Power House And Turbine Setting
According to the location of the hydel power station, the power houses are classified as surface power house or underground power house. As the name implies, the underground power house is one which is built underground. A cavity is excavated inside earth surface where the sound rock is available to house the power station. A surface power house is one which is founded on earth’s surface and its superstructure rests on the foundation.
The surface power house has been broadly divided into three subdivisions which is separated from the intake as mentioned below :
(a) Substructure ; (b) Intermediate structure ; (c) Super-structure.
(a) Substructure. The substructure of a power-house is defined as that part which extends from the top of the draft tube to the soil or rock. Its purpose is to house the passage for the water coming out of the turbine. In case of reaction turbines, the hydraulic function of the sub-structure is to provide a diverging passage (known as draft tube) where the velocity of the exit water is gradually reduced in order to reduce the loss in pushing out the water. In case of impulse turbine, such a draft tube is not required and only an exit gallery would serve the purpose.
The structural function of substructure is dual. The first function is to safely carry the superimposed loads of machines and other structures over the cavities. The second function is to act as transition foundation member which distributes heavy machine loads on the soil such that the obtainable ground pressures are within safe limits.
(b) Intermediate structure. The intermediate structure of a power house may be defined as that part of the power house which extends from the top of the draft tube to top of the generator foundation. This structure contains two important elements of the power house, one is the scroll case which feeds water to the turbine. The generator foundation rests on the scroll-case which is embedded in the concrete. The other galleries, adits and chambers also rest on the same foundation. Scroll or spiral case is a part of the turbine and it distributes water coming from penstock uniformly and smoothly through guide vanes to the turbine. The scroll case is required only in case of reaction turbine. In case of impulse turbine the place of scroll case is taken by the manifold supplying water to the jets.
(c) The structural function of the concrete around scroll case would depend upon the type of scroll case used. If the scroll case is made of steel and strong enough to withstand internal loads including the water hammer effects, the surrounding concrete acts more or less as a space fill and a medium to distribute the generator loads to the sub-structure. If it is a concrete scroll case then this concrete should be strong enough to withstand the internal hydrostatic and water hammer head as well as the external superimposed loads on account of the machine etc. Many times, the steel scroll case is used as water linear and in this case the surrounding concrete must be strong enough to withstand the internal hydraulic pressures in addition to the superimposed loads. The structural function of the generator foundation is to support the generator. Arrangements may be made either to transmit the load 'directly to the substructure through steel barrel or through a column beam or slab arrangement.
(c) Superstructure. The part of the power house above the generator floor right upto the roof is known as superstructure. This part provides walls and roofs to power station and also provides an overhead travelling crane for handling heavy machine parts.
The arrangement of the power house is shown in Fig.
Arrangement of Reaction and Impulse Turbines. Factors affecting the choice between horizontal and vertical setting of machines are : relative cost of plant, foundations, building space and layout of the plant in general.
Vertical machines offer many advantages over horizontal especially when there are great variations in tail-race level. Horizontal machines turbine-house should be above the tail-race level or the lower part of the house must be made watertight. In vertical machines, the weight of rotating parts acts in the same direction as axial hydraulic thrust. This requires a thrust bearing capable of carrying considerable heavy load. The efficiency of the vertical arrangement is 1 to 2% higher than for a similar horizontal arrangement. This is due to the absence of a suction bend near the runner. As the alternator being mounted above the turbine, it is completely free from flooding.
With the horizontal machines, there may be two turbines driving one generator and turbines would operate at a higher speed bringing about a smaller and lighter generator. The horizontal machines would occupy a greater length than the vertical but the foundations need not be so deep as required for vertical machines. The horizontal shaft machines require higher settings to reduce or to eliminate the cost of sealing the generator, the auxiliary electrical equipment and cable ducts against water.
In actual cases, the arrangement of the machine (vertical or horizontal) is so chosen which will give the lowest cost of the station. The majority of impulse turbines are of the horizontal shaft types. The horizontal arrangement is simpler than vertical from constructional and maintenance point of view. The overall height and width of the station will be relatively greater in case of vertical arrangement. The floor space occupied by horizontal shaft units is in general greater than that required for vertical shaft machines. Horizontal shaft arrangement is adopted in most cases, for Pelton wheels, mainly because this type of setting lends itself readily to the use of multiple runner units and secondly, because the resulting hydraulic conditions are not favourable with vertical machines.
There are mainly two principal types of setting as : (1) open flume and (2) cased turbines.
The open flume setting as shown in Figs (Rewalls power plant on black river at Watertown in U.S.A.) are chiefly used for low heads with concentrated falls or with a short canal. Open penstock setting is one where the entry to the runner has no casing but is placed in an open forebay. The runner should be placed at a convenient depth below the water surface such that eddies and suction of air through vertices will not take place. The turbine is completely submerged which results in a simple and comparatively cheap plant. The disadvantage of this arrangement is that the pit must be drained to enable inspection and maintenance to be carried out on the turbine and guide vane mechanism. The turbine should have an adequate water head above it, otherwise a sudden increase in load may draw the water to a dangerous level and allow air to enter. Such condition would break the vacuum in the draft tube and stop the turbine.
The cased turbines are further divided as concrete casing or steel plate casing as mentioned earlier. The width of the concrete flume should be kept as small as possible as design permits because the concrete approach flume often fixes the machine spacing. The concrete scrolls are limited to low head installations upto 20 metre heights. The complicated form work and reinforcement required for a concrete flume makes it expensive so that other methods of construction have to be used.
Steel plate scrolls are used for heads ranging from 10 m to 120 m. The arrangement of steel scroll is shown in Fig.
Underground Power House. The conventional hydro-electric power stations are usually located over-ground at the foot of a dam or a hill slope on the banks of a river. The first underground power station Nerayaz was built in 1897 in Switzerland. The high capacity underground power plants were built only after second world war. The idea of locating powerhouse underground was suggested not only with the intention of protecting them against air raids but also technical and economical considerations were mainly considered. After second world war, the immunity against air attacks was unquestionably regarded as an important. advantage-of underground power station. A large number of underground power stations have been installed in U.K., U.S.A., Russia, Canada, Japan after second world war and recently in India also.
In all, there are about 300 such stations in service with a total installed capacity of 31 million kW Fig. Horizontal setting of Pelton wheel with penstock upto the end of 1963.
The considerations supporting the construction of underground power stations are stated below :
1. Non-availability of a suitable site for a conventional surface station and good slope for penstock.
2. Danger of falling rocks and snow avalanches particularly in narrow valleys.
3. Availability of underground sound rock and avoidance of a long pressure tunnel and facility for a convenient tail-race outlet.
4. Possibility of elimination of surge tank required for surface station due to long pressure tunnel
5. The rugged topographical features and the difficulties in finding a suitable short and steep slope for pipe lines make it more economical to install the water conduit, the machine, transformer hall and tailrace system underground.
6. Foundation costs for over ground power house become excessive in case of poor quality surface layers. The construction of draft tube, spiral case and separating floors in loose weathered rock is again more expensive than the excavation of corresponding parts underground. The costs of underground machine hall are lower than those of the superstructure of a surface powerhouse of similar dimensions.