Electric Power Transmission and Distribution Systems
Electric Power Transmission and Distribution Systems
ELECTRIC POWER TRANSMISSION AND DISTRIBUTION SYSTEMS
The North American electric power transmission system has been described as the largest, most complex machine ever built by humanity. It is a massive network of generating stations, transmission lines, substations, distribution lines, motors, and other electrical loads all interdependently linked for the conversion, transportation, and control of electrical energy. Approximately 60 percent of all energy utilized in the United States passes through the interconnected electric power system. The major goal of the system is to most efficiently and reliably deliver electric power from generating stations to residential, commercial, and industrial consumers.
A small portion of the power system is depicted in Figure 1. The flow of energy is as follows: at generating stations, mechanical, chemical, or some other form of energy is converted into electricity, most often using a synchronous generator. The electrical output from the generator is converted, through a transformer, to a very high voltage, to be conveyed through transmission lines to transmission substations. Within the transmission substations, the voltage is stepped down to the subtransmission system, through which the power is conveyed to the distribution substations and into the distribution system. The distribution system delivers the power to residential, commercial, and industrial users, where the power is converted to light, heat, motion, or other desired forms of energy.
HISTORICAL CONTEXT AND TECHNOLOGICAL DEVELOPMENTS
The earliest commercial power system, believed to be Thomas Edison's Pearl Street station opened in 1882, consisted of a simple generator and a number of users. Within twenty years, more than 3,000 small electric generating stations were built in cities across the United States, each serving relatively local loads and with no interconnection. With the advent of the transformer, in about 1885, and the recognition that voltage drop and losses will be significantly reduced by stepping the voltage up and the current down, power transmission systems were created. The first demonstration of an ac power transmission line, in 1886, operated at 3,000 V over a distance of 4,000 ft (1,220 m). The first commercial transmission line in the United States was a 13-mile (21-km) transmission line operating at 3,300 V. As insulation systems improved and the technology of transformers was advanced, transmission voltage levels increased to 40 kV by 1907. This was a practical limit for the pin-type insulators (Figure 2) that were used to support the line on the towers, due to the structural stresses in the support. The voltage level was only able to increase further with the invention of the suspension insulator (Figure 2), which is in common use at the beginning of the twenty-first century. This increased the practical limit to about 150 kV, which was the limiting case because of corona. (Corona is a phenomenon in which the air in the vicinity of the energized surface is ionized because of the intensity of the electric field. It results in significant energy losses to the system.) The intensity of the electric field is reduced by having a larger-diameter conductor. The voltage was again increased in the 1960s after the realization that forming bundles of two, three, or four conductors could also mitigate the problem of corona. Power systems in the late 1990s operated at voltages as high as 765 kV in the United States, and as high as 1,100 kV in some parts of Europe.
Through the first several decades of commercial power systems, one generator or a small cluster of generators would provide power to a group of users in a given region. In the 1930s, systems began to be interconnected for reliability and economic reasons. In the operation of any engineering system, equipment occasionally malfunctions due to degradation of the equipment itself or because of outside influences. When this happens in small systems, the load is no longer served and the region is subjected to blackout. Relatively minor changes to the system, such as the addition or removal of a single large load, also cause a significant impact on system operating frequency or other parameters. The interconnection of a large number of generators over a wide area avoids both types of problems. The loss of one generator is made up quickly by the controls on the other generators on the system. A change in load has a much smaller impact when the total load is many orders of magnitude larger. Additionally, the interconnection of several local systems permits economic transactions, so that a utility requiring more power may purchase from another generating company rather than utilizing its own generation facilities, which may not be available or may be more expensive to operate.
The North American power system (covering continental Canada, the conterminous United States, and parts of northern Mexico) is made up of four independent power systems, with special connections among them. The independent systems are the Eastern interconnection, the Western interconnection, Quebec, and Texas. Within any of the four systems, there is a high level of connectivity, and all generators have coordinated control systems to enable them to work together fully. Among the systems are one or more high-voltage direct current (HVDC) links, which permit the flow of power for commerce and reliability. Several other smaller power systems also operate in North America but do not have any interconnection to the larger systems. There are a large number of other interconnections internationally in Europe and elsewhere.
Despite the very strong advantages to interconnection, there is an inherent weakness in the interconnection of large systems covering vast distances. Under some operating conditions, the system becomes unstable, and a small change may have a large impact. This occurs most frequently when the system is heavily loaded, such as during the summer, under heavy air-conditioning loads. Interarea oscillations may occur as the internal control systems for the various generators respond at slightly different times, so that power flow through transmission lines may widely fluctuate and even change direction. Careful monitoring and control must be maintained to avoid unstable operating regimes, and extensive efforts have gone into the development of simulation techniques to predict potentially unstable operating conditions so they may be avoided. Computational analyses are typically done that assess the impact of single and double contingencies—that is, the widespread effect of the failure of a single piece of equipment or generating unit or the loss of a single transmission line, or some combination of events. Naturally, inasmuch as the system has so many elements all working together, it is impossible to predict every possible contingency. Because of this, occasionally unstable operating regimes are entered, which may result in local or even widespread outages.
The most infamous outage in U.S. history was the Northeast blackout of November 9–10, 1965, in which thirty million people lost power for as long as thirteen hours. In this case, large quantities of power were being transmitted over long lines to New York City. The initiating event was the tripping of a single transmission line on the Ontario–New York border. This resulted in several other transmission lines having to pick up the load that had previously been carried by that line, and those lines overheated and tripped, removing 1,800 MW of generation (at Niagara Falls) from the system. The entire Northeast power system became unstable and separated into a number of different isolated systems, none of which had a balance between generation and loads. This resulted in the remaining generation tripping off line, and a widespread outage covering much of New York, Ontario, New England, New Jersey, and Pennsylvania. The blackout was widespread, and the damages that resulted from looting and panic were extraordinary. In response to the outage, the National Electric Reliability Council (now the North American Electric Reliability Council—NERC) was created, with the responsibility to ensure system security and reliable operation. NERC is owned by ten regional coordinating councils consisting of various utilities, power producers, power marketers, and customers. While many other blackouts have occurred since the one in 1965, some affecting millions of customers, none has affected as many customers, and none has had an impact on the industry that was so widespread. This is largely due to the efforts of NERC to study and promote reliability and establish policies, guidelines, and standards conducive to reliable operation.
TRANSMISSION LINE CHARACTERISTICS AND THEIR IMPLICATIONS
The performance of a transmission line, and its limitations, are directly related to physical parameters that come from its design, construction, and even its location. Those parameters are common to many electrical circuits: resistance, inductance, and capacitance.
The series resistance of a transmission line is closely related to the losses that will be dissipated when current passes through the line (proportional to the square of the current magnitude). The resistance is proportional to the length of the line but inversely proportional to the cross-sectional area of the conductor. The losses, and hence the effective resistance, are also increased by passing the line in close proximity to a noninsulating surface—for example, passing the line over seawater. The metal from which the conductor is made is also very important—for example, copper has a lower resistance, for the same geometry, than aluminum does. Also related to the losses of the transmission line is the shunt resistance. Under most circumstances, these losses are negligible because the conductors are so well insulated; however, the losses become much more significant as the insulators supporting the transmission line become contaminated, or as atmospheric and other conditions result in corona on the line.
The series inductance of the transmission line is a measure of the energy stored in the magnetic field of the conductor. High inductance is usually the limiting factor on the ability to transmit power over long distances, because the stability limit for power transfer is inversely proportional to the line inductance. Inductance increases for conductors that are farther apart, and decreases for conductors having a larger diameter.
The shunt capacitance of the transmission line is related to the energy stored in an electric field between conductors and/or earth. Capacitance negatively influences the operation of the transmission line by requiring higher currents from the generators (charging current), and by inducing a voltage rise (sometimes well in excess of safe operating limits) in a lightly loaded transmission line (the Ferranti effect). The capacitance increases when conductors are brought closer together, and it decreases for conductors having a smaller diameter.
The actual flow of power through the transmission system from one point to another is dictated by the operation of all of the generators and by the resistance, inductance, and capacitance of the system. Because those parameters are mainly characteristics of design, in the past very little flexibility in control was provided to change the path of power flow. In one well-known example, there is a lot of power generation that occurs in Niagara Falls, New York, and New York City consumes a lot of power. It has frequently happened that the flow of the majority of the power from Niagara Falls to New York City, rather than being along the most direct route, has been from Niagara Falls into Ontario, through Michigan, Ohio, Pennsylvania, and New Jersey before getting to New York City. Despite economic transactions and contractual agreements that would call for a direct route with minimized losses, the path of power flow is dictated by the laws of physics and the system parameters.
There are several techniques, however, that modify the system parameters to manipulate the power flow in localized regions. The one that has been in use the longest is modifying the parameters of generator operation, the quantity of power being produced, and the voltage at the terminals of the machine. This sometimes requires operation of less efficient and more costly generating stations, instead of optimizing efficiency and cost, and other limitations frequently come into play as well. The second approach is to install a very costly type of specialized transformer, a phase angle regulating transformer, at crucial points in the system. This device provides limited control, but can of modify power flows by imposing a change in phase angle in the voltage and current going through it. Switched series capacitor banks are sometimes used to reduce the effective inductance of a transmission line, permitting more power to flow through it, but these are not highly controllable devices either.
A fairly new technology that provides a high level of control for power flows and system stability is a class of devices known as flexible alternating current transmission systems (FACTS). A number of FACTS devices have been used at all different voltage levels. On the high-voltage system are devices such as static var compensators (SVC), thyristor controlled series capacitors (TCSC), static synchronous compensators (STATCOM), and universal power flow controllers (UPFC). These devices work by canceling out the inductance and/or capacitance of the transmission lines and the system loads. They are operated with high-power electronic devices and are fully controllable, either manually or automatically. Because of their dynamic nature and quick response time, they can respond to system disturbances and can provide an extraordinary increase to system stability.
Still another approach, which has been used since the 1960s to control power flow and transmit power over very long distances at high efficiencies, is high-voltage direct current transmission (HVDC). While they fill the same role as high-voltage ac transmission lines (bulk power transmission), HVDC lines are impervious to the effects of system inductance and capacitance, so power flow and system stability are not influenced by those parameters. Each end of the HVDC transmission line is located in a special converter station where high-power electronic devices convert alternating current to direct current at one end and reverse the process at the other end. HVDC power transmission is more efficient and more controllable, but the converter stations are very costly, so the planning and design of such a line includes a careful cost/benefit analysis in which typically the HVDC system is only chosen over a high-voltage ac line for very long distances or as an interconnection between independent systems.
TYPES OF TRANSMISSION LINES
For the ac or dc interconnections that span great distances, high-voltage transmission lines are almost exclusively overhead. These lines are typically constructed with aluminum wrapped around a reinforcing steel core. The conductors are connected to the support structures with suspension insulators that may be made of porcelain or polymers. The type of structure that supports the transmission line, the distance between structures, the arrangement of conductors, and so forth are functions of operating voltage, terrain, climate, right-of-way, aesthetic concerns, cost, etc. Common types of structures include treated wood, steel lattice, tubular steel, and concrete. Overhead power transmission lines are less expensive than underground transmission lines and are easier to install, inspect, and maintain. However, they are also more exposed to the hazards of environmental conditions such as severe winds, ice formation, and corrosion and contamination due to airborne pollutants.
Underground transmission lines are preferred in places where rights-of-way are severely limited because they can be placed much closer together than overhead lines. They are also favored for aesthetic reasons. They may be directly buried in the soil, buried in protective steel or plastic pipes, or placed in subterranean tunnels. The conductors are usually contained within plastic insulation encased in a thin metallic sheath. The conductors enclosed in steel pipes may be immersed in oil, which may be circulated for cooling purposes. For all types of underground lines, the capacitance is higher than for overhead lines, and the power transfer capability is usually limited by the resistive losses instead of the inductance. While not exposed to environmental hazards, underground cables are at risk of damage by rodents, construction, or geological instabilities.
In the 1990s, significant efforts have gone into the development of transmission lines made from super-conductors, which are materials that have essentially no resistance when operating at extremely low temperatures. While a significant amount of energy is required to operate the cryogenic systems, the reduction in power loss and the increase in power transfer capability have made the technology appealing. Several prototype installations in various stages of design and preparation have provided encouraging preliminary results, but as of the end of the twentieth century, no full-scale applications have been deployed.
ENVIRONMENTAL IMPACT OF TRANSMISSION SYSTEMS
While many people are concerned about the aesthetic impact of transmission lines, several other aspects of the transmission line environmental impact must also be considered. There is a danger to wildlife imposed by the presence of high-voltage surfaces such as overhead transmission lines. For small birds that land on transmission lines there is very little hazard because there is no path for current to flow through them. However, birds with large wingspans or climbing rodents will often reach between surfaces energized at different voltages, killing them and damaging system equipment.
Overhead transmission lines require that the area beneath them be cleared of trees or tall shrubs, which may result in erosion. When the transmission line right-of-way is not kept clear, the transmission line may come into contact with vegetation, causing a fault on the system and possibly starting a fire. Chemical contamination of soil may result from some types of transmission structures, such as treated wood. Burial of underground cables also can impact the environment due to erosion.
Another environmental concern that has been raised is the fear that electromagnetic fields (EMF) may cause negative physiological effects. Various epidemiological studies have purported to find an association between the presence of transmission lines and different types of cancer, especially in children. These studies have found at most a very weak association, and were based on estimated and not measured electromagnetic fields. While the issue has been given significant media attention, inciting grave public concern and much new research, the medical and scientific communities have been unable to definitively confirm that EMF has any measurable physiological impact.
TRANSMISSION SYSTEM ORGANIZATIONAL STRUCTURE
Until recently, all functions related to the generation, transmission, and distribution of electric power in the United States were executed under the umbrella of monopolies regulated by state utility boards (under a variety of titles). Through the 1990s, legislation has resulted in the deregulation of that industry and many changes in the nature of commerce in the energy industry. The details of the new system are still in development, but the deregulated system as presently designed consists of a plan to separate the utilities into independent entities consisting of regulated businesses and unregulated businesses. The regulated businesses will include the transmission companies and the distribution companies. It is necessary to regulate these companies to utilize the assets that exist to provide power to individual users and to transfer bulk power without the proliferation of an excessive quantity of new infrastructure. The deregulated businesses will consist of generating companies responsible for the production of power, and retail companies responsible for sale, billing, and other energy-related services. Sale and marketing transactions will be conducted by power marketers, with transactions being conducted on the power exchange. The entity responsible for observing and operating the overall system, ensuring that all functions are executed within the safe and reliable operating limits, will be the independent system operator (ISO).
John A. Palmer
See also:Capacitors and Ultracapacitors; Electric Motor Systems; Electric Power, Generation of; Electric Power, System Protection, Control, and Monitoring of; Electric Power, System Reliability and; Electric Power Substations; Environmental Problems and Energy Use; Insulation; Transformers.
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