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1、山东建筑大学毕业设计外文文献及翻译本科毕业设计外文文献及译文文献、资料题目：Special English for Architectural Electric Engineering and Automation文献、资料来源：期刊（著作、网络等）文献、资料发表（出版）日期：2006.4院 （部）： 信息与电气工程学院学院专 业： 电气工程与自动化班 级： 电气093姓 名： 孙德艺学 号： 2009081102指导教师： 陈红艳翻译日期： 2013.6.30外文文献一：Transformer One of the most valuable apparatus in electric po
2、wer system is the transformer, for it enables us to utilize different voltage levels across the system for the most economical value. Generation of power at the synchronous machine level is normally at a relatively low voltage，which is most desirable economicallyStepping up of this generated voltage
3、 to high voltage，extra-high voltage or even to ultra-high voltage is done through power transformers to suit the power transmission requirement to minimize losses and increase the transmission capacity of the linesThis transmission voltage level is then stepped down in many stages for distribution a
4、nd utilization purposes A transformer is a static device for transferring electric energy from one circuit to another electromagnetically，that is，by induction instead of by conductionIts usual function is to transfer energy between circuits of different voltageA transformer has a magnetic core on wh
5、ich there are two or more windingsThese windings are insulated from each other and from groundIn autotransformers，however，the windings are connected togetherThe assembly of core and coils is normally insulated and cooled by immersion in mineral oil or other suitable liquid within an enclosing tankCo
6、nnection to the windings is by means of insulating bushings，usually through the cover.The “ratio of transformation” is determined by the relative number of turns in each of he windingsThis is known as the “turn ratio” and it is the ratio of the no-load voltagesWhen the unit is carrying load, the rat
7、io of the actual voltages is slightly different because of the drop caused by the flow of 1oad current through the impedance of the transformer windingsAt rated load, this drop is known as the voltage regulation. The amount of voltage drop varies with the power factor of the output even when the kil
8、ovolt-amperes remain constantPractically all power transformers and many distribution transformers have taps in one or more windings for changing the turn ratioChanging the ratio is desirable for two reasons to compensate for varying voltage drop in the system and to assure that the transformer oper
9、ates as nearly as possible at the correct core densityFor the latter purpose, the taps should be in the winding subject to the voltage variation. De-energized tap changers are used when it is expected that the ratio will need to be changed only infrequently, because of load growth or some seasonal c
10、hangeThe desired tap is selected by means of a ratio adjuster(no load taps)Load tap changer(LTC)is used when changes in ratio may be frequent or when it is undesirable to de-energize the transformer to change a tapIt seldom makes much difference to the user which winding or windings are tapped；there
11、fore，the choice is usually made by the designer on the basis of cost and good design. Both winding current and voltage must be considered when applying LTC equipmentHigh voltage and high current applications require special considerations to arrive at an optimum location for the LTC equipmentStep do
12、wn units usually have LTC in the low-voltage winding and de-energized taps in the high voltage windingWhen energy is to be transferred between two circuits of nearly the same voltageThe use of autotransformers affords cost savings over two-winding unitsThe nearer the voltages are to each other, the
13、smaller will be the autotransformer per kilovolt-ampere of output，and the greater the savingsThe simplicity of phasing out systems has increased its useMost autotransformers are Y-connected，and it has been a standard American practice to add a low capacity, delta windingThis is frequently referred t
14、o as a “delta tertiary“Its primary purpose has been to provide an internal path for the third harmonic currents (required for excitation)，thus reducing those currents on the power systemIt also helps to stabilize the neutral and to ground the system betterIn recent years，the use of shielded telephon
15、e cable has reduced the requirements for the delta tertiary. Because an autotransformer does not afford electrical separation between the two circuits, disturbances originating on one circuit can be communicated to the otherThis difficulty is minimized by solidly grounding the neutral of the autotra
16、nsformer. Solidly grounding the neutral，however, causes(among other things)current of short-circuit magnitude to flow through the delta-connected tertiary winding during ground faults on either system. Autotransformers are not inherently self-protecting and, therefore，all windings must be examined for mechanical strength as applied to the system where they will be used . Tertiaries are nor
