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Hydroelectric Power Plant : Layout, Working And Types


Hydro Electric Power Plant - As the name suggest that hydro power plant suggest that " A plant where electric energy  generation is associated with water.hydro electric power plant harness water power for generation of electric energy.when water drops through a height its energy is able to rotate turbines which are coupled to alternators.
There are many termilogy used to describe the hydro power plant which is called hydrology.the term which is used are describe below:-

Hydrology

 Run-Off 

Only a small part of rain fall can be used for power generation.A major part of rain water evaporates,another parts seeps into soil and form the under ground water and some part is taken up by the vegetation. the remaining part of water flows on the ground surface of the catchment area to  form the stream and is know as run-off.investigation of run-off form the first phase in the planning of a hydro power project.the factor affecting run-off are rain fall pattern,geology of the area,shape and size of the catchment area,the topography and nature of the soil in the catchment area amount of vegetation and the weather condition in the catchment area.

Stream Flow  

During the working on hydro power plant project it is essential to calculate the total energy available in the water.For this calculation a knowledge of the quantity of water flowing in the stream and its variation with time over a period of many years is necessary.From the estimates of the stream flow the following estimates can be prepared:-
  • Average annual available 
  • Minimum annual energy available and firm capacity
  • Additional energy and increase in storage capacity possible with a storage reservoir.
  • Capacity of storage reservoir to equalise the flow to a given minimum
  • Minimum daily output without storage.
  • Capacity of flood control reservoir.
the stream flow measurement are generally done by stream gauging at site and expressed in cubic meter per second.The investigation over a long period about of 20 years are necessary to cover the minimum flow and maximum flow conditions.A factor of safety is also applied to the results to cover the inaccuracies of measurement and unforeseen circumstances.The minimum flow and the storage fix the amount of primary power available.Primary power is that capacity of hydro power plant which is available at all the times when the power is needed.

Hydrograph

Hydrograph is a simple graph which is plotted between stream flow in cubic meter per second and time .Hydrograph is plotted for a particular river site. Generally time is in month and year.it is similar to the chronological load curve.The graph given below is a yearly hydrograph of a river of almost steady flow.the following information is provided by the hydrograph:-
  • The discharge at any time during the period under consideration 
  • The maximum and minimum run off during the period
  • The mean run-off during the period
  • Total volume of flow upto any time is given by the area under the curve upto that time

FLow Duration Curve

Flow duration curve is a rearrangement of all the stream flow element of a hydrograph in descending order.it is similar to the load duration curve.Each point on a flow duration curves shows the percentage time during the period when the flow was equal to or greater than the given flow.Yearly flow duration curve may be prepared from mean monthly flows or daily flows, the curve obtained from daily flows being obviously more accurate.The difference between daily and monthly curve would be very small  for steady streams but may be considerable for flashy stream.

Mass Curve

The mass curve gives the total volume of run-off in cubic meter upto a certain time.the abcissa can be a day month or year.A mass curve is obtained from the record of the average monthly flows.the slope of the curve at any point shows the rate of the flow at any time.If the rain fall is uniform through out the year the mass curve would be a straight line having uniform slope. A typical mass curve is shown in the figure given below.

Storage

In most of the streams the flow is deficient during a part of the year and regulation through artificial storage is necessary.the main function of storage is to make more water available during deficient flow time thus increasing the firm capacity of the station.In addition to increase in firm capacity storage also means that the plant would produce more energy.Availability of storage may convert an uneconomic hydro project into an economic one because increase in firm capacity means that the alternative steam capacity can be dispensed with.

Pondage

The power demand fluctuates with time.if power plant is away from the storage reservoir, a small pond is necessary near the power plant to meet the hourly changes in demand.the capacity of pond should be sufficient to cope with hourly changes for 24 hours .If the week end load conditions are very different from week day condition the capacity of pond should be determined from a study of load and stream flow for one week.the capacity of the pond can be determined in a manner similar to that for determining the capacity of reservoir.

Site Selection for Hydro Electric Plants

The hydro electric power plant is only a small part of the whole project.the power station should be near the dam and storage reservoir.Such a location reduces the length of the penstock and loss of head in penstock.In view of this  several structure such as dam,intake,surge tank power house are involved in the site selection.The essential requirements for hydro scheme are large cathment area,high rain fall steep gradient favourable site for reservoir,solid sub soil etc.Many factors have to be considered in the selection but the following are most important.

Availability of water

The river run off data pertaining to many years should be available so that estimate of the power potentialof the power project can be made. The data should include the average flow minimum and maximum flow and its period .The estimate of water availability should also take into account the evaporation loss and seepage loss.

Water Storage 

Because of wide fluctuation in stream flow storage is needed in most of hydro project to store water during high flow period and use it during lean flow period.The storage capacity can be calculated from the hydro graph.Storage arrangements require topographical and geological investigations so that natural foundations can be used to be the best advantage.The storage reservoir may be located near the power plant or some distance away from it.

Head of  Water

An increase in effective head reduces the quantity of water to be stored and handled by penstocks.The head of water depend on topography of the area.

Geological Investigation

Geological investigations are needed to see that the foundation rock for the dam and other structure is firm,stable, imprevious and strong enough to withstand water thrust and other stresses.The area should be free from earthquakes.

Water Pollution

polluted water may cause excessive corrosion and damage to the metallic structures.this may render the operation of the plant unreliable and uneconomic.As such it is necessary to see that that water is of good quality and will not cause any troubles.
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