Various Components in Hydroelectric Power Plants – Functions

Hydroelectric power plant requires various components for generating electrical power. Some of the major components in hydroelectric power plants are: Reservoirs, Dam, Trash Rack, Forebay, Surge Tank, Penstock, Spillway, Prime Mover and Generator, Draft Tube. The functions of all major components are discussed.
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The basic requirement of a hydroelectric power station is a reservoir where large quantity of water is stored during rainy season and used during the dry season. The reservoir is built by constructing a dam across the river. The water from the reservoir is drawn by the forebay through an open canal or tunnel. The water from the forebay is supplied to the water prime mover through the penstock which is located at the much lower level than the height of the water in the reservoir. Thus potential energy of water stored in reservoir is converted into kinetic energy and made to rotate the turbine. Turbine shaft is connected to synchronous generator or alternator for generating electricity. This generated power is stepped up using step-up transformer and delivered to load centers or grid. The regulation of water flow to the turbine depending on the electrical load demand is carried out by the governor system.

Some of the components of hydroelectric power plants and their functions are given below:

Hydraulic Power plant diagram

Reservoir:

The function or purpose of reservoir is to store the water during rainy season and supply the same during dry season. This is in simple, water storage area.

Dam:

The function of dam is to increase the height of the water level (increase in the potential energy) behind it which ultimately increases the reservoir capacity. The dam also helps in increasing the working head of the power plant. Dams are generally built to provide necessary head to the power plant.

Trash Rack:

The water intake from the dam or from the forebay are provided with trash rack. The main function of trash rack is to prevent the entry of any debris which may damage the wicket gates and turbine runners or choke-up the nozzles of impulse turbine. During winter season when water forms ice, to prevent the ice from clinging to the trash racks, they are often heated electrically. Sometimes air bubbling system is provided in the vicinity of the trash racks which brings warmer water to the surface of the trash racks. 

Forebay: 

The function of forebay is to act as regulating reservoir temporarily storing water when the load on the plant is reduced and to provide water for initial increment of an increasing load while water in the canal is being accelerated. In many cases, the canal itself is large enough to absorb the flow variations. In short, forebay is naturally provided for storage of water to absorb any flow variations if exist. This can be considered as naturally provided surge tank as it does the function of the surge tank. The forebay is always provided with some type of outlet structure to direct water to penstock depending upon the local conditions.

Surge Tank:

The main function of surge tank is to reduce the water hammering effect. When there is a sudden increase of pressure in the penstock which can be due sudden decrease in the load demand on the generator. When there is sudden decrease in the load, the turbine gates admitting water to the turbine closes suddenly owing to the action of the governor. This sudden rise in the pressure in the penstock will cause the positive water hammering effect. This may lead to burst of the penstock because of high pressures.

When there is sudden increase in the load, governor valves opens and accepts more water to the turbine. This results in creation of vacuum in the penstock resulting into the negative water hammering effect. Therefore the penstock should have to withstand both positive water hammering effect created due to close of governor valve and negative water hammering effect due to opening of governor valve.In order to protect the penstock from these water hammering effects, surge tank is used in hydroelectric power station.

A surge tank is introduced in the system between dam and the power house nearest. Surge tank is a tank provided to absorb any water surges caused in the penstok due to sudden loading and unloading of the generator. When the velocity of the water in the penstock decreases due to closing of turbine valves, the water level in the surge tank increases and fluctuating up and down till its motion is damped out by the friction. Similarly when the water accelerates in the penstock, water is provided by the surge tank for acceleration. Surge tank water level falls down and fluctuates up and down absorbing the surges.

Penstock:

Penstock is a pipe between the surge tank and the prime-mover. The structural design of the penstock is same as for any other pipe expect it has to bear high pressure on the inside surface during sudden decease in the load and increase in the load. Penstocks are made of steel through reinforced concrete. Penstocks are usually equipped with the head gates at the inlet which can be closed during the repair of the penstocks, A sufficient water head should be provided above the penstock entrance in the forebay or surge tank to avoid the formation of vortices which may carry air in to the penstock and resulting in lower turbine blade efficiency.

Spillway:

The function of spillway is to provide safety of the dam. Spillway should have the capacity to discharge major floods without damage to the dam and at the same time keeps the reservoir levels below some predetermined maximum level.

Power House:

A power house consists of two main parts, a sub-structure to support the hydraulic and electrical equipment and a superstructure to house and protect this equipment. 

The superstructure of most power plants is the buildings that house all the operating equipment. The generating unit and the exciter is located in the ground floor. The turbines which rotate on vertical axis are placed below the floor level while those rotating on a horizontal axis are placed on the ground floor alongside of the generator.

Prime movers or Hydro Turbines:

The main function of prime movers or hydro turbines is to convert the kinetic energy of the water in to the mechanical energy to produce the electric power. The prime movers which are in common use are pelton wheel, francis turbine and kaplan turbines.

Draft tube:

The draft tube is a part of the reaction turbine. The draft tube is a diverging discharge passage connecting the running with tailrace. It is shaped to decelerate the flow with a minimum loss so that the remaining kinetic energy of the water coming out of the runner is efficiently regained by converting into suction head., thereby increasing the total pressure difference on the runner. This regain of kinetic energy of the water coming out from the reaction turbine is the primary function of the draft tube. The regain of static suction head in case where the runner is located above the tail water level is the secondary purpose of the draft tube.