CONTENTS AT A GLANCE Overview
Energy for Electricity Generation North American Power Grid Single- versus Three-Phase Power Power Generating Stations AC Generators
Auxiliary Power Station Equipment Generator Synchronization
Wye- and Delta-Connected Loads AC Transmission Systems
Transmission Towers, Poles, and Frames High-Voltage DC Transmission
Overview
The first commercial power plant was opened in San Francisco in 1879. It was fol-lowed in 1882 by the opening of Thomas Edison’s Pearl Street station in New York City, which delivered direct current (DC) electric power. In 1893 alternating current (AC) generation and transmission were displayed at the Chicago Worlds Fair. By 1896 an AC transmission line had delivered power generated by a Niagara Falls hydroelec-tric plant some 20 miles to Buffalo, New York. After a contentious battle between Thomas Edison and other proponents of DC power, the advocates of AC power such
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as Nikola Tesla and George Westinghouse prevailed. Alternating current became the accepted national power standard. Demonstrations had proven that AC transmitted over long distances sustained lower power losses than DC transmitted over the same distances.
Over the next hundred years a North American power grid, a major development in power generation and transmission, evolved from the consolidation of separate AC power generation and distribution networks. This grid now stretches across the coun-try from New York to California, with parts extending into Canada and Mexico. More recently, computer-controlled switching systems with advanced software have been introduced into the power grid.
Meanwhile, electric power generating capacity has fallen behind the ever-increasing demand for electricity in North America. The power shortage has been traced to the complications brought on by deregulation, a shortfall in the construction of new power plants, and strong environmental activism that has inhibited new plant construction.
The deregulation of the electric power industry in the 1990s has resulted in immense changes in the industry since that time. While traditional electric utilities still gener-ate, transmit, and distribute electricity much as they did before deregulation, many others have taken advantage of deregulation to divest themselves of their power gen-eration facilities, which had long served their local areas. Of these utilities, some acquired newer, more efficient generation plants in other locations, while others aban-doned generation altogether. Some utilities that gave up on generation claimed that they wanted to concentrate their resources on transmission, distribution, and improv-ing customer service, but others admitted that they just wanted to be free from the con-stant public complaints about the air pollution produced by their power stations.
As a direct result of deregulation, the electric power industry has seen the entry into the market of small and independent power producers, so-called merchant generators, and power marketers. Moreover, there has been a significant increase in mergers and acquisitions within the industry, along with the entry of some power utilities into other, more lucrative commercial enterprises. The objective of some of this diversification has been the formation of integrated energy services.
Power marketers act as independent middlemen who buy and sell electricity in the wholesale market at market prices. Most of this power is traded in the growing elec-tricity commodity market. In the past, power marketers did not own electric generation, transmission, or distribution facilities, but recently even this has changed. They are now acquiring generation plants under various ownership and leasing arrangements.
Electric utilities now bid for electricity from various generation plants in two auc-tions, one that occurs every day before the power is scheduled for use and another that happens an hour before use.
The U.S. electric power industry today is a complex mix of organizations consisting primarily of shareholder-owned, cooperative-owned, and government-owned utilities engaged in power generation, transmission, and distribution. There are, however, other participants classed as nonutility producers and suppliers. As one of the nation’s largest industries, the revenues generated by the U.S. electric power industry surpass those of the telecommunications, airline, and natural gas industries.
Demand for electricity in the United States has historically been closely correlated with economic growth. Since the end of World War II, electric power demand has
matched the growth in the gross domestic product (GDP), the indicator of economic health. The reasons for this increasing demand for electricity include the population expansion, the surge in the use of electrically powered labor-saving machines, tools, and appliances, wider acceptance of air conditioning in all parts of the country, and the popularity of home entertainment electronics and computers.
Newer models of TVs, stereo systems, and computers consume more power than their predecessors, and the Internet has attracted nearly around-the-clock home com-puter operation. Deregulation introduced competition and the price of electricity with respect to the cost of living index has fallen, encouraging even more consumption.
The electric power industry recognizes three major customer groups: residential, commercial, and industrial. As one might expect, the number of residential customers far exceeds the number of commercial and industrial customers. The commercial tomer base includes retail stores, hotels, offices, and restaurants, but the ratio of cus-tomers to the total sales of electricity is relatively small. The customer base in the industrial sector is the smallest, accounting for less than 1 percent of all electric utility customers. The sector consists primarily of large corporations engaged in manufac-turing, mining, and the processing of oil, chemicals, metals, and food.
Surprisingly, each of these groups buys about one-third of the total power generated in the United States. However, there is yet another smaller group of customers, not classed among the big 3 because it consumes less than 3 percent of all electricity gen-erated. This group includes railroads, national, state, and local government agencies, and the state and municipal authorities that pay for street and highway lighting.
Energy for Electricity Generation
More than 85 percent of all electric power generated in North America is produced by AC generators that are driven by steam turbines. Of this amount, more than 65 percent of the steam is produced by burning of fossil fuels, primarily coal and natural gas. The pie chart Fig. 2-1 illustrates the distribution of energy sources for electrical power gen-eration in the United States. The proportions hold for North America and many European countries as well.
Coal is the dominant fossil fuel consumed to produce steam, accounting for more than 50 percent of all energy consumed. Despite its reputation as a constant threat to its neighbors, nuclear energy accounts for only about 20 percent of the energy consumed for electric power generation. The nuclear reactors function only as steam generators.
Natural gas is in third place among energy sources for steam generation. Oil is also a fossil fuel accounting for only about 3 percent of the energy consumed for electric generation, but most of it is used to power gas turbines in turbine generators or as fuel for the diesel engines in engine–generator sets.
Coal remains the dominant fuel worldwide for producing the steam required for electric power generation, despite efforts toward using the so-called renewable resources:
water power, wind power, and solar power. Coal retains its importance because it is plentiful and relatively inexpensive and because many industrialized countries have
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adequate domestic sources. The United States, for example, does not have to depend on foreign sources for coal.
The burning of coal, oil, and natural gas provides 85 percent of the world’s commer-cial energy and 80 percent of all human-caused carbon dioxide emissions. Energy demand has nearly doubled in the past 30 years, and it is expected to increase another 60 percent by 2020. At present only about 10 percent of the world’s total energy is supplied by renewable energy, although in some countries its use is said to be growing rapidly.
This figure is comparable to the approximately 10 percent figure for North America.
Hydroelectric power dominates among the renewable sources, but such alternative sources as wind turbines, solar cells, biomass fuels, and hydrogen fuel cells still account for only a few percentage points. Nevertheless, some studies have predicted that renewable energy sources could provide half the world’s energy needs by 2050.
The burning of fossil fuels has been identified as the source of most of the world’s pollutants—sulfur dioxide, nitrogen dioxide, particulates, and ozone. These emissions have been blamed for air pollution, smog, and acid rain, and they have been identified as a major cause of death and serious health problems. However, motor vehicles pro-duce far more of these pollutants than electric power plants.
Because it is easier to focus on power plants than vehicles, citizen groups, envi-ronmentalists, and health professionals have demanded more government regula-tions to control and possibly eliminate objectionable emissions from power plants.
Despite the fact that the electric power industry has done much to reduce its emis-sions over the past 20 years, primarily as a result of new plant construction, friction between the government and the industry still exists.
Some power plant owners have argued that some pollution control regulations are excessive, impractical, and too costly to implement in older plants. They say that com-pliance would be so expensive that they would either have to shut down the plants or raise electric rates. They add that by shutting down the plants they would deprive many people living nearby of a reliable local power source, and that low-income families would be unable to pay the higher rates.
Some have said that the obvious solution is to build more nuclear power plants because they do not produce pollutants, but this argument does not seem to be a viable
Figure 2-1 The use of different energy sources to generate electrical power in the United States.
option. Nuclear power plants have long been controversial because they pose a threat to public safety and health due to the possibility of nuclear accidents caused by equipment failure or operator error. This hazard was amply demonstrated by the well-publicized reactor meltdown at Chernobyl in the Ukraine and the many casualties it caused.
More recently, the public has become alarmed over the accumulation of spent nuclear fuel at existing nuclear power plants and the hazards that are presented by transporting large quantities of radioactive waste material from those sites over the nation’s highways to a storage facility in the Nevada desert.
Another serious consideration has been the security at commercial nuclear plants, because of the threat of terrorist attacks on the reactors that could release radioactive materials into the air. All of these factors have led to more legal constraints on the operation of existing nuclear plants and any construction of new ones, along with pres-sure to decommission more existing plants.
As a result of all of this controversy, natural gas is reemerging as the fuel of choice for new power plants in the United States because its combustion by-products are lower in polluting gases and particulates than coal-fired plants. This means that the scrubbing and filtering systems need not be as comprehensive as those required for coal-fired plants.
Federal laws prohibiting the use of both natural gas and petroleum products as fuels for power generation were passed during the energy crisis of the 1970s. That prohibi-tion was only lifted years later, in 1987. Many of the new power plants being built or planned will be capable of generating steam from either natural gas or coal. The choice will depend on the price and availability of natural gas.
Despite high hopes for the renewables, the most important of these sources, hydro-electric generation, has proven to be unreliable in times of drought. Moreover, environ-mental concerns about the damming of bodies of water large enough to produce electric power reliably and cost-effectively have led to public protests against new dam con-struction. Here again, there is pressure to decommission many existing dams to improve the water flow in rivers and restore now submerged lands to a natural condition.
Complaints about the unsightly appearance of wind turbines and the threats they present to migrating birds have cast a shadow on that technology. Hopes for econom-ical power generation from large arrays of solar panels have been dashed, and research into power generation by ocean waves and tides has yet to prove its viability.
Coal-fired, hydroelectric, and nuclear power plants remain the most economical sources for electric generation on an hourly basis for 24-hr periods. Because oil-fueled turbine and diesel engine generators have a higher hourly cost, their operation is reserved for peak periods or as backup when other power plants are offline for repairs.
Newer technologies have been introduced to correct the pollutant emissions from exist-ing coal-fired power plants. Improved fabric filters and electrostatic precipitators are removing particulates, the dust and smoke that affect air quality. An electrostatic charge is applied to the particulates in precipitators, and the particulates are then passed through an electric field where they are attracted to collecting electrodes. The electrodes are then mechanically jolted, causing the particulates to drop into collecting hoppers.
Various flue-gas desulfurization (FGD) processes including lime/limestone wet scrubbers and dry scrubbers are being installed to remove sulfur dioxide, the industrial
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pollutant that forms acid rain. In addition, catalytic reduction systems (SCRs) are reducing the emission that reacts with sunlight to create ground-level ozone, or smog.
North American Power Grid
The North American power grid consists of interconnected grids of generating plants, transmission lines, and distribution facilities that blanket the United States and extend into both Canada and Mexico. Transmission lines link generators to substations that distribute electricity to local customers throughout this vast region. These grids pro-vide electric utilities with alternative power paths in emergencies, and allow them to buy and sell electricity from each other and from other power suppliers.
The U.S. power grid consists of three networks: the large Eastern and Western, and the smaller Ercot within Texas. Essentially independent, they are connected by high-voltage DC lines in only a few locations. In emergencies, power can be transferred from one connection to another, but power failures cannot spread between them.
There are more than 700,000 mi of high-voltage transmission lines in the three interconnected networks of the grid. Each of the three networks in the grid pro-duces and distributes AC, but they are connected by DC links, which are easier to control. Today 138 control areas monitor the grid with computers that predict energy flow and anticipate reactions to power failures. Within each of these networks, the amount of electricity consumed must equal the amount of electricity produced at all times.
The Eastern grid covers the entire East coast from Maine to Florida and extends westward to the Continental Divide and northward into Canada. The Western grid cov-ers the western states from the Continental Divide to the Pacific coast, also extending into Canada and Mexico. The Ercot grid covers eastern Texas.
Each grid is composed of a tangle of transmission lines operated by a diversified group of owners from regulated utilities to government agencies and private power marketers. A disparate set of state, regional, and federal regulators governs the opera-tion of the networks. Far from a perfect system, it still requires that restraints be applied to avoid overloading; consequently, it has been called a “work in progress.”
Transmission operators in strategically located control substations monitor:
■ Electricity flowing from their own regional networks
■ Changes in customer demand
■ Transfers of electricity between the grids
■ Power from transfers flowing through their own grids A computerized system permits the operators to:
■ Control the network
■ Find alternate sources when generation plants are offline
■ Verify that power transfers follow orderly procedures
The transmission lines within the networks operate at voltage levels of 765, 500, 345, and 230 kV. The ever-increasing demand for power in the United States has not been matched by the construction of needed extensions to the existing transmission infra-structure. To remedy this shortcoming, engineers have turned to computer science and electronics for controlling the grids and making them work faster and more efficiently.
The development of specialized high-power silicon thyristors has made it possible to switch high levels of power faster than could be done earlier, compensating, in part, for the lack of needed transmission line extensions. Thyristors, like transistors, can turn the flow of electrons ON and OFF, but they can handle larger power loads more effectively than transistors because of their higher electrical ratings. Moreover, once turned ON, thyristors stay ON. This characteristic allows energy to flow continuously.
However, stock thyristors are unable to switch electrons as rapidly as transistors, which are orders of magnitude faster. This has limited their capabilities for high-speed power switching in the grid. This was overcome with the development of the insulat-ed-gate bipolar transistor(IGBT), a four-layer discrete power transistor that combines the characteristics of a power MOSFET and a thyristor. MOSFET transistors open and close the thyristor’s latch electronically. These devices have also been used to control electric motors and low-power generators.
The IGBTs make the grid less vulnerable to voltage sags, surges, and noise in the power signal. Without electronic control of high-power transmission, power-line loads must be limited to as little as 60 percent of their rated thermal capacity, the tempera-ture at which overheated wires sag into trees or onto the ground, and short out.
Computer-based controllers can bypass surges or sags automatically and much more quickly than would be possible with the manual adjustment of transformers or depen-dence on automatic circuit breakers that sense a disturbance and simply “trip” a trans-mission cable offline. That action can send surges of power through neighboring cir-cuits, tripping them as well, leading to massive regional outages.
The efficient operation of the North American electrical grid now depends on com-puters, software, and solid-state power electronics capable of handling heavy current loads. These systems have improved the reliability of power distribution throughout North America. They have also made it possible to increase the efficiency of existing power plants while reducing the urgency for the construction of the thousands of power plants that will be needed in the United States alone in the near future.
It is expected that electronic controls will eventually be located throughout the nation’s power grids. Integrated network controls could synchronize all of the system’s electronics to optimize flow over the entire grid. The Electric Power Research Institute estimates that integrated control could boost the overall transmission capacity of the existing infrastructure by 30 to 40 percent.
Single- versus Three-Phase Power
The principal elements of an electric power system are the generating stations, the transmission lines, the substations, and the distribution networks. The generators
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