水力发电与水轮机简介

troduction of hydro-electric power andhydraulicturbinesPower may be developed from water by three fundamental processes : by action of its weight, of its pressure, or of its velocity, or by a combination of any or all three. In modern practice the Pelton or impulse wheel is the only type which obtains power by a single process the action of one or more high-velocity jets. This type of wheel is usually found in high-head developments. Faraday had shown that when a coil is rotat ed in a magnetic field electricity is generated. Thus, in order to produce electrical ener gy, it is necessary that we should produce mechanical energy, which can be used to rot ate the coil. The mechanical energy is produced by running a prime mover by the ene rgy of fuels or flowing water. This mechanical power is converted into electrical powe r by electric generator which is directly coupled to the shaft of turbine and is thus run by turbine. The electrical power, which is consequently obtaind at the terminals of the generator, is then transited to the area where it is to be used fordoing work.he plant or machinery which is required to produce electricity is collectiv ely known as power plant. The building, in the entire machinery along with other aux iliary units is installed, is known as power house.Keywords hydraulic turbines hydro-electric power classification of hydel plants head schemeThere has been practically no increase in the efficiency of hydraulic turbines sinc e about 1925, when maximum efficiencies reached 93% or more. As far as maximum efficiency is concerned, the hydraulic turbine has about reached the practicable limit o f development. Nevertheless, in recent years, there has been a rapid and marked increa se in the physical size and horsepower capacity of individual units.In addition, there has been considerable research into the cause and prevention of cavitation, which allows the advantages of higher specific speeds to be obtained at hig her heads than formerly were considered advisable. The net effect of this progress wit h larger units, higher specific speed, and simplification and improvements in design h as been to retain for the hydraulic turbine the important place which it haslong held at one of the most important prime movers.1. types of hydraulic turbinesHydraulic turbines may be grouped in two general classes: the impulse type which utilizes the kinetic energy of a high-velocity jet which acts upon only a small part of t he circumference at any instant, and the reaction type which develops power from the combined action of pressure and velocity of the water that completely fills the runner and water passages. The reaction group is divided into two general types: the Francis, sometimes called the reaction type, and the propeller type. The propeller class is also f urther subdivided into the fixed-blade propeller type, and the adjustable-blade type of which the Kaplan is representative.1.1 impulse wheelsWith the impulse wheel the potential energy of the water in the penstock is transf ormed into kinetic energy in a jet issuing from the orifice of a nozzle. This jet dischar ge freely into the atmosphere inside the wheel housing and strikes against the bowl-sh aped buckets of the runner. At each revolution the bucket enters, passes through, and p asses out of the jet, during which time it receives the full impact force of the jet. This produces a rapid hammer blow upon the bucket. At the same time the bucket is subjec ted to the centrifugal force tending to separate the bucket from its disk. On account of the stresses so produced and also the scouring effects of the water flowing over the w orking surface of the bowl, material of high quality of resistance against hydraulic we ar and fatigue is required. Only for very low heads can cast iron be employed. Bronze and annealed cast steel are normally used.1.2 propeller runnersnherently suitable for low-head developments, the propeller-type unit has effecte d marked economics within the range of head to which it is adapted. The higher speed of this type of turbine results in a lower-cost generator and somewhat smaller powerh ouse substructure and superstructure. Propeller-type runners for low heads and small outputs are sometimes constructed of cast iron. For heads above 20 ft, they are made of cast steel, a much more reliable material. Large-diameter propellers may hav e individual blades fastened to the hub.1.3 Francis runnersWith the Francis type the water enters from a casing or flume with a relatively lo w velocity, passes through guide vanes or gates located around the circumstance, and flows through the runner, from which it discharges into a draft tube sealed below the t ail-water level. All the runner passagesare completely filled with water, which acts up on the whole circumference of the runner. Only a portion of the power is derived from the dynamic action due to the velocity of the water, a large partof the power being obt ained from the difference in pressure acting on the front and back of the runner bucket s. The draft tube allows maximum utilization of the available head, both because of th e suction created below the runner by the vertical column of water and because the ou tlet of he drafttube is larger than the throat just below the runner, thus utilizing a part of the kinetic energy of the water leaving the runner blades.1.4 adjustable-blade runnersThe adjustable-blade propeller type is a development from the fixed-blade propel ler wheel. One of the best-known units of this type is the Kaplan unit, in which the bla des may be rotated to the most efficient angle by a hydraulic servomotor. A cam on th e governor is used to cause the bladeangle to change with the gate position so that hig h efficiency is always obtained at almost any percentage of full load.By reason of its high efficiency at all gate openings, the adjustable-blade propell er-type unit is particularly applicable to low-head developments where conditions are such that the units must be operated at varying load and varying head. Capital cost an d maintenance for such units are necessarily higher than for fixed-blade propeller-type units operated at the point of maximum efficiency.2.Hydro-plants may be classified on the basis of hydraulic characteristics as foll ow:① run-off river plants .②storage plants.③pumped storage plants.④tidal plants.They are described below.(1) Run-off river plants.These plants are those which utilize the minimum flow in a river having no appre ciable pondage on its upstream side. A weir or a barrage is sometimes constructed acr oss a river simply to raise and maintain the water level at a pre-determined level withi n narrow limits of fluctuations, eithersolely for the power plants or for some other pur pose where the power plant may be incidental. Such a scheme is essentially a low hea d scheme and may be suitable only on a perennial river havingsufficient dry weather f low of such a magnitude as to make the development worthwhile.Run-off river plants generally have a very limited storage capacity, and can use water only when i comes. This small storage capacity is provided for meeting the hourly flu ctuations of load. When the available discharge at site is more than the demand the ex cess water is temporarily stored in the pond on the upstream side of the barrage, whic h is then utilized during the peak hours.he various examples of run-off the river pant are: Ganguwal and Kolta power ho uses located on Nangal Hydel Channel, Mohammad Pur and Pathri power houses on Ganga Canal and Sarda power house on Sarda Canal.The various stations constructed on irrigation channels at the sites of falls, also fa ll under this category of plants.(2) Pumped storage plants.A pumped storage plant generates power during peak hours, but during the off-pe ak hours, water is pumped back from the tail water pool to the headwater pool for futu re use. The pumps are run by some secondary power from some other plant in the syst em. The plant is thus primarily meant for assisting an existing thermal plant or some o ther hydel plant.During peak hours, the water flows from the reservoir to the turbine and electricity is generated. During off-peak hours, the excess power is available from some other plant , and is utilized for pumping water from the tail pool to the head pool, this minor plant thus supplements the power of another major plant. In such a scheme, the same wateris utilized again and again and no water is wastedFor heads varying between 15m to 90m, reservoir pump turbines have been devis ed, which can function both as a turbine as well as a pump. Such reversible turbines c an work at relatively high efficiencies and can help in reducing the cost of such a plan t. Similarly, the same electrical machine can be used both as a generator as well as a motor by reversing the poles. The provision of such a scheme helps considerably in i mproving the load factor of the power system.(3) Storage plantsA storage plant is essentially having an upstream storage reservoir of sufficient si ze so as to permit, sufficient carryover storage from the monsoon season to the dry su mmer season, and thus to develop a firm flow substantially more than minimum natur al flow. In this scheme, a dam is constructed across the river and the power house may be located at the foot of the dam such as in Bhakra, Hirakud, Rihand projects etc. the power house may sometimes be located much away from the dam . In such a case, the power house is located at the end of tunnels which carry water from the reservoir. Th e tunnels are connected to the power house machines by means of pressure pen-stocks which may either be underground or may be kept exposed .When the power house is located near the dam, as is generally done in the low he ad installations ; it is known as concentrated fall hydroelectric development. But when the water is carried to the power house at a considerable distance from the dam throu gh a canal, tunnel, or pen-stock; it is known as a divided fall development.(4) Tidal plantsTidal plants for generation of electric power are the recent and modern advancem ents, and essentially work on the principle that there is a rise in seawater during high t ide period and a fall during the low ebb period. The water rises and falls twice a day; each fall cycle occupying about 12 hours and 25 minutes. The advantage of this rise a nd fall of water is taken in a tidal plant. In other words, the tidal range, i.e. the differe nce between high and low tide levels is utilized to generate power. This is accomplish ed by constructing a basin separated from the ocean by a partition wall and installing t urbines in opening through this wall.Water passes from the ocean to the basin during high tides, and thus running the t urbines and generating electric power. During low tide，the water from the basin runs back to ocean, which can also be utilized to generat e electric power, provided special turbines which can generate power for either directi on of flow are installed. Such plants are useful at places where tidal range is high. Ran ce power station in France is an example of this type of power station. The tidal range at this place is of the order of 11 meters. This power house contains 9 units of 38,000 kW.4.Hydro-plants or hydroelectric schemes may be classified on the basis of operati ng head on turbines as follows:① low head scheme (head<15m),②medium head scheme (head varies between 15m to 60 m) ,③high head scheme (head>60m). They are described below:(1) Low head scheme.A low head scheme is one which uses water head of less than 15 meters or so. A r un off river plant is essentially a low head scheme, a weir or a barrage is constructed t o raise the water level, and the power house is constructed either in continuation with the barrage or at some distance downstream of the barrage, where water is taken to the power house through an intake canal.(2) Medium head schemeA medium head scheme is one which used water head varying between 15 to 60 meters or so.This scheme is thus essentially a dam reservoir scheme, although the da m height is mediocre. Thisscheme is having features somewhere between low had sch eme and high head scheme.(3) High head scheme.A high head scheme is one which uses water head of more than 60m or so. A dam of s ufficient height is, therefore, required to be constructed, so as to store water on the ups tream side and to utilizethis water throughout the year. High head schemes up to heigh ts of 1,800 meters have been developed. The common examples of such a scheme are: Bhakra dam in (Punjab), Rihand dam in (U.P.), and Hoover dam in (U.S.A), etc.。