Power Electrical System

Most of mankind's energy needs, except transportation, are met by electric power. It is a willing servant in home, office, factory, school, and theater. Electric energy is available almost everywhere. Its ease of production, distribution, and utilization coupled with stable costs has accounted for the rapid rise of electrical devices during the 20th century. Most electric power is generated in large plants that use coal, gas, oil, or nuclear energy. These are generally called central stations, and they require only a few workers. Central stations can be located at any convenient site, generally as long as cooling water is available. Sometimes it is convenient to place a plant near the source of fuel and at other times near the majority of users. The only exception is for hydroelectric plants, which must be on a body of water with a large enough water drop, or head, and a steady flow. Power is brought from the generating plant to the user through a network of wires called transmission and distribution lines. The power can be used as needed simply by turning on a switch.

Electric Utility Industry

The electric utility industry began when Thomas Edison opened the Pearl Street station in New York City on Sept. 4, 1882. That station contained six direct current (DC) generators of about 100 kilowatts (KW), each of which could serve an area of about 1 square mile (2.6 square kilometers). Following the development of polyphase alternating current (AC) systems by Nikola Tesla around 1885 and the introduction of the improved transformer by George Westinghouse in 1887, the way was opened for the modern AC power generation and distribution system. Early generators were driven by steam engines. These were replaced by large steam turbines, and the size of power plants grew to the point where the typical modern plant can generate from 500 to 4,000 megawatts (MW), or millions of watts. There are about 3,500 individual power systems in the United States. They comprise investor-owned public utilities, cooperatives, government-owned systems, and manufacturing industries that also produce power. The federal government owns about 10 percent of the generating capacity including such large organizations as the Tennessee Valley Authority (TVA) but few distribution facilities (see Tennessee Valley Authority). Nonfederally owned systems have few generators but focus on transmission and distribution systems to account for another 10 percent of the total power. About 1,000 cooperatives are usually small distributors of power to rural areas. Industry-owned systems generate power mostly for their own use but may sell surplus power to utilities. Nearly 80 percent of the nation's power generation comes from the approximately 400 investor-owned public utilities. A utility is a monopoly; that is, the customer has no choice but to buy electricity from the local utility company. To assure that the customer is charged a fair rate for electricity and that the utility has a fair return on its investment, state governments (and sometimes the federal government) determine rates through appropriate regulatory commissions.

Generation, Transmission, and Distribution

Conventional steam power plants.

Most modern power plants generate high-pressure (from 2,400 to 4,500 pounds per square inch), high-temperature steam (about 1,000 to 1,200 F, or 540 to 650 C) in boilers up to ten stories high that may be fired by coal, natural gas, or oil. The steam expands through the turbines, which are connected to generators, to do useful work. The steam is then turned back into water in a condenser and finally is fed back to the boiler. The large amount of water that is needed to condense steam demands access to cooling water either from rivers or from large lakes. If neither is close by, cooling towers must be constructed. Some of the water is evaporated in these while it cools the remainder and also condenses the steam. In desert areas "dry" cooling towers, in which air is passed over the condenser in large towers, may be used. This, however, increases the cost. In the early 1990s about 56 percent of electricity in the United States was generated in coal-fired plants, and the percentage was rising. Only 4 percent was generated from oil, about 9 percent from natural gas, and about 21 percent from nuclear power. Some plants are able to switch from gas to oil, depending on the price of the fuel.

Nuclear plants.

The energy source in a nuclear reactor is the heat released by the splitting of uranium-235 or plutonium-239 atoms when they are bombarded by neutrons. The heat is transferred to a coolant, which also serves to control the reaction. The coolant then retransfers the heat to water and converts it into steam. This in turn drives a conventional turbine-alternator combination. Most plants in the United States use either pressurized water or boiling water as the coolant. Other systems may use liquid sodium, a pressurized gas (usually carbon dioxide), or heavy water (deuterium oxide) as the coolant. The coolant is kept separate from the steam to minimize radiation dangers. By the early 1990s there were about 110 operable nuclear reactors with a total capacity of more than 577,000 MW. There were a number of units under construction, but no new orders were placed after 1978 as a result of public concern about reactor safety and the high costs of construction.

Hydroelectric plants.

Electric energy may also be obtained from waterpower. About 10 percent of United States generating capacity is in hydroelectric plants. Water passes downward through a hydraulic turbine that is connected to a generator. Large plants, which depend on large volumes of water dammed upstream, can generate more than 2,000 MW. There are many small plants on rivers, some generating only a few hundred kilowatts. (See Waterpower.) Among the largest plants in the United States are Hoover Dam, which forms Lake Mead on the Colorado River at the Arizona-Nevada border, and the Grand Coulee Dam on the Columbia River in eastern Washington state. Hoover Dam has an installed capacity of 1,244 MW. Grand Coulee has an installed capacity of 2,025 MW. It is part of the Columbia River Basin project, which has a planned capacity of 9,770 MW.

Other commercial plants.

The inability to store electric energy cheaply and efficiently requires that every utility have the capacity to adjust its power output instantaneously to meet the demand. While there is considerable flexibility in operating large steam-power plants, such a plant might not be able to meet a sudden peak demand. It may take as long as eight hours to start another boiler. Many utilities, therefore, have installed gas-turbine peaking plants to produce additional power within a few seconds. These are only about half as efficient as large plants and are used for peak production only. Small and remote villages and towns may rely on diesel engine-driven generators. These may not even be connected to larger power systems.

The generator.

Steam turbines, diesel engines, and hydraulic turbines are connected to alternators that generate AC electricity. Most generators produce three-phase current at voltages ranging from 2,000 to 4,000 volts. Three separate but connected windings are used within each unit. Alternators are rated by the number of volt-amperes they can supply. This is the sum of the products of the winding voltage multiplied by the full load current for the three windings. Modern units are rated from 1 to 1,300 million volt-amperes (MVA), with a typical rating of 500 to 600 MVA for a large generator. Large units are preferred, as they are more efficient, they initially cost less per MVA, and they have fewer parts to maintain than do several smaller units. The frequency of the alternating current must be kept constant by assuring that the turbines and the generator turn at an exactly fixed speed. Considerable engineering effort is devoted to the controls on the steam or water flow to bring this about. In the United States the standard frequency is 60 hertz (cycles per second); in parts of Canada it is 50 hertz. Some electric railroads run on 16 2/3 hertz. Power systems with different frequencies cannot be interconnected directly. Constant frequencies make electric AC motors run at essentially constant speeds. If variable speed control is desired, it can be achieved by electronically varying the frequency just ahead of the motor.

Load management.

Power plants run most economically when each unit operates near its maximum capacity. In this case the cost per kilowatt-hour (KWH) becomes a minimum. Sometimes this is best achieved when the utility either purchases some power from, or sells it to, a neighboring utility. This interchange is usually computer controlled to achieve minimum generating costs. Wilmington, Del., for example, is an industrial center that needs maximum power during the day, while nearby Atlantic City, N.J., is a resort area with its maximum power demand at night. By interchanging power between the two locations, both benefit from lower rates and reduce the need for additional generating units that would not be fully utilized.

Transmission and distribution systems.

The electricity generated in the power plant must be transformed to higher voltages for long-distance transmission. It is most efficient to use the maximum voltage, especially for long distances. Modern transmission systems operate at voltages of from 66,000 to 765,000 volts. The interconnection of transmission systems forms a so-called power grid, which permits the interchange of electricity between utilities. A failure in one part of the system, however, can lead to a power outage for the whole system unless emergency disconnect devices can be actuated. Transmission lines terminate at substations in which the voltage is reduced to the primary distribution voltage, usually 23,000 volts. This voltage is then supplied directly to large industrial users or further transformed down to 2,300 or 4,100 volts for local distribution. To this point all transmission takes place as three-phase power using four wires. Residences usually require only lower voltage single-phase power, or one phase of the three-phase system. Most residential customers are supplied with 220 to 240 volts (nominally 230 volts) using three wires. The secondary winding of the distribution transformer has a center tap that is usually grounded at both the transformer and at the customer's service entrance, resulting in a voltage from the center tap to the end of either winding of 115 volts (see Transformer). This is the voltage generally used at outlets for small appliances and lights. Large appliances such as water heaters, large air conditioners, and ovens use 230 volts directly. A different distribution system is sometimes used in high-density residential areas such as New York City, where power is taken directly from a three-phase system. Here the so-called line-to-line voltage is 208 volts, while the line-to-neutral voltage is 120 volts. Although most large appliances are designed for 230 volts, they will work at 208 volts with some loss of performance.

In the Home

Single-phase power is brought to the home either aboveground (aerial) or below the ground (buried) through three wires. Two of these are covered with insulation and carry the power, while the third often bare is the ground wire. Before entering the house, the wires go through a watt-hour meter. It measures the power consumed, forming the basis for billing electric charges. On entering the home the wires are fed to a circuit breaker or fuse box. This contains a disconnect switch to isolate the home from the power line, a main fuse or circuit breaker, and breakers for the various circuits in the house. Separate breakers protect the 230-volt lines for large appliances. Modern homes are equipped with three wire connections to each outlet to provide full grounding protection. Such 115-volt devices as microwave ovens should only be connected to grounded outlets. Other small appliances are only supplied with two-pronged connections and can be safely operated on a two-wire outlet. In most homes the wall outlets in two or more rooms share a common fuse or circuit breaker. The total use of electricity in residences accounts for about 34 percent of the national electric power output.

Industrial, Commercial, and Agricultural Uses

The introduction of small electric motors in the 1920s allowed factories to couple a motor to each machine. Before that time all machines were powered from one central steam engine or large motor, which drove a maze of shafts, pulleys, and leather belts to each machine. This resulted in uneconomical, noisy, and unsafe operation. Today motors can be built in a variety of sizes and speeds to meet almost any requirement. Many industries notably the aluminum and steel industries use large amounts of power. Electricity is required to produce aluminum from its ore. Much of the hydroelectric power produced at Niagara Falls, for example, is used by the Aluminum Company of America. Electric arc furnaces are common in steel production. They readily provide the controlled high temperatures needed to produce many special alloys. The total industrial use of electricity in the United States accounts for about 36 percent of the national output. Stores, businesses, banks, theaters, hospitals, and other nonmanufacturing organizations account for about 25 percent of the national output. Initially electric power was limited to high-density areas such as cities, where distribution costs were lower than in rural areas. Small farms especially were generally not served. By 1935 only about 11 percent of United States farms had electricity. In 1935 the Rural Electrification Administration (REA) began to extend long-term loans to utilities for constructing power lines and wiring farms, and, more significantly, to about 1,000 rural cooperatives. These were formed especially to distribute electric energy to rural areas and usually to purchase power from investor-owned or public-owned utilities. Today more than 95 percent of the nation's farms are connected to electric utilities, with only a few isolated farms depending on their own power generation. Electric motors on the farm grind feed, pump water, and run milking machines. Electric power pasteurizes milk, refrigerates food products, and keeps newborn livestock and chickens warm. Electricity has allowed farmers to increase their output and their productivity just as it has helped industry.

Direct Current

The most common source of DC, which is required in some applications, is the battery, though AC can also be rectified, or made one directional. The voltage per cell is usually low between 1 and 2 volts as is the maximum amount of current that can be drawn for any length of time. Very high-voltage 300,000 to 750,000 volts DC is sometimes used for very long-distance more than 500 miles (800 kilometers) power transmission. Transmission of DC also permits the interconnection of power systems at different frequencies such as those in parts of Canada and the United States. The high-voltage AC is rectified by using thyristers, or silicon-controlled rectifiers, into high-voltage DC, which is then transmitted. At the other end thyristers in an inverter circuit convert the DC back to AC, which can then be at a different frequency.

Additional Developments

Alternative energy sources have recently gained prominence, but their total power output is still small. Photovoltaic research promises to make solar cells eventually competitive with other energy sources. Here the light of the sun is converted directly into low-voltage DC electricity, which is then converted to AC. The wind has also been used to harness energy and to produce electricity. About 500 MW of generating capacity are available, mostly in California. Few locations, however, have steady winds at appropriate speeds, and costs have been very high. Steam for conventional power plants can also be produced from geothermal reservoirs. Another source is from the incineration of municipal trash. Fuel cells use electrochemical reactions such as between hydrogen and oxygen to produce power directly. Several are under development in Japan. In the United States fuel cells have been used in a few special-purpose applications. Cogeneration. An organization that requires low-pressure steam for heating finds that it is not much more expensive to produce high-pressure steam instead. This is then expanded through a turbine to generate some electric power and is subsequently available at low pressure. If this power is not needed by the plant, it can be sold to a utility. Since 1978 federal legislation in the United States requires utilities to buy such excess power that is "cogenerated" by qualified customers. Since this cogeneration usually takes place during periods of high energy demand, it can be an advantage to both the customer and the utility. The customer receives income for the power sold, while the utility finds less need to build additional power plants.

Read also :

- Tennessee Valley Authority
- Transformer
- Water power
- Wind power
- Turbine

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