1、 外文资料翻译 Electric Power System Introduction Electric Power System,components that transform other types of energy into electrical energy and transmit this energy to a consumer.The production and transmission of electricity is relatively efficient and inexpensive,although unlike other forms of energy,
2、electricity is not easily stored and thus must generally be used as it is being produced. Components of an Electric Power System A modern electric power system consists of six main components:(1)the power station,(2)a set of transforms to raise the generated power to the high voltages used on the tr
3、ansmission lines,(3)the transmission,(4)the substations at which the power is stepped down to the voltage on the distribution lines,(5)the distribution lines,and(6)the transformers that lower the distribution voltage to the level used by the consumers equipment. Power StationThe power station of a p
4、ower system consists of a prime mover,such as a turbine driven by water,steam,or combustion gases that operate a system of electric motors and generators.Most of the worlds electric power is generated in steam plants driven by coal,oil,nuclear energyor gas.A smaller percentage of the worlds electric
5、 power is generated by hydroelectric(waterpower),diesel,and internal-combustion plants. TransformersModern electric power systems use transformers to convert electricity into different voltages.With transformers,each stage of the system can be operated at an appropriate voltage.In a typical system,t
6、he generators at the power station deliver a voltage of from 1,000 to 26,000 volts(V).Transformers step this voltage up to values ranging from 138,000 to 765,000 V for the long-distances.At the substation the voltage may be transformed down to levels of 69,000 to 138,000 V for further transfer on th
7、e distribution system.Another set of transformers step the voltage down again to a distribution level such as 2,400or 4,160 Vor 15,27,or 33 kilovolts(KV).Finally the voltage is transformed once again at the distribution transformer near the point of use to 240 or 120 V. Transmission LinesThe lines o
8、f high-voltage transmission systems are usually composed of wires of copper,aluminum,or copper-clad or aluminum-clad steel,which are suspend from tall latticework towers of steel by strings of porcelain insulators.By the use of clad steel wires and high towers,the distance between towers can be incr
9、eased,and the cost of the transmission line thus reduced.In modern installations with essentially straight paths,high-voltage lines may be built with as few as six towers to the kilometer.In some areas high-voltage lines are suspended from tall wooden poles spaced more closely together. For lower vo
10、ltage distribution lines,wooden poles are generally used rather than steel towers.In cities and other areas where open lines create a safety hazard or are considered unattractive,insulated underground cables are used for distribution.Some of these cables have a hollow core through which oil circulat
11、es under low pressure.The oil provides temporary protection from water damage to the enclosed wires should the cable develop a leak.Pipe-type cables in which three cables are enclosed in a pipe filled with oil under high pressure(14 kg per sq cm/200psi)are frequently used. These cables are used for
12、transmission of current at voltage as high as 345,000 V(or 345 KV). Supplementary Equipment Any electric-distribution system involves a large amount of supplementary equipment to protect the generators,transforms,and the transmission linesthemselves.The system often includes devices designed to regu
13、late the voltage or other characteristics of power delivered to consumers. To protect all elements of a power system from short circuits and overloads,and for normal switching operations,circuit breakers are employed.These breakers are large switches that are activated automatically in the event of
14、a short circuit or other condition that produces a sudden rise of current. Because a current forms across the terminals of the circuit breaker at the moment when the current is interrupted,some large breakers(such as those used to protect a generator or a section of primary transmission line)are imm
15、ersed in a liquid that is a poor conductor of electricity, such as oil, to quench the current. In large air-type circuit breakers, as well as in oil breakers, magnetic fields are used to break up the current.Small air-circuit breakers are used for protection in shops, factories,and in modern home in
16、stallations.In residential electric wiring, fuses were once commonly employed for the same purpose. A fuse consists of a piece of alloy with a low melting point, inserted in the circuit,which melts,breaking the circuit if the current rises above a certain value. Most residences now use air-circuit b
17、reakers. Power Failures In most parts of the world, local or national electric utilities have joined in grid systems.The linking grids allow electricity generated in one area to be shared with others. Each utility that agrees to share gains an increased reserve capacity, use of larger, more efficien
18、t generators, and the ability to respond to local power failures by obtaining energy from a linking grid. These interconnected grids are large, complex systems that contain elements operated by different groups. These systems offer the opportunity for economic savings and improve overall reliability
19、 but can create a risk of widespread failure. For example, the worst blackout in the history of the United States and Canada occurred August 14, 2003, when 61,800 megawatts of electrical power was lost in an area covering 50 million people.(One megawatt of electricity is roughly the amount needed to
20、 power 750 residential homes.)The blackout prompted calls to replace aging equipment and raised questions about the reliability of the national power grid. Despite the potential for rare widespread problems, the interconnected grid system provides necessary backup and alternate paths for power flow,
21、 resulting in much higher overall reliability than is possible with isolated systems .National or regional grids can also cope with unexpected outage such as those caused by storms, earthquakes,landslides,and forest fires, or due to human error or deliberate acts of sabotage. Power Quality In recent
22、 years electricity has been used to power more sophisticated and technically complex manufacturing processes, computers and computer networks, and a variety of other high-technology consumer goods. These products and processes are sensitive not only to the continuity of power supply but also to the
23、constancy of electrical frequency and voltage. Consequently, utilities are taking new measures to provide the necessary reliability and quality of electrical power, such as by providing additional electrical equipment to assure that the voltage and other characteristics of electrical power are const
24、ant. Voltage RegulationLong transmission lines have considerable inductance and capacitance.When acurrent flows through the line, inductance and capacitance have the effect of varying the voltage on the line as the current varies. Thus the supply voltage varies with the load. Several kinds of device
25、s are used to overcome this undesirable variation in an operation called regulation of the voltage. The device include induction regulators and three-phase synchronous motors(called synchronous condensers), both of which vary the effective amount of inductance and capacitance in the transmission cir
26、cuit. Inductance and capacitance react with a tendency to nullify one another. When a load circuit has more inductive than capacitive reactance, as almost invariably occurs in large power systems, the amount of power delivered for a given voltage and current is less than when the two are equal. The
27、ratio of these two amounts of power is called the power factor. Because transmission-line losses are proportional to current, capacitance is added to the circuit when possible, thus bringing the power factor as nearly as possible to 1. For this reason, large capacitors are frequently inserted as a p
28、art of power-transmission systems. World Electric Power ProductionOver the period from 1950 to 2003, the most recent year for which data are available, annual world electric power production and consumption rose from slightly less than 1 trillion kilowatt-hours(kW.h) to 15.9 trillion kW.h. A change
29、also took place in the type of power generation. In 1950 about two-thirds of the worlds electricity came from steam-generating sources and about one-third from hydroelectric sources.In 2003 thermal sources produced 65 percent of the power, but hydropower had declined to 17 percent, and nuclear power accounted for 16 percent of the total. The growth in nuclear power slowed in some countries,notably the United States, in response to concerns about safety. Nuclear
