電力系統中英文外文文獻翻譯(編輯修改稿)

　

【文章內容簡介】 passes into a condenser after leaving the turbine. Condensation is effected by the circulation of large quantities of cold water 4 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 connected directly to large electricity generators. In turbines the action of the steam is kiic. 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 burned, and the heat given off during bustion 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 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 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 kiic energy before 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 provided 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 kiic form and partly in the pressure form. The diesel engine is an excellent prime mover for electricity generation in plant below about 10,000 KVA. It has the advantage of low fuel cost, a brief warmingup 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 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 considerable 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, 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 voltages the insulation cost also rises and to find the optimum voltage we must strike a balance between this cost and the saving throug