Hydro-Electric Power Plant

Presented By:Harsh Bhavsar 03Parth Contractor-06

Introduction

The hydro-electric power plant plays very important role in development of the country as it provides power at cheapest rate.

About 20% of the world power is generated by hydro-power staion.

In hydro-electric power plants, hydraulic turbines converts the kinetic and potential energy of the water into mechanical power and is thus a prime mover, which is coupled to a generator, produces electric power.

The power plants designed to produce electric power from water , flowing continuously under pressure are known as hydro-electric power plants.

the electrical energy produced by plant is given by

E = gQH *

where, = density of water,

g = gravitational constant,

Q = volumetric flow rate of water,

H = height of water from turbine,

= Overall efficiency of turbine.

General layout and essential components

Reservoir : The function of reservoir is to store the water during rainy season and supply the water during dry season.

Dam : Its function is to increase the height of water level and also increase the working head of the power plant.

Trash rack : Its function is to prevent the entry of debris which might damage the fixed blades and runner of the turbine.

Surge tank : it is a small reservoir which provides better regulation of water in the system during variable load conditions.

Penstock : The function of penstock is to carry water under the pressure from the large reservoir to the turbine.

Turbine : It is the hydraulic machine through which hydraulic energy is converted into mechanical power by the dynamic action of water particles flowing over the blades or vanes mounted on wheel which free to rotate about the axis.

Power house : The power house consists of substructure to support the hydraulic and electric equipments and superstructure to house and protect

this equipments.

Draft tube : The function of draft tube is to discharge water from exit of turbine to tail race. The draft tube is essential part of reaction turbine.

Classification of Hydro-Electric power plant

(A) According to the availability of head :

(1) Low head plants : when the head of water available is less then 50m, the power plant is called low head plant.

(2)Medium head : When available head usually lies between 50m to 300m, the plant is called medium head plant.

(3)High head plants : When available head is usually greater than 300m, plant is called high head plant.

(B) According to the quantity of water available

(1)Run off river plants : When a river flowing through a hilly region, the flowing water is directly fed to turbines & the water is not being stored, the power plant is known as run-off river plants.

(2)Run-off river plant with reservoir : The utility of run-off river plant is increased by providing a reservoir in the plant.

(3)Storage type plants : In this type of plant water is stored during rainy season and supply same during dry season.

(4)Pump storage plants : Where the less amount of water available, there water after passing through turbine is pumped back from the tail race to reservoir.

(5)Mini and micro-hydro plants : When power develops from low head as 5m to 20m, plant is known as mini hydro plant. When power develops from head less than 5m, plant is known as micro- hydro power plant.

(C) According to the nature of load

(1)Base load plants : This type of power plant generate power output continuously. They run without stop.

(2)Peak load plants : This type of plants, generate power during peak load hours. This plant do not run continuously and generates power to meet the demand of electricity.

Advantages

Less operating cost

No requirement of fuel

Totally economical

Its speed of turbine is low compared to thermal power plant. Hence, less mechanical problem and no required any special materials

Its efficiency is higher than others and not changes with age of plant

It is simple in design and maintenance is easy

Disadvantages

The investment cost is high.

Power generation is depends on water availability which depends on natural phenomenon of rain.

The sites are mostly far away from load center. So long transmission line is required.

Time of construction is much more than thermal power plant.

Hydraulic Turbines

Impulse turbine : In this type of turbine, the water from dam is brought to the turbine inlet through pipes ending in one or more fixed nozzles.

The entire pressure energy of water is converted into the kinetic energy of water jet in the nozzle. The water jets then strikes the blades of the runner and loses all its kinetic energy.

Reaction turbine : in this type of turbine, the water pipe feeds water to a row of fixed blades through a casing.

The fixed blades convert a part of the pressure energy into kinetic energy before water enters the runner. Hence water entering the runner of a reaction turbine has kinetic energy as well as pressure energy.

The static pressure at the inlet to the runner is greater then the static pressure at outlet of runner.

The rotation of the runner is partly due to impulse action as well as partly due to reduction of pressure in runner blades.

Pelton wheel impulse turbine

The Pelton wheel is a water impulse turbine. It was

invented by Lester Allan Pelton in the 1870s. The Pelton wheel extracts energy from the impulse of moving water.

The water flows along the tangent to the path of the runner. Nozzles direct forceful streams of water against a series of spoon-shaped buckets mounted around the edge of a wheel.

As water flows into the bucket, the direction of the water velocity changes to follow the contour of the bucket. When the water-jet contacts the bucket, the water exerts pressure on the bucket and the water is decelerated as it does a "u-turn" and flows out the other side of the bucket at low velocity.

In the process, the water's momentum is transferred to the turbine. For maximum power and efficiency, the turbine system is designed such that the water-jet velocity is twice the velocity of the bucket.

A very small percentage of the water's original kinetic energy will still remain in the water. Often two buckets are mounted side-by-side, thus splitting the water jet in half. This balances the side-load forces on the wheel, and helps to ensure smooth, efficient momentum transfer of the fluid jet to the turbine wheel.

Francis Turbine It is an inward flow reaction turbine having radial discharge at

outlet. It was the first inward flow reaction turbine. It was developed by James B. Francis.

Components :

(1) Spiral Casing: The spiral casing around the runner of the turbine is known as volute casing . All throughout its length, it has numerous openings at regular intervals to allow the working fluid to impound on the blades of the runner. these openings convert the pressure energy of the fluid into momentum energy just before the fluid impound on the blades.

To maintain a constant flow rate despite the fact that numerous openings have been provided for the fluid to gain entry to the blades, the cross-sectional area of this casing decreases uniformly along the circumference.

(2) Guide or Stay Vanes: The primary function of the guide or stay vanes is to convert the pressure energy of the fluid into the momentum energy.

it also serves to direct the flow at design angles to the blade runners.

(3) Runner Blades :Runner blades are the heart of any turbine as these are the centers where the fluid strikes and the tangential force of the impact causes the shaft of the turbine to rotate and hence electricity is produced.

In this part one has to be very careful about the blade angles at inlet and outlet as these are the major parameters affecting the power production.

(4) Draft tube: The draft tube is a conduit which connects the runner exit to the tail race where the water is being finally discharged from the turbine. The primary function of the draft tube is to reduce the velocity of the discharged water to minimize the loss of kinetic energy at the outlet. This permits the turbine to be set above the tail water without any appreciable drop of available head.

Working principle :

Francis turbine has a purely radiate flow runner. Water under pressure, enters the runner from the guide vanes towards the center in radial direction and discharges out of the runner axially. Francis turbine operates under medium heads.

Water is brought down to the turbine through a penstock and directed to a number of stationary orifices fixed all around the circumference of the runner. These stationary orifices are called as guide vanes.

The head acting on the turbine is transformed into kinetic energy and pressure head. Due to the difference of pressure between guide vanes and the runner (called reaction pressure), the motion of runner occurs. That is why a Francis turbine is also known as reaction turbine.

Kaplan Turbine

Kaplan Turbine is designed for low water head applications. Kaplan Turbine has propeller like blades but works just reverse. Instead of displacing the water axially using shaft power and creating axial thrust, the axial force of water acts on the blades of Kaplan Turbine and generating shaft power.

Most of the turbines developed earlier were suitable for large heads of water. With increasing demand of power need was felt to harness power from sources of low head water, such as, rivers flowing at low heights. For such low head applications Viktor Kaplan designed a turbine similar to the propellers of ships. Its working is just reverse to that of propeller

Working Principle :

The working head of water is

low so large flow rates are allowed in the Kaplan Turbine. The water enters the turbine through the guide vanes which are aligned such as to give the flow a suitable degree of swirl determined according to the rotor of the turbine.

The flow from guide vanes pass through the curved passage which forces the radial flow to axial direction with the initial swirl imparted by the inlet guide vanes which is now in the form of free

vortex.

The axial flow of water with a component of swirl applies

force on the blades of the rotor and looses its momentum, both linear and angular, producing torque and rotation in the shaft.

The scheme for production of hydroelectricity by Kaplan

Turbine is same as that for Francis Turbine.

Draft Tube

In Reaction turbine as Francis turbine and Kaplan turbine a diffuser tube is installed at the exit of the runner known as Draft Tube.

It is used to increase pressure of water coming out the runner and to discharge water from the exit of the turbine to the tail race.

Functions of draft tube :

It converts kinetic energy into potential energy.

It makes possible the installation of turbine above the trail race without loss of head. This help to do inspection and repair work of the turbine easily.

Using draft tube, the net head on the turbine increases and work developed per kg of water increases. Therefore the turbine efficiency is increased.

Classification : Depending on the shape and alignment,

draft tubes are classified as follows,

(1) Conical draft tube : This type of draft tube consists of a conical diffuser with half angle generally less than equal to 10 to prevent flow separation. It is usually employed for low specific speed, vertical shaft francis turbine. Efficiency of this type of draft tube is 90%.

(2) Simple elbow type draft Tube : It consists of an extended elbow type tube. Generally, used when turbine has to be placed close to the tail-race. It helps to cut down the cost of excavation and the exit diameter should be as large as possible to recover kinetic energy at the outlet of runner. Efficiency of this kind of draft tube is less almost 60%.

(3) Moody spreading tube : This turbine reduces thrilling action of discharge water. It is suitable where water leaves runner with whirl velocity. The efficiency of this draft tube is 85%.

(4) Elbow draft tube with circular inlet and rectangular outlet : The efficiency of this tube is 85%.

Cavitation

Cavitation is formation of vapor bubbles in the liquid flowing

through any Hydraulic Turbine. Cavitation occurs when the static pressure of the liquid falls below its vapor pressure. Cavitation is most likely to occur near the fast moving blades of the turbines and in the exit region of the turbines.

Causes : => The liquid enters hydraulic turbines at high pressure; this

pressure is a combination of static and dynamic components.

=> Dynamic pressure of the liquid is by the virtue of flow velocity and the other component, static pressure, is the actual fluid pressure which the fluid applies and which is acted upon it.

Static pressure governs the process of vapor bubble formation or boiling. Thus, Cavitation can occur near the fast moving blades of the turbine where local dynamic head increases due to action of blades which causes static pressure to fall.

Cavitation also occurs at the exit of the turbine as the liquid has lost major part of its pressure heads and any increase in dynamic head will lead to fall in static pressure causing Cavitation.

Effects of Cavitation :

The formation of vapor bubbles in cavitation is not a major problem in itself but the collapse of these bubbles generates pressure waves, which can be of very high frequencies, causing damage to the machinery.

The bubbles collapsing near the machine surface are more damaging and cause erosion on the surfaces called as cavitation erosion.

The collapses of smaller bubbles create higher frequency waves than larger bubbles. So, smaller bubbles are more detrimental to the hydraulic machines.

With further decrease in static pressure more number of bubbles is formed and their size also increases. These bubbles coalesce with each other to form larger bubbles and eventually pockets of vapor. This disturbs the liquid flow and causes flow separation which reduces the machine performance sharply.

Avoiding cavitation :

To avoid cavitation while operating, Hydraulic Turbines parameters (like Pressure head, flow rate and exit pressure) should be set such that at any point of flow static pressure may not fall below the vapor pressure of the liquid.

Using stainless steel runner of the turbine.

Providing highly polished blades to the runner.

Governing of Hydraulic Turbines=> The load on turbine (demand of power) is not constant throughout the day or hour, hence speed of turbine varies with respect to load at constant head and discharge.

When load on the generator increases, the speed of generator-turbine unit decreases beyond the normal speed. But it is necessary to run the generator at constant speed, therefore the rate of water flow to the turbine should be increased upto the speed becomes normal.

So governing of turbine may be defined as:

The process of providing any arrangement, which will keep the speed constant and will regulate the rate of flow (as per variation in load)

Governing of Pelton wheel

Governing of Francis turbines

Governing of Kaplan turbines

Thank you