电气英文文献翻译.doc

Elements of electrical power station design = 电站设计基础 / M. V. Deshpande. Wheeler Publishing, 1981 Where water resources are available, hydro-electrie power station are used to supply electrical energy to consumers.This type of station，however, cannot be located everywhere. First,there must be an ample quantity of water at sufficient hard, and suitable site must be available.Stream flow date for a number of years should be studied.The possibility of constructing a dam at a suitable site to store water in the catchment area and the availability of water throughout the year are decisive factors. The amount of power that can be developed depends on the quantity of water available,the rate at which it is available,the head,etc.Hydro-electric projects involve a large amount of civil engineering construction work .Often the project is a multipurpose one embracing irrigation and power,flood control and power,or flood control,navigation and power.One might imagine that hydro-electric power should be very cheap as water does not cost anything and there is no fuel cost,This ,however,is not true.To store water at enough head,it is necessary to have a dam and civil engineering construction works. This costs money,and increases investment costs and fixed costs which are not required for other types of power plants,Whenever a hydro-electric project is considered for power purposes ,it is always necessary to work out the economics comparing hydro-electric power costs with the costs for other forms of station.7.2HydrologyWhen considering the possibility of a hydro-electric project,the first requirement is to obtain data regarding the stream flow of water that would be available,and to predict the yearly possible flow from the data ,The water cycle in general consists of evaporation from seas or oceans and/or other water surfaces on the earth due to heat from the sun,the formation of moist air and clouds ,the circulation in form of rain ,hail or snow .Determination of the amount of stream flow and its variation involves a study of hydrography (or hydrology) which deals with the occurrence and under the earths surface.The science of hydrology is based on meteorology ,geology,agricultural physics,chemistry,botany and data obtained by observation and measurement.It is necessary to calculate the mean annual rainfall in the area under consideration from a record of the annual rainfall for a number of years,say 25to30,and also to note the frequency of dry years.The precipitation or rainfall in a particular area can be measured by rain gauges at various places in the area.A rain gauge consists of a collecting cylinder which exposes a circular gauge consists of a collecting cylinder which exposes a circular surface for collecting the rain and a storage vessel in which the water is retained until measured .Recording rain gauges ,giving a continuous record,are use where possible.Frequency of precipitation is also care fully noted.A curve of precipitation in millimetres plotted against percentage of time gives a range of able for a number of years,the range of distribution of mean monthly precipitation is obtained the general character of the periodic distribution of the rainfall tends to be similar each year in each locality .There is some variation in the occurrence of rainfall from year to year ,but when the record of a number of seasons is available ,it is usually found that the occurrences of rainfall for a given month in a given locality are similar.The frequency of occurrence of a dry year or two consecutive dry years is also seen when records for a long period are available.Precipitation in a particular locality depends to a certain extent on altitude,The general and fundamental tendency is for the precipitation to increase as the altitude increases on the coastal side of a mountain range,if the locality is not far from the general sources of water vapour.Date regarding maximum rainfall in a particular area and the nature of the drainage area help in the study of possible floods in the area.Knowledge of the frequency of high rainfall in the are is also helpful in the above studies.The mean precipitation on a drainage area over a period of time can be obtained by plotting isohyets,or lines of equal precipitation,from the records available and then finding a weighted average by planimeter.Of the total precipitation ,some part of the water is lost.Sources of loss are evaporation from the water are ,soil evaporation ,interception and transpiration losses from land area.The stream flow is known when the water losses have been subtracted from the precipitation .Stream flow consists of surface flow and percolation through the ground.The various factors which affect the distribution of precipitation are meteorological conditions, drainage area characteristics,topography of land,geology of land and vegetation in the area.Evaporation is effected by heat from the sun.Temperature and relative humidity affect evaporation,and so too does wind velocity.Plants absorb moisture from the ground,giving it off to the atmosphere,the process being known as transpiration.During the process ,some water is lost,this is known as transpiration loss.Drainge area characteristics are the size and shape of the are .A small area tends to have a higher unit flood discharge than a large area.The topography of land and surface slopes affects the surface flow of water.Steep slopes tend towards a rapid and large surface flow.The size of the water area affects the losses .A large percentage of water area tends to a diminished run-off.The geological characteristics of the watershed affect run-off. Evaporation varies inversely with the porosity of soil.Percolation and water losses depend on whetner the soil is pervious or impervious.A large surface run-off is experienced when pervious ground is saturated with rainwater or when the ground is of impervious material such as rock or clay.Evaporation and percolation are increased by cultivation.Transpiration and interception losses occur owing to the presence of vegetation.To find the evaporation from a water surface a shallow pan is partly immersed in a lake or reservoir ,the evaporation being measured by the change in the water level in the pan, If the pan were placed on land, the temperature would differ from that of the lake.Humidity and wind,air and water temperatures are noted because evaporation varies with all these factors.Transpiration is estimated in a particular area in an approximate,empirical way.Changes in ground water level are found by a water-level recorder.The amount of moisture in the soil above the water table is found by measuring the electrical resistance of gypsum blocks buried at different depths in the ground.The resistance of gypsum increases as it gets drier.The soil temperature is measured by means of thermocouples placed near the gypsum blocks. In considering the factors affecting losses,the stream flow and the run-off available are determined.A drainage area and profile map is prepared.This shows the length of the river,the size and shape of the watershed,tributaries,lakes,reservoirs,etc.The watershed or drainage area is sometimes called the catchment area with respect to precipitation.7.3Stream Flow :Hydrographs:Flow Duration CurvesFor water-power studies,it is essential to know the streamflow date,over a long period if available.Daily,weekly or monthly flow data for number of years at a site help in finding the average flow data for a number of years at a site help in finding the average flow and output power that are likely to be available from the stream flow.The head available is determined by surveying the site.The minimum or low water flow data help in estimating the firm power that will be available at the site .The maximum flow helps in estimating the floods and in designing the spillway.Estimating the stream flow helps in determining the capacity of storage reservoir for equalising the flow to a given minimum.Knowing the maximum stream flow conditions, it is possible to estimate the necessary capacity of a flood-control reservoir for limiting the discharge to a pre-determined maximum under all conditions except extreme floods. When studing stream flow in a river,it is necessary to find the depth of the river at various sections by river gauging.The depth of the river is measured by wading in shallow water,by wire weight gauge,etc.Measurement of discharge in open channels is done by the velocity-area method or by the flow of water over a weir of known dimensions. In the velocity-area method,the discharge is obtained as the sum of the products of partial cross-sectional areas of the flowing water by the respective measured velocities in such areas.The cross-sectional areas are found by measuring the width and depth at suitably spaced points to show the shape of the river bed ,and summation of the partial areas is computed from these measurement.Velocities in different areas are measured by float or current meters.Thee other method of computing the discharge is by observing the head on a weir and the known dimensions of the weir.The discharge ,or volume rate of flow ,Q,in cubic metres per second,is given by Q=CLh3/2Where L is the length of the weir in metres ,h is the head in metres,and C is a constant.For a sharp-crested weir ,C is 1.85 approximately. Hydrographs. When a river discharge has been measured ,a curve can be plotted showing discharge in cubic metres/second against time in hours.The curve is known as a hydrograph.The ordinates may be plotted in terms of the gauge height ,the number of cubic metres per second per square kilometre,the power in kilowatts that can be developed theoretically per metre of fall,or the energy in kilowatt-hours,days or weeks.A hydrograph shows the variation of flow with time.It will also indicate the power available from the stream at different times of the day or year.It is similar to the load curve used in the study of electrical power.Hydrographs help in noting the extremes of flow more readily than the inspection of tabular values of the discharge.Hydrographs are essential in studies of the effect of storage on flow. Flow duration curve. The flow duration curve is a very convenient form of hydrograph for determining the available power at the site.It indicates the daily ,weekly or monthly flows available as ordinates plotted against percentage of time.The flow is available,one can convert the cubic metres per second per week,or other unit of time.Knowing the head at which the flow is available,one can convert the cubic metres per second per weeks,or the discharge for the period,into power in kilowatts,so that the flow duration curve becomes the load duration curve for a hydro-electric plant .If storage is available at a site upstream from the power plant ,a flow duration curve as modified by the use of storage,will be necessary.Low water flow decides the primary power that can be developed.The flow duration curve also shows the possible heavy flood flow and the time during which it may occur .The data are useful in designing a spillway on the dam to allow flood water to escape from the reservoir.It is easy to convert a hydrograph into a flow duration curve.The following examples illustrate the plotting of a hydrograph and flow duration curve from the flow data in terms of discharge per week.7.5 Investigation of Site Preliminary investigations are made when selecting the site for a dam,The dam should be as close to the turbines as possible,and should have maximum size of pondage and the shortest length of conduit.The ideal site will be one where the dam will have the largest catchment area to store water at a high head,and yet be economical in construction.A general reconnaissance survey is first made from the air .Topographical mapping is then carried out by detailed surveying of the area,Geological studies are made.Methods of exploring the sub-surface are by digging pits ,shafts and/or tunnels,etc.Supplementary methods used are the measurement of electrical resistivity and seismic methods of prospecting.The electrical resistivity method depends on the difference between the resistivities of the ledge rock and the overburden.The seismic method requires the measurement of the ratio of propagation of waves caused by explosion and is dependent on the difference in the elastic properties of the ledge rock and the overburden.Other methods of investigating the geological properties of rock are wash borings,churn drilling,rotary core drilling and diamond drilling.Sub-surface explorations should be made ,and condition of soil mechanics helps a great deal in this respect.Ti is necessary to investigate the possibility and availability of construction materials in the neighbourhood,namely,earth,gravel,concrete aggregates,etc.Foundation conditions have to be studied.水电站：初步设计I7.1介绍 水电站建在有水资源的地方，用于向消费者提供电能。 然而，这种类型的站，可以不设随处可见。 首先，必须有足够数量的水，足够硬，必须提供合适的地点 。应研究了数年的流量资料。在一个合适的集水区兴建水坝和一年四季水的可用性是决定性的因素。可开发的发电量依赖于水量，它是可用率，源头等水电项目涉及大量的土建施工工作。常见项目是一个多用途的包含灌溉电源，防洪电源，或者防洪，航运电源。可以想象，水力发电应该是很便宜的，水不消耗什么，也没有燃料成本。然而，这并不正确。要在源头储存足够的水，它是需要有一个水坝和土建施工工程。这些资金投入，增加投资成本，而不是其他类型电厂所需的固定成本。每当一个水电项目作为电力用途考虑时，水力发电站与其他发电站的费用成本的经济性比较是很必要的。7.2水文当考虑一个水电项目的可能性，第一个要求是获取数据流的水流量，数据流将提供，并预测每年可能的水流量。一般的水循环包括从海或大洋和/或其他地球上的水表面由于来自太阳，潮湿的空气和云层的形成，暴雨，冰雹或雪的形式循环加热蒸发。流流及其变化量的测定涉及发生在地球表面的水文学研究（水文）。水文科学是建立在气象，地质，农业物理学，化学，植物学，通过观察和测量获得的数据基础上的。在该地区的年平均降雨量计算中，要通过数年的年均降雨量纪录考虑，25至30年，并要注意干旱年的发生频率。在某一特定地区的降水或降雨量可以通过在该地区各个地方的雨量计测量。雨量计由一个敞开的收集缸，一个圆形仪表组成。收集缸有一个敞开的圆弧面用来收集雨水并在容器中储存，通过圆形仪表测量。雨量计在可用处记录，并给出连续记录。降水频率也被仔细考虑到了。时间的百分比绘制毫米的降水曲线能够数年的范围内给出，月平均降水量的分布范围，并得到定期的降雨分布的一般性质，每年相同地方，往往是相似的。有一些是每年的降雨量发生变化，但当季节纪录时，它通常在一个给定的地方给定月份的降雨量出现类似的。干旱一年或两年连续干几年发生的频率也可从很长一段时间的记录时得出。在某一特定地区的降水在一定程度上取决于海拔高度。一般的和基本的趋势是降水增加，沿海边的山脉海拔增加如果地方不远处就是一般水汽来源。对于在一个特定的区域和流域最大降雨量的数据，可以帮助该地区洪水性质的研究。在该地区的高频降雨量信息在上述研究也很有帮助。利用现有的记录对流域面积超过一段时间的平均降水量绘制等雨量线，或平等降水线，然后通过所求积仪加权平均可以得到的。在总降水量中，水有某些部分损失。损失的的来源是从水的蒸发，土壤水分蒸发，截留和土地面积的蒸腾损失。水流量中，水的损失已经从降水量减去。水流量包括通过地面的地表径流和渗流。影响降水分布的各种因素有气象条件，流域面积的特点，土地的地形，地质土地，在该地区的植被。蒸发,来自太阳的热量。温度、相对湿和风速度影响蒸发。植物吸收来自地面的水分，排出给大气，这一过程被称为蒸腾。在这个过程中，失去了一些水，这被称为蒸腾损失。流域特点的是大小和形状。小面积的泄洪能力较大面积更强。土地和表面的斜坡地形影响表面流动的水。陡峭的山坡上倾向于迅速和大量的地表径流。水面积的大小影响的水损失。一个面积很大水域很大比例减弱径流损失。流域的地质特征影响径流损失。蒸发与土壤的孔隙率成反比。渗流和水的损失取决于土壤透水与否。一个大的地表径流流过透水地面与雨水或地面时，像岩石或粘土防渗材料会饱和。通过种植增加蒸发和渗透。由于植被的存在减少蒸腾和拦截损失的发生。将一个浅盘