Question

With a neat diagram explain medium head hydro-electric power plants.

Answer 1

Diagram of medium head hydro-electric power plants:

Explanation:-
 If head of water is between 30 and 100 m, the plant is called a medium-head plant.
Potential energy of stored water is medium.
Larger volume of water is required
 Catchment area of medium capacity is required as water requirement is more.
 The power plant is situated at medium distance from dam.
 There is no surge tank forebay acts as a surge tank.
 Penstocks are of medium length and comparatively medium in diameter
 Francis turbines are normally used.
Alternator required in these plants is of low speed and large in diameter.

Answer 2

Medium-Head Hydro-Electric Power Plant

A medium-head hydro-electric power plant operates with a "head" (the vertical distance the water falls) typically ranging from 30 meters to 300 meters. These plants represent a common and efficient design, balancing the high-pressure characteristics of high-head plants with the large water flow requirements of low-head plants.

They are often built in hilly or mountainous regions where a river can be dammed to create a reasonably high reservoir without needing an excessively tall dam structure. The most common type of turbine used in these plants is the Francis Turbine, which is highly efficient under the pressure and flow conditions found in this head range.

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Working Principle

The operation follows a clear energy conversion process: Potential Energy → Kinetic Energy → Mechanical Energy → Electrical Energy.

1. Water Storage (Potential Energy): A dam is constructed across a river to create a reservoir. The water stored in the reservoir has high potential energy due to its height (the "head").

2. Water Conveyance (Kinetic Energy): When power is needed, an intake gate is opened. Water from the reservoir flows through the intake, which is protected by a trash rack to filter out debris like logs and rocks. The water then enters the penstock, a large-diameter pipe that channels it down towards the powerhouse. As the water flows down the penstock, its potential energy is converted into kinetic energy (energy of motion).

3. Pressure Regulation: A surge tank is often connected to the penstock. Its purpose is to absorb sudden pressure rises (water hammer) if the turbine gates are closed quickly and to provide extra water during a sudden increase in load demand, stabilizing the system.

4. Mechanical Energy Conversion: The high-velocity water jet from the penstock strikes the blades of the turbine (typically a Francis turbine). The force of the water causes the turbine runner to spin at high speed. This converts the water's kinetic energy into rotational mechanical energy.

5. Electrical Energy Generation: The spinning turbine is connected by a shaft to a generator. As the shaft turns the generator's rotor, it creates a rotating magnetic field within a set of stationary copper coils (the stator). This process of electromagnetic induction generates electricity.

6. Water Discharge: After passing through the turbine, the water has lost most of its energy. It flows out through a widening tube called the draft tube, which helps to recover some remaining kinetic energy and reduce pressure, thereby increasing the turbine's efficiency. The water is then discharged into the tailrace, from where it rejoins the river downstream.

7. Power Transmission: The electricity produced by the generator is at a relatively low voltage. A transformer in the powerhouse's switchyard steps up the voltage to a high level for efficient long-distance transmission over power lines to consumers.

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Key Components and their Functions

• Dam & Reservoir: Stores water and creates the necessary head.
• Intake & Trash Rack: The entry point for water into the penstock, with a screen to block debris.
• Penstock: The pipe that carries water from the reservoir to the turbine. It is built to withstand high pressure.
• Surge Tank: A vertical pipe or chamber that protects the penstock from dangerous pressure fluctuations. It's a key feature in medium and high-head plants where penstocks are long.
• Francis Turbine: The "heart" of the plant. It is a reaction turbine, meaning it works using both impulse and pressure from the water. Its design is perfectly suited for the moderate pressure and flow rates of medium-head plants.
• Generator: Converts the mechanical energy from the spinning turbine into electrical energy.
• Draft Tube: An expanding tube that slows the water exiting the turbine, recovering energy and improving overall efficiency.
• Powerhouse: A building that houses and protects the turbine, generator, and other control equipment.
• Tailrace: The channel that carries water away from the powerhouse and back to the river.