电气毕业设计外文文献翻译.doc

附录： 外文资料与中文翻译 外文资料： Selection and Cooling of Power Transformer The selection of the transformer can have a major impact on the cost of a substation,since the transformer represents the major cost item.Nameplate rating is only a guide to transformer application, and should only be used as a first step in the selection process.The transformer is available as a self-cooled unit, or it can be purchased with additional steps of forced cooling that use fan or fans and oil pumps.The nameplate ratings can be increased from 25%to 66% by the addition of fans pumps. The nameplate rating is based on a continuous load producing a 55 0C to 65 0C conductor temperature rise over ambient. Since many transformers do not carry continuous loads,advantage can be gained from the thermal time lag to carry higher peak loads without exceeding the temperature limits. Transformer ratings are based on the assumption that only an extremely slow deterioration of insulation will take place with normal operation. A substantial increase in rating can be achieved by accelerating the loss of insulation life, This increase in rating might approach 200%for an hour or two, and approximately 120% for 24 hours. For substations that are designed to carry full load under the outage of any one transformer, a high emergency rating for a 24-hour period (e.g. Until the failed unit can be replaced) could mean the selection of smaller transformer and a substantial saving in substation cost. The selection of the transformer should involve a careful evaluation of a number of other factors: Impedances should be selected considering their effect on short-circuit duties and low-side breaker ratings both for initial and future station developments. In addition, impedance is important to achieve a division in the parallel operation of transformer. No load tap ranges should be selected to provide an adequate low-side bus voltage. If the high-side or low-side voltages vary over a wide range during the load cycle, it ma