performance of a modern grid.
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Complicating matters further, costs have been escalating sharply in recent years for large-scale T&D construction, as for other energy projects. Trans mission investment is anticipated to continue to increase to meet load growth and replace aging equipment, but additional investment will be needed over the next few decades to modernize the T&D system.
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.40 Brattle estimates that $675 billion will be needed for distribution versus EPRI's $640 billion. Neither report explicitly accounted for the construction of new transmission lines to bring power from remote wind or other renewable energy sources to load centers.
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, which funded the Brattle study, have suggested that the split would be approximately one-third during the first 10 years and the remaining two-thirds over the second 10 years.41 The AEF Committee has assumed that 40 percent of the expenditures should be made before 2020, with the remaining 60 percent between 2020 and 2030. Thus invest ments averaging $9 billion per year would be needed in the transmission system from 2010 to 2020, with approximately $2 billion per year of this total dedicated to modernization.42 From 2020 to 2030, an average of approximately $14 billion per year will be needed, including $3 billion per year for modernization.
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There will also be a reduction in grid congestion and forced power outages. A modernized grid will enable a wide array of new options for load management, distributed generation, energy storage, and
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A modern grid could improve the diversity of energy supplies by allowing larger proportions of renewable energy into the U.S. energy supply.
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Efficiency can be improved in the T&D system as well as in end-uses, reducing the need for new generation and the siting of new transmission lines. A modern T&D system can enable intermittent renewable electricity sources (particularly wind power)
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Approximately 10 percent of the total power produced in the United States is lost in the process of delivering it to the end user. For example, reactive power flow over a transmission line not only increases losses in the transmission line but also significantly reduces the power carrying capacity of the line; the use of power electronics, however, can reduce such flow of reactive power.
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In addition, by reducing the risk of long-term outages following terrorist attacks or natural disasters, modern ization could help prevent public health and safety catastrophes. BARRIERS TO DEPLOYING A MODERN T&D SYSTEM It should be clear from the previous section that a modernized electric grid is very much in the nation's best interest.
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Even though the additional investment would eventu ally pay off, financial markets and regulatory constraints drive utilities to mini mize investments. In addition, some of the benefits of modern grid technologies are societal (higher quality, more reliable power)
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Also, time-of-day rates for consumers that reflect actual wholesale market conditions are not yet widely implemented, thereby preventing the level of demand-side involvement needed in the modern grid. Net metering policies that provide customers with retail credit for energy generated by them are also not widely deployed, which reduces the incentive for end users to install rooftop photovoltaics or other generating technologies.
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Given these factors, a broad vision and an accompanying road map are required to achieve consensus on common goals and to guide the integrated deployment of modern technologies that meet the performance requirements of the modern grid, as described previously in this chapter. The complexity of the transmission system suggests that the development of clear metrics to measure societal benefits will be essential to measuring prog ress.
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The key components of the modern grid (FACTS devices, custom power, HVDC and HVAC technologies, and storage) have largely been developed, as noted earlier, and measurement, communications, and control technologies to manage these components will be deployable on a large scale, along with the associated decision-support tools, before
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Recently, many factors, including changes in the regulatory structure of the power industry, have lowered the reli ability of this critical national infrastructure. While it is encouraging that the industry has greatly increased its T&D investment in the past several years, more is still needed to implement a modern grid capable of meeting future challenges -- such as enabling power markets, inter mittent renewable-electricity sources, and modern efficiency technologies -- while maintaining reliability and security in the systems.
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The modernization of the transmission system will ben efit greatly from a comprehensive national vision based on consensus among the many stakeholders. The transmission system is national in scale, and the major benefits of a modern system come from the opera tion of many technologies in concert across the entire system rather than from technologies deployed in isolation.
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Costs: The estimated cost to modernize the T&D system is modest relative to the investments that will be required simply to meet load growth and replace or upgrade aging equipment. Transmission.
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This objective can be met with backup generation (such as gas-fired power plants) or by large-scale storage technologies, such as compressed air energy storage (CAES)
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1997. 1997 System Disturbances: Review of Selected Electric System Disturbances in North America.
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2007b. The NETL Modern Grid Initiative Powering Our 21st Century Economy -- Barriers to Achieving the Modern Grid.
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Presentation at the American Public Power Association National Conference, Anaheim, Calif., June 18–22. U.S.-Canada Power System Outage Task Force.
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Next is a more detailed description of the characteristics of a modern grid than could be discussed in the section "A Modern Electric T&D System" of the chapter, followed by a more detailed description of some of the technologies discussed in the section "Key Technologies for a Modern Electric T&D System." Finally, the cost analysis in the section "Costs of Modernization" in the main text is elaborated upon. Reliability Measures in the Distribution System The reliability of the distribution system is often measured using three indexes: the Customer Average Interruption Duration Index (CAIDI)
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Source: Ohio Public Utility Commission. 1.6 1.4 1.2 1.0 SAIFI 0.8 0.6 0.4 0.2 0 2000 2001 2002 2003 2004 2005 2006 2007 Year FIGURE 9.A.2 System Average Interruption Frequency Index (SAIFI)
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Optimization. The modern grid provides advanced tools for compre hending conditions, evaluating options, and exerting a wide range of control actions to optimize grid performance, whether from reliability, environmental, efficiency, or economic perspectives.
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In addition, the distribution system must accommodate smaller distributed-energy sources. Large-scale baseload generation resources may require backup generation and, possibly, also power electronics to ensure that power flows are accommodated.
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Major improvements must be made to the transmission system to achieve well-designed and operating markets, especially as industrial, commercial, and even residential consumers will generate and sell power. These contributors will be enabled by emerging generation and storage technologies; the modern grid will allow for two-way power flow on the distribution system and thereby provide self-generation opportunities for the end user to also participate in power markets.
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For transmission, the needed measures include: effective and advanced moni toring; methods for very quickly determining the cause and location of a fault or instability; probability-based contingency analysis; rapid system alignment for the next contingency; effective use of flexible alternating current transmission system (FACTS) devices and HVDC to stabilize system voltages and power flows; remotely dispatchable storage near generators and load centers; effective use of customer-generated power and storage; intelligent load-shedding; effective island ing; fast restoration means; strict reliability standards; and predictive maintenance of key components (NETL, 2007b)
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The aggressive introduction of technologies such as intelligent metering and real-time pricing could create incentives to shift energy use to off-peak times, thereby reducing demand for peak-load power generation and decreasing stress on the T&D system overall. For example, 20 percent of California's electricity demand is used to move water, which can be done predominantly at night.
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The modern transmission system must also reduce the consequences of a successful attack by devoting resources to recovery. Many of the technologies described previously for advanced components, measurements, communications and controls, and improved decision-support technology (IDST)
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Accordingly, many industrial and commercial users install equipment -- such as uninterruptible power supplies, alternate utility feeders with high-speed transfer switches, standby generators, or a variety of power electronics devices, depending on cost and benefit -- to attain the needed power quality. But with proper monitoring of the network condition and anticipation of changes, power-quality problems can be avoided at the system level through the use of existing technologies.
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In principle, a deficit or surplus of reactive power should be corrected at or near where it occurs -- namely, generators, transmission lines, and loads or load areas. Reactive power flow over a transmission line not only increases losses in the transmission line but also significantly reduces the line's power carrying capacity.
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Decision makers can decide more economically where, what, and how to invest in future grid improvements. Whether from optimizing assets or operating efficiently, the real-time information from the modern grid sensors, coupled with communicating it widely and processing it effectively, will significantly enhance the system.
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are used primarily to control active and reactive power flow through a line as well as to adjust fre quency drift. Hundreds of SVCs, and a few STATCOMs, TCSCs, and VFTs, are currently deployed in the T&D system.
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transmission system today is almost entirely AC, transmitting electricity via HVDC involves converting AC to DC, transmitting the DC electricity, and then converting it back to AC at the other end. Most HVDC projects to date have been based on current source converter technology, in which the DC current flows in the same direction and power reversal involves reversal of voltage.
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Pumped hydroelectric power, currently the only proven means of large-scale energy storage, is unlikely to be expanded greatly because few sites are both economically and environmentally acceptable. Other near-term candidates are compressed-air energy storage (CAES)
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A 300 MWe CAES storage facility can utilize wind power to compress air, and then, during low wind periods, the compressed air can provide the combustion air for a natural-gas-fired combustion turbine providing up to 10 hours of backup capability for the wind farm. Another possible storage technology for use in the grid is batteries, which rely on electrochemical processes to store electricity.
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They have very long life as well as very high efficiency compared to bat teries. A second example is superconducting energy storage (SES)
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Source: Adapted from a presentation by Dan Rastler, Electric Power Research Institute, to the Panel on Electricity from Renewable Resources, March 11, 2008. setting standards for transformer efficiency can be important in lowering the T&D losses.
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These measurements, made at each substation, are used to drive controls and protective relays. In the early days, all the measurements and controls were hardwired within the substation, and a few -- very few -- of the measurements from high-voltage transmission substations were hardwired all the way back to a central control center.
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. This configuration remains the architecture of most control centers in place today.
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These meters can also bring control signals from the power company directly into appliances and other equipment on the customer side. The ubiquity of more and faster measurements throughout the T&D system raises the issue of how to handle this proliferation of measurement data.
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The Eastern Interconnection also has about 10 second-level control centers, known as reliability coordinators, that supervise larger areas of the T&D systems; each of these facilities has to process data at rates that are an order of magnitude higher than those of the substations. But these data rates cannot be handled by the communication system used today between control centers and substations.
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If indeed they are not available, the costs of alternative technologies are likely to be higher than that amount and/or the benefits of modernizing the grid could be lower. The committee also considered the investment that would be required to meet load growth and replace aging equipment.
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Substation automation 10 Other equipment (power electronics, storage, HV lines and equipment, 55 superconducting lines) Emergency operation and restoration tools and equipment 12 IDSTa software 3 Dynamic thermal circuit rating 1 Predictive maintenance 20 Total 110 aIDST = Improved decision-support technology.
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In addition, technologies can meet multiple purposes. For example, dynamic thermal circuit rating can help to meet load growth by increas ing the capacity of existing lines, but this is also an important part of a modern transmission system.
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Implementation of the modern T&D system alone makes up a small portion of this total, as shown in Table 9.A.3: $50 billion for transmission and $170 billion for distribution.7 The committee assumed that 40 percent of the transmission improvements involved in implementing the modern grid, meeting load growth, and correcting deficiencies would be made before 2020, while the remaining 60 percent would need to be implemented between 2020 and 2030. Thus an investment of $9 bil lion per year would be needed in the transmission system from 2010 to 2020 7The $50 billion for transportation and $170 billion for distribution are the total costs for a modern T&D system, less the listed "synergies."
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2008. Compressed Air Energy Storage Scoping Study for California.
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2007b. A Systems View of the Modern Grid.
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