【正文】 e 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 economically． Stepping up of this generated voltage to high voltage， extrahigh voltage or even to ultrahigh voltage is done through power transformers to suit the power transmission requirement to minimize losses and increase the transmission capacity of the lines． This transmission voltage level is then stepped down in many stages for distribution and utilization purposes． A transformer is a static device for transferring electric energy from one circuit to another electromagically， that is， by induction instead of by conduction． Its usual function is to transfer energy between circuits of different voltage． A transformer has a magic core on which there are two or more windings． These windings are insulated from each other and from ground． In autotransformers， however， the windings are connected together． The assembly of core and coils is normally insulated and cooled by immersion in mineral oil or other suitable liquid within an enclosing tank． Connection 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 windings． This is known as the “turn ratio” and it is the ratio of the noload voltages． When the unit is carrying load, the ratio of the actual voltages is slightly different because of the drop caused by the flow of 1oad current through the impedance of the transformer windings． At 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 kilovoltamperes remain constant． Practically all power transformers and many distribution transformers have taps in one or more windings for changing the turn ratio． Changing the ratio is desirable 山東建筑大學畢業(yè)設計外文文獻及翻譯 for two reasons to pensate for varying voltage drop in the system and to assure that the transformer operates as nearly as possible at the correct core density． For the latter purpose, the taps should be in the winding subject to the voltage variation. Deenergized 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 change． The 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 deenergize the transformer to change a tap． It seldom makes much difference to the user which winding or windings are tapped； therefore， 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 equipment． High voltage and high current applications require special considerations to arrive at an optimum location for the LTC equipment． Step down units usually have LTC in the lowvoltage winding and deenergized taps in the high voltage winding． When energy is to be transferred between two circuits of nearly the same voltage． The use of autotransformers affords cost savings over twowinding units． The nearer the voltages are to each other, the smaller will be the autotransformer per kilovoltampere of output， and the greater the savings． The simplicity of phasing out systems has increased its use． Most autotransformers are Yconnected， and it has been a standard American practice to add a low capacity, delta winding． This is frequently referred to 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 system． It also helps to stabilize the neutral and to ground the system better． In rece