Hydroelectric power is a form of renewable source of electricity generation. There are various advantages of hydroelectric power plants over gas-turbine power plants, thermal power plants and diesel-engine power plants. These advantages and disadvantages are described in “Hydroelectric Power Plant Layout and Working” along with the layout and working principle of the hydroelectric power plant. In this post, we’ll explore hydroelectric power plant components and a few differences between different types of turbines.
Hydroelectric Power Plant Components
Reservoir
The reservoir is the basic requirement of a hydro plant. The purpose of the reservoir is to store water. This stored water will be utilized to run the prime mover to produce electric power. It holds water during the rainy season and supplies the same during the dry season.
Dam
The function of the dam is to provide a head to the water. Dam increases the capacity of the reservoir. They are built of concrete or stone. The type of dam depends on foundation condition, local materials, transportation availability, the occurrence of earthquakes, and other hazards.
Spillways
In heavy rainfall, river flow exceeds the storage capacity of the reservoir. The spillway is an arrangement to discharge excess water. It is designed to discharge flood water without damage to the dam. Chute, side-channel, shaft, and siphon are some of the different types of spillways.
Intake Gate
The function of intake is to provide a passage to water to flow into the water conduit, channel, or penstock. The intake gates are shut when we need to empty the water from the conduit.
Forebay or Surge Tank
The load on the generator keeps changing. So, water intake to the turbine has to be regulated according to load. Reduction in load on the alternator causes the governor to close the turbine gates. The sudden closure of turbine gates creates pressure increment in the penstock. In case of an increment of load, the governor opens the turbine gates suddenly to admit more water which creates a vacuum.
The function of a surge tank is to absorb these sudden changes in water requirements. To prevent sudden increments in pressure, due to a decrease in load results in the rising of water level in the surge tank. It creates retarding head and decreases water velocity. On the other hand, at the time of load increment surge tank helps stabilise velocity and pressure in the penstock.
Simply put Forebay or Serge Tank serves as a regulating reservoir, storing water temporarily when the load on the plant is reduced. And provide water when the load increases. It can be considered as an enlarged body of water just above the intake.
Penstock
It is a huge pipe which carries water from the storage system to the turbine. It may be low or high-pressure type. A low-pressure penstock may be a canal or steel pipe while a high-pressure penstock consists of thick steel pipes. Each turbine has its own separate penstock.
Power House
The powerhouse is located at the foot of the dam. It consists of two main parts, firstly a sub-structure to support hydraulic and electric equipment, and secondly, a super-structure to house to protect this equipment.
Water Turbines
Pelton Turbine
The Pelton turbine is an impulse turbine and is used for the large head (head above 300m) and low quantity of water. The potential energy of water in penstock is converted into kinetic energy in a jet of water issuing from the nozzle. The water jet impinges on buckets fixed on the periphery of the rotor and causes movement of the rotor. After performing work, water discharges into the tailrace. Each bucket is hemispherical with a rigid centre. The rotor is made of steel and the buckets are of cast iron. Pelton turbines have horizontal alignment. This turbine is not suitable for heads below 200m.
Francis Turbine
It is a reaction turbine suitable for the medium head (range from 30m to 400m) and medium quantity of water. They are large in size and of vertical type. Alternator mount above the turbine. It consists of the outer ring of stationery guide blades forming a runner. Guide blades control the flow of water. Water flows radially inwards and changes its direction to down while passing through the runner. As water passes over rotating blades, both the pressure and velocity of water reduce which causes a reaction force to drive the turbine.
Kaplan Turbine
It is a propeller (reaction) turbine with adjustable blades. Due to adjustable blades, it operates at high efficiency even under partial load. Some parts of the Kaplan turbine are similar to the Francis turbine. It is suitable for the low head (range from 10m to 60m) and large quantity water.
Differences between Francis and Kaplan turbine
- Kaplan runner has 3 to 6 blades and Francis has 6 to 24 blades
- In Kaplan turbine water strike blades axially while in the Francis turbine water enters radially.
- The speed of the Kaplan turbine lies between 600-1000rpm while for the Francis turbine, it lies between 300-600rpm.
- The efficiency of the Kaplan turbine is higher as compared to the Francis turbine.
- Kaplan turbines are compact, unlike Francis turbines.
Governor
The function of the governor is to keep speed constant when loading increases or decreases. To maintain the supply frequency constant, the speed of the alternator driven by the hydraulic turbine must remain constant. As the load changes, the governor regulates the rate of flow of water to keep speed constant and preserves the power supply balance.