外文翻译--电力系统介绍.doc

外文翻译Electric Power SystemElectric 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, electricity is not easily stored ad thus must generally be used as it is being produced.Components of an Electric Power SystemA modern electric power system consists of six main components: (1) the power station, （2）a set of transformers to raise the generated power to the high voltages used on the transmission lines, (3) the transmission lines, (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 Station, the power station of a power system consists of a prime mover, such as a tribune 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 hydroelectric (water power), diesel, and internal-combustion plants.Transformers, Modern 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, the generators at the power station deliver a voltage of from 1,000 to 26,000volts (v). Transformers step this voltage up to values ranging from 138,000 to 765,000 V for torture transfer on the distribution system. Another set of transformers step the voltage down again to a distribution level such as 2,400 or 4,160 V or 15, 27, or 33 kilovolts (kV). Finally the voltage is transformed once again the distribution transformed near the point of use to 240 or 120 V.Transmission Lines, The lines of high voltage transmission system are usually composed of wires of copper, aluminum-clad steel, which are suspended form 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 increased, 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 voltage distribution lines, wooden poles are generally used rather than steel towers. In cites 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 circulates 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 transmission of current at voltage as high as 345,000V (or 345 kV).Supplementary Equipment Any electric-distribution system involves a large amount of supplementary equipment to protect the generators, transformers, and the transmission lines themselves. The system often includes devices designed to regulate 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 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 where the current is interrupted, some large breakers (such as those used to protect a generator or a section of primary transmission line) are immersed 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 installations. In residential electric wiring, fuses were once commonly employed for the same purpose. A fuse consists of piece of alloy with a low melting point, inserted in the circuit, which melts, breaking the circuit if current rises above a certain value. Most residences now use air-circuit breakers.Power FailuresIn 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 efficient 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 saving and improve overall reliability 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 megawatts of electricity is roughly the amount needed to 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, resulting in much higher overall reliability than is possible with isolated systems. National or regional grids can also cope with unexpected outages such as those caused by storms, earthquakes, landslides, and forest fires, or due to human error or deliberate acts of sabotage.Power qualityIn recent years electricity has been used to power more sophisticated and technically complex manufacturing processes, computers and computer network, 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 constancy of electrical frequency and voltage. Consequently, utilities are taking new measure to provide the necessary reliability and quality of electrical power, such as by providing additional electrical equipment assure that the voltage and other characteristics of electrical power are constant.Voltage Regulation long transmission lines have considerable inductance and capacitance. When current flows through the lines, 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 devices are used to overcome this undesirable variation in an operation called regulation of the voltage. The devices include induction regulation and three-phase synchronous motors (called synchronous condensers), both of which vary the effective amount of inductance and capacitance in the transmission circuit.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 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 part of power-transmission systems.World Electric Power Production Over the period from 1950 to 2003, the most recent year for which data are available, annual world electrical power production and consumption rose from slightly less than 1 trillion kilowatt-hours () to 15.9 trillion A change 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 hydro electric sources. In 2003thermal 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 grown in nuclear power showed in some countries, notably the United States, in response to concerns about safety. Nuclear plants generated 20 percent of U.S. electricity in 2003; in France, the world leader, the figure was 78 percent.ConservationMuch of the worlds electricity is produced from the use of nonrenewable resources, such as natural gas, coal, oil, and uranium. Coal, oil, and natural gas contain carbon, and burning these fossil fuels contributes to global emissions of carbon dioxide and other pollutants. Scientists believe that carbon dioxide is the principal gas responsible for global warming, a steady rise in Earths surface temperature.Consumers of electricity can save money and help protect the environment by eliminating unnecessary use of electricity, such as turning off lights when leaving a room. Other conservation methods include buying and using energy-efficient appliances and light bulbs, and using appliances such as washing machines and dryers, at off-peak production hours when rates are lower. Consumers may also consider environmental measures such as purchasing “green power” when it is offered by a local utility, “Green power” is usually more expensive but relies on renewable and environmentally friendly energy sources, such as wind turbines and geothermal power plants.电力系统介绍电力系统把其它形式的能源转化为电能并输送给用户。尽管不同于其它形式的能源，电能不容易储存，一旦生产出来，必须得到使用，但是电力的生产和传输相对高效和廉价。电力系统的组成当今的电力系统由六个主要部分组成：电站，升压变压器（将发出来的电升压至传输线所需高电压），传输线，变电站（电压降至配电线电压等级），配电线路和降压变压器（将配电电压降至用户设备使用的电压水平）。 1、电站。电力系统的电站包括原动机，如由水，蒸汽驱动的涡轮，或者燃烧气体操控的电动机和发电机系统，世界上大多数的电能由煤炭、石油、核能或者燃气驱动的蒸汽发电厂产生。少量电能由水力，柴油和内燃机发电厂产生。2、变压器。现代电力系统使用变压器把电能转换为不同的电压。有了变压器，系统的每个阶段都能在合适的电压等级下运行。在典型的系统中，电站发电机发出的电压范围是1000伏到26000伏。变压器把电压升至138000到765000伏后，送至主传输线上。因为对于长距离传输，电压越高，效率越高。在变电站，电压被降至69000到138000伏，以便在配电系统中传输。另外一组变压器把电压进一步降至配电等级，如2400到4160伏，或者15，27，33KV。最终，在使用端，经配电变压器，电压再次被降至240V或120V。 3、传输线。高压传输系统通常由铜线、铝线或者镀铜、镀铝的钢线组成，它们悬挂在高大钢格构塔架上成串的断瓷质绝缘体上。由于含镀层钢线和铁塔的使用，增大了塔与塔之间的距离，降低了传输线的成本。在当前的直线安装中，每公里高压线只需建立6个铁塔。在一些地区，高压线悬挂于距离较近的木质电线杆上。对于低压配电线路，更多的使用木质电线杆，而不是铁塔。在城市和一些地区，明线存在安全危险或者被认为影响美观，所以使用绝缘地下电缆进行配电。一些电缆内核中空，供低压油循环。油可以为防止水对封闭线路的破坏提供临时保护。通常使用管式电缆，三根电缆放入线管中，并填满高压油。这些电缆用于传输高达345KV的电流。4、辅助设备。每个配电系统包含大量辅助设备来保护发电机、变压器和传输线。系统通常还包括用来调整电压或用户端其它电力特性的设备为了保护电力系统设施，防止短路和过载，对于正常的开关操作，采用断路器。断路器是大型开关，在短路时或者电流突然上升的情况下自动切断电源。由于电流断开时，断路器触点两端会形成电流，一些大型断路器（如那些用来保护发电机和主输电线的断路器）通常浸入绝缘液体里面，如油，以熄灭电流。在大型空气开关和油断路器中，使用磁场来削弱电流。小型空气开关用于商场，工厂和现代家庭设备的保护。在住宅电气布线中，以前普遍采用保险丝。保险丝由熔点低的合金组成，安装在电路中，当电流超过一定值，它会熔断，切断电路。现在绝大多数住宅使用空气断路器。供电故障世界上大多数地方，局部或全国电力设施都连成电网。电网可以使发电实现区域共享。同意共享的每个电力企业可以获得不断增加的储备功率