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1、3 文献翻译学院名称电气工程学院指导教师职称教授班级学号学生姓名名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 1 页，共 17 页 - - - - - - - - - 南华大学电气工程学院毕业设计第 1 页，共16 页Electric Power Systems Introduction of Electric Power Systems As the power industry grows, so do the economic and engineering problems
2、 connected with the generating, transmission and distribution systems used to produce and handle the vast quantities of electrical energy consumed today. These systems together form an electrical power system. It is important to note that the industry that produces electrical energy is unique in tha
3、t it manufactures its product at the very instant that it is required by the customer. Energy for the generation of electricity can be stored in the form of coal and oil, and of water in reservoirs and lakes, to meet future requirements, but this does not decrease the need for generator capacity to
4、meet the customers demands.It is obvious that the problem of the continuity of service is very important for an electrical power system. No service can be completely protected from the possibility of failure and clearly the cost of the system will depend on its requirements. However a net reliabilit
5、y gain is obtained by employing a certain number of generating units and by using automatic breakers for the separation into sections of the bus bars in generating stations and of the transmission lines in a national or international grid system. In fact a large system comprises numerous generating
6、stations and loads interconnected by high-capacity transmission lines. An individual unit of generation or set of transmission lines can usually cease to function without interrupting the general service. The most usual system today for generation and for the general transmission of power is the thr
7、ee-phase system. In favor of this system are its simplicity and its simplicity and its saving with respect to other a.c. system. In particular, for a given voltage between conductors, with a given power transmitted, with a given distance, and with a give line loss, the three-phase system requires on
8、ly 75 per cent of the copper or aluminium needed in the single-phase system. Another important advantage of the three-phase system is that three-phase motors are more efficient than single-phase ones. The sources of energy ofr large-scale electricity generation are: 名师资料总结 - - -精品资料欢迎下载 - - - - - -
9、- - - - - - - - - - - - 名师精心整理 - - - - - - - 第 2 页，共 17 页 - - - - - - - - - 南华大学电气工程学院毕业设计第 2 页，共16 页1. steam obtained by means of a conventional fuel (coal, oil or natural gas), the combustion of city refuse or the employment of nuclear fuel; 2. water; 3. diesel power from oil. There are other poss
10、ible sources of energy such as direct solar heat, wind power, tidal power, etc., but none of these has yet gone beyond the pilot-plant stage. In large steam power plants, the thermal energy stored in steam is converted into work by means of turbines. A turbine consists essentially of a shaft or roto
11、r fixed in bearings and enclosed in a cylindrical casing. The rotor is made to turn smoothly by means of jets of steam from nozzles around the periphery of the turbine cylinder. These steam jets strike blades attached to the shaft. Central power stations employ condensing turbines in which the steam
12、 passes into a condenser after leaving the turbine. Condensation is effected by the circulation of large quantities of cold water through the tubes of the condenser,thus increasing the expansion ratio of the steam and the consequent efficiency and work output of the turbine. The turbines are connect
13、ed directly to large electricity generators. In turbines the action of the steam is kinetic. There is progressive expansion of the steam from the high pressure and relatively small volume at which it leaves. Steam is made by heating water in a boiler. The usual boiler has a furnace in which fuel is
14、burned, and the heat given off during combustion is conducted through the metal walls of the boiler to generate steam at a pressure within the boiler vessel. In nuclear plants, steam is generated with the aid of a reactor in which the controlled fission of uranium or plutonium supplies the necessary
15、 heat for the vaporization of water. Thus the reactor rep; aces the steam generator of conventional plants. Use is made of the energy possessed by water in hydroelectric stations. In order to transform this energy into work, hydraulic turbines are used. Modern hydraulic turbines may be divided into
16、two classes: impulse turbines and pressure or reaction turbines. Of the former, the Pelton wheel is the only type used in important installations; of the latter, the Francis turbine or one of its modifications is universally employed. In an impulse turbine, the whole head of water is converted into
17、kinetic energy before 名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 3 页，共 17 页 - - - - - - - - - 南华大学电气工程学院毕业设计第 3 页，共16 页the wheel is reached, as the water is supplied to the wheel through a nozzle. In the pressure or reaction turbine the wheel (or runner) is provi
18、ded with vanes into which water is directed by means of a series of guide vanes around the whole periphery. The water leaving these guide vanes is under pressure and supplies energy partly in the kinetic form and partly in the pressure form. The diesel engine is an excellent prime mover for electric
19、ity generation in plant below about 10,000 KV A. It has the advantage of low fuel cost, a brief warming-up period and low standing losses. Moreover it requires little cooling water. Diesel generation is generally chosen for small power requirements by municipalities, hotels and factories; hospitals
20、often keep and independent diesel generator for emergency supply. The transmission of electrical energy by means of lines is a great problem in electrical power systems. Transmission lines are essential for three purposes: 1.To transmit power from a hydroelectric site to a load center perhaps a cons
21、iderable distance away; 2.For the bulk supply of power from steam stations to load centers a relatively short distance away; 3.For interconnection purposes to transfer energy from one system to another in case of emergency. The transmission voltage is determined largely by economic factors. In fact,
22、 in a transmission line, if the distance, the power and the power loss are fixed, the total weight of the conductor varies inversely as the square of the transmission voltage. For the economic transmission of power over considerable distances the voltage must therefore be high. Naturally with higher
23、 voltages the insulation cost also rises and to find the optimum voltage we must strike a balance between this cost and the saving through the reduction of the cross-section of the conductors. For high voltages, overhead-line construction is generally used with suspension-type insulators. Steel towe
24、r, called pylons serve to carry the insulators, with each conductor suspended from the bottom of a group or string of insulator units. The following types of conductor are those most commonly used: stranded copper conductors, hollow copper conductors and ACSR (aluminum cable, steel reinforced) condu
25、ctors. 名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 4 页，共 17 页 - - - - - - - - - 南华大学电气工程学院毕业设计第 4 页，共16 页Distribution includes all the parts of the electricity system between the power substations supplied from high-voltage transmission lines and the consumers swi
26、tch. Electric power is received from substations and distributed to the consumers at the voltage levels and with the degree of continuity that are acceptable to the various types of consumer. In large metropolitan systems both over head and underground distribution methods are used. Although undergr
27、ound distribution is more expensive than an overhead system, it is virtually a necessary in heavily urbanized areas. In smaller towns and in the less congested districts of large cities, the entire distribution system is usually overhead. Electric System Monitoring A modern electric power system is
28、an assembly of many components each of which influences the behavior of every other part. Proper functioning of the system as a whole makes it necessary to monitor conditions existion at many different points on the system in order to assure optimum operation. The concern of the customers is primari
29、ly that the frequency and voltage of the supply are held within certain rather narrow limits. Since frequency of the system is the same everywhere, it may be monitored by a single frequency meter located at any convenient point. In contrast the voltage of the system may be quite different at differe
30、nt points. Consequently, it is necessary to make continuous observation of the voltage at certain key points on the system in order to provide acceptable service. Efficient operation of the system is obtained by assigning proper load schedules to each of the generators on the system. Newer plants, a
31、lthough individually more efficient, may be located at points on the system where their loading occasions large system losses. It is desirable to operate with a division of the load between generators so that the total cost of fuel consumed is minimized. To provide reliability of the power supply in
32、 tile event of unexpected conditions, it is desirable to have the total kilowatt rating of all machines in operation somewhat greater than the total load plus losses. This excess of generation, known as spinning reserve, is then available for picking up suddenly applied customer loads or to pick up
33、the load dropped by a generator that must be removed from service for emergency maintenance. 名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 5 页，共 17 页 - - - - - - - - - 南华大学电气工程学院毕业设计第 5 页，共16 页Instrumentation is necessary to permit billing of Customers for energy us
34、ed. Many interconnections exist between different power systems. Instruments must be provided at interchange points to permit billing for energy transferred from one system to another. The continuous monitoring of energy transfer is necessary to assure that interchanged power is within the limits of
35、 contract agreements. The continuous measurement of conditions on major pieces of equipment is necessary to avoid damage due to overload. As load increases from month to month, points at which additional capacity of equipment is required may be recognized and provision made for the installation of a
36、dditional equipment. Thus instrumentation serves as a guide for future construction in a growing power system. Occasionally, under emergency conditions, a system operator observes that his system load exceeds the ability of the available generating and transmission equipment. He is then faced with t
37、he problem of load shedding or ,more properly, load conservation. It is then necessary to drop selected loads where service interruption is least objectionable. In such an event, he relies on the many instruments which provide information relative to system-operation conditions. Instruments may soun
38、d alarms as advance warnings of conditions requiring action to avoid damage to equipment operating beyond its design limitation. In the event of extreme conditions such as power-system faults, defective equipment is switched out of service automatically. Instruments that continuously monitor current
39、 voltage, and other quantities must be able to identify the faulted equipment and to bring about operation of the circuit breakers which remove it from service, while leaving in service all other equipment on the operating system. The many different electrical devices on a power system and those own
40、ed by the customers are designed for operation within certain specified ranges. Operation, excessive deterioration, or (in extreme cases ) the destruction of the device. Careful attention to the conditions under which equipment is operating may indicate corrective action that must be taken. Overcurr
41、ent on all electrical devices is undesirable, as it produces excessive temperatures, inefficient operation, and reduced service life. Overcurrent in residential 名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 6 页，共 17 页 - - - - - - - - - 南华大学电气工程学院毕业设计第 6 页，共16 页circu
42、its may bring about disconnection of the circuit by fuse or breaker action. Overcurrent in motors may damage insulation, with possible early insulation failure. Undervoltage considerably reduces the efficiency of incandescent lamps and may result in nonoperation of fluorescent lamps. Undervoltage of
43、 the power supply to motors may result in excessive currents in the motors, with possible damage to windings. Overvoltage increases the light output of lamps but in many instances seriously shortens useful life. Overvoltage applied to motors and transformers may result in excessive losses within the
44、 iron, with possible damage to the iron or to the adjacent winding insulation. Overspeed of rotating machines may result in structural damage to rotating parts. The overspeed of the customers production equipment may result in an inferior quality of the product. An out-of-step condition existing bet
45、ween two generators or between a generator and a synchronous motor results in an interruption of useful power transfer between the two machines. An out-of-step condition should be recognized promptly and the machines separated from each other. They may then be resynchronized and brought back into se
46、rvice. Instruments of many different types must be installed at many locations on a power system and on the premises of the many customers. With such instruments, conditions existing on the system may be continuously monitored. Faults on Power SystemsEach year new design of power equipment bring abo
47、ut increased reliability of operation. Nevertheless, equipment failures and interference by outside sources occasionally result in faults on electric power systems. On the occurrence of a fault , current an voltage conditions become abnormal, the delivery of power from the generating station to the
48、loads may be unsatisfactory over a considerable area, and if the faulted equipment is not promptly disconnected from the remainder of the system, damage may result to other pieces of operating equipment. 名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 7 页，共 17 页 - - -
49、 - - - - - - 南华大学电气工程学院毕业设计第 7 页，共16 页A faulty is the unintentional or intentional connecting together of two or more conductors which ordinarily operate with a difference of potential between them. The connection between the conductors may be by physical metallic contact or it may be through an arc
50、. At the fault, the voltage between the two parts is reduced to zero in the case of metal-to-metal contacts, or to a very low value in case the connection is through an arc. Currents of abnormally high magnitude flow through the network to the point of fault. These short-circuit currents will usuall