Introduction

Introduction:
This is the era of development. Electricity has played a vital role for the socioeconomic
development of the country. Now almost all the devices are operated by
electricity. So to generate electricity, power plant is very necessary.
Power plant:
Power plant is an assembly of equipment that produces and delivers mechanical and
electrical energy. A power plant is also referred to as a generating station or
powerhouse. It is an industrial facility for the generation of electric power.
River water screening System:
Water is the most important thing in a steam power plant. Water supplies generally are
divided into two major categories: surface water and ground water. Surface water is
available from rivers lakes and other impoundments as well as the sea. Ground water is
present is below the earth’s surface.
In Ashuganj Power plant, water is collected from the river Meghna. This water
comes through 3 channels. One channel is for CCPP, ST-1 & ST-2 and one channel is
for ST-3, ST-4 & ST-5 and other channel is for demi water.
This water is used mainly for generation of steam, for condenser, cooling of
overall system, cooling of lubricating oil, for chillers etc. So to use the water efficiently
water screening system is essential. There are three steps of water screening system.
They are described in below:
1. Heavy-duty Bar: It’s the first stage of screening system. In this stage water
is screened by trust rack. Heavy wastes like wooden piece, dead animals,
water hyacinth etc. are removed in this stage.
2. Rotating Bar: In this stage water is screened by scrapers. Smaller waste
than heavy-duty bar waste are removed in this stage.
Figure: Water screening system with Rotating and Band screening bar.
3. Band Screening Bar: It is a process where 57 baskets are arranged in such
a way that screens the water by passing water through net to remove very
small wastes like small pieces of stones, sand etc. It is the most effective and
modern screening process. A tray is placed below the baskets (net) that takes
the waste which is separated from nets by spraying water

Transformer

A transformer is an electrical device that takes electricity of one voltage and changes it into another voltage.

Types of transformer:                    

Ø      Step up

Ø      Step down

Ø      Isolating

Ø      Variable etc.

In Ashuganj power station, step up transformers are used to up the voltage and minimize transmission loss.

Step up transformer:

            The transformer in which voltage is increased is called step up transformer. Here the secondary has more turns than the primary winding.Figure: Transformer.

Main parts of transformer: 

                       

Ø      Transformer shell

Ø      Inner core

Ø      Oil reservoir

Ø      Cooling system

Ø      Bushing

Transformer shell:

It is a shell generally made of cast iron or steel. It holds all the equipments inside of shell. In this sense it is an outer casing of transformer.

Inner core:    

Inner core means the main core of parallel plates arranged into the transformer shell. There are two types of windings, primary and secondary.

                        

                Figure: transformer core and windings.

Oil reservoir:

            It is a reservoir which keeps the oil reserved for cooling of transformer.

Cooling system:

            In cooling system, oil is used to cool the inner core of transformer by absorbing internal heat. Then this oil is cooled by circulating air. In air cooling system, there are fans and radiators to cool the hot oil. Oil flow through the radiators and air is blown across the radiators to cool the oil. Then the cooled oil is returned to transformer oil reservoir.

Bushing:

            Bushings are insulated bars or wires that connected the transformer with other electrical equipments. It is usually filled with oil to provide extra insulation. To prevent faults, non-conducting gaskets are used with the bushing of transformer.

Generator cooling system:

Normally generators are cooled by hydrogen gas. A cooling system consist some hydrogen cylinders, two pressure regulating valves, two isolation valves and a manifold.

                       

Figure: Generator cooling system.

Working principle:

Ø      Hydrogen gas is supplied from gas cylinders.

Ø      Gas pressure is maintained at 100-125 psig by the 1^st pressure regulating valve.

Ø      Gas pressure is maintained at lower value of 30 psig by 2^nd pressure regulating valve.

Ø      Two insulation valves are used for insulation the supply.

Ø      Finally hydrogen enters into generator through a manifold and cools the entire temperature.

Megawatt and Mega VARs:

Megawatt:

            The megawatt is equal to one million (10⁶) watts. Many events or machines produce or sustain the conversion of energy on this scale. For example: lightning strikes, large electric motors, large warships, such as aircraft carriers, cruisers, and submarines, engineering hardware, and some scientific research equipment, such as supercollider’s, and in the output pulses of very large lasers. A large residential or commercial building may consume several megawatts in electric power and heat.

The productive capacity of electrical generators operated by a utility company is often measured in MW.

Mega VARs:

            Mega VAR stands for Mega Volt*Amps Reactive. Although reactive power is not 'real' (i.e. it is not considered power at the source or destination), it still consists of current flowing in the transmission lines. When current flows in the line, power (real power) is lost due to I²R losses. When you are talking about Mega VARs, this power loss is significant and is a direct loss for the power company. This reactive power is usually caused by a company having a large inductive load (lots of motors). If it is bad enough, sometimes companies will put large capacitor banks in or near their factory to try and balance things out. If they don't, the power company may ask them to pay for the losses.

 

Here cosθ is known as power factor. It is the ratio of real power to apparent power. It lies between the number 0 and 1.

Kilowatt-hour (KWH):

            A kilowatt-hour is the amount of energy equivalent to a steady power of 1 kilowatt running for 1 hour. Generally it is known as unit.

Hence,

            1 unit electricity = 1 KWH power.

Generator:

In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. The source of mechanical energy may be a reciprocating or turbine steam engine, water falling through a turbine or waterwheel, an internal combustion engine, a wind turbine, a hand crank, compressed air or any other source of mechanical energy.

           

                                         

 Figure: Electric generator.

           

Working principle:

Electromagnetic induction is the production of voltage across a conductor moving through a magnetic field. So to get voltage, induction is very necessary. There are three conditions of proper induction.

1.      Conductor

2.      Magnetic fields and

3. Relative motion between them. 

 

 

3. Relative motion between them.    

Figure: Electromagnetic induction.

A metallic wire is warped around a metal bar called rotor. Rotor is connected with an external dc source for excitation. By the excitation, a magnetic field is produced where fluxes are flows from North Pole to South Pole.  When the rotor rotates the magnetic field also rotates. As a result the fluxes are cut by the conductors and voltage is induced from there. This is the Faraday’s law of electromagnetic induction.

Faraday's law of electromagnetic induction states that:

            “The electromotive force (EMF) produced around a closed path is proportional to the rate of change of the magnetic flux through any surface bounded by that path.”

Mathematically,

                       

Where, is the electromotive force (emf) in volts.

Φ_B is the magnetic flux in webers.

For the common but special case of a coil of wire, composed of N loops with the same area, Faraday's law of electromagnetic induction states that

Where,

 is the electromotive force (emf) in volts.

N is the number of turns of wire.

Φ_B is the magnetic flux in webers through a single loop.

Here, the negative sign indicates the direction which opposes the direction of motion according to Lenz’s law.

Applications of Faraday’s law:

Ø      Electrical generator

Ø      Moving coil microphone etc.

Power:

            Electric power is the rate at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt. It is denoted by P.

Mathematically, P = VI = I²R

Where, V = voltage in volts (V)

             I = current in ampere (A)

             R = resistance in ohm (Ω)

In every power plant there are two types of power is generated.

1.      Megawatt (MW)

2.      MegaVARs (M-VAR)

Piping system:

Piping system is an essential part of a steam power plant. It is employed to transmit water, steam, air, oil and vapor from one position to others.

Types of piping system:

A power uses many fluids like water, air, oil, steam, gas etc during its operation. This requires a variety of integrated piping system mentioned below:

1.      Water piping: Raw water, feed water, condensate and condenser cooling water.

2.      Steam piping: Main, reheat, bleed exhaust steam.

3.      Blow of piping: Boiler, evaporator, feed treatment.

4.      Miscellaneous piping: Water treatment, service water, lubricating oil, drains compressed air etc.

                                                Figure:  Steam piping system

Materials for pipes:

Ø      Cast iron: Cast iron pipes are used underground for water and drainage system and in order to place where problem of corrosion is excessive. Cast iron pipes are used for water service up to a pressure of 15 kg/cm².

Ø      Wrought iron: Wrought iron pipes are used for condensate, feed water and blow of lines. Such pipes are used for low and medium pressure range and should not be used when pressure is more than 250 psi.

Ø      Wrought steel: Most of the pipes used in power station are made of wrought steel. It is cheaper than others.

Ø      Alloys steel: For high temperature flow pipes are made of alloys steel. Chromium steel pipes are used for the temperature higher than 525⁰C. For temperature between 400⁰C-525⁰C carbons molybdenum steel may be used.

Ø      Copper and brass: Pipes made up of copper and brass are costly and are mostly used for oil lines. Brass pipe are used up to pressure 20 kg/cm².

Lubrication:

Lubrication means apply of lubricating element at right position, right amount and right time. In case of bearing lubrication is very necessary to minimize the bearing resistance, frictions and power losses.

Purpose of lubrication:

Ø      To reduce the sliding frictions.

Ø      To remove heat from contact surfaces.

Ø      To prevent corrosions.

Ø      To keep contaminants out of surface.

Lubricants:

            The elements which are used for lubrication are known as lubricant. There are mainly two types of lubricants:

1.      Oil

2.      Grease

Properties of good lubricants:

Ø      Higher wear resistance

Ø      Medium viscosity

Ø      Having unbroken film

Ø      Transparent and chemically stable

Ø      Prevent frictions

Ø      Removes heats from contact surface

Ø      Remain stable under several operations

Lubricant selections: It depends upon the parameters –

Ø      Load

Ø      Speed

Ø      Temperature

Ø      Bearing types

Oil lubrication:

It is used for light and moderate loads operating at high speed with 30^oF – 200^oF.

Applications:

Ø      Antifriction bearings

Ø      Generators

Ø      IC engines

Ø      Machine tools

Ø      Power transmission equipments

Ø      Pumps

Grease lubrication:

Grease is a mixture of lubricating oil and thickening substances like soap, graphite etc. It is used at low temperature under heavy loads. It also prevents the dust particles and other contaminants.

Grease selection:

 It depends upon the factors-

Ø      Hardness

Ø      Stability

Ø      Water resistance

Hardness is expressed in terms of number. Higher the number indicates higher hardness.

Generally greases are classified by number.

When number = 6, then most hard grease

When number = 0, then softer.

Softer grease is easy to apply anywhere and more efficient.

Relation with temperature:

v     Grease life becomes half with every 25^oF temperature increase.

v     Grease life becomes double with every 25^oF temperature decrease.

Applications:

Ø      Motor bearings

Ø      Power transmission devices.

Comparison of gas and steam turbine:

Comparison of gas and steam turbine:

Serial No:	Gas turbine	Steam turbine
01	Flue gas acts as working fluid.	Steam acts as working fluid.
02	Main components are compressor and combustor.	Main components are boiler and accessories.
03	Running cost is less and starts quickly.	Running cost is less and starts quickly
04	Its efficiency is lass.	Its efficiency is more.
05	It requires less space for installation.	It requires more space for installation.
06	The mass of gas turbine per KW developed is less.	The mass of gas turbine per KW developed is more.
07	It does not depend upon water supply.	It depends upon water supply.

Gas turbine:

A gas turbine, also called a combustion turbine, is a rotary engine that extracts energy from a flow of combustion gas. It has an upstream compressor coupled to a downstream turbine, and a combustion chamber in-between. Gas turbine may also refer to just the turbine component. Energy is added to the gas stream in the combustor, where fuel is mixed with air and ignited. In the high pressure environment of the combustor, combustion of the fuel increases the temperature. The products of the combustion are forced into the turbine section. There, the high velocity and volume of the gas flow is directed through a nozzle over the turbine's blades, spinning the turbine which powers the compressor and, for some turbines, drives their mechanical output. The energy given up to the turbine comes from the reduction in the temperature of the exhaust gas.

                                                Figure: Gas turbine chamber.

Gas turbine works on the basis of Bray ton cycle. Brayton cycle is called the backbone of the gas turbine. Here the T-S and P-V diagram is shown in below:

All four processes of the Brayton cycle are executed in steady flow devices so they should be analyzed as steady-flow processes.

When the changes in kinetic and potential energies are neglected, the energy balance for a steady-flow process can be express, on a unit-mass basis, as −

                       

                                                                                               

           Figure: 1^st stage blade of GT

                                        Figure: T-S and P-V diagram of Brayton cycle.

(Q _in –Q _out) + (W_in –W _out) = H _exit – H _inlet

Therefore, heat transfers to and form the working fluid are

                        Q _in = H₃ – H₂ = C_p (T₃- T₂)

            And, Q _out = H₄ – H₁ = C_p (T₄ – T₁)

Then the thermal efficiency of the ideal Brayton cycle is-

 Brayton efficiency = 1 – (Q _out / Q _in).

           

                       

Turbine blades:

The energy conversion takes place through the turbine blades. A turbine consists of alternate rows of blades. This blades convert the chemical or thermal energy of working fluid into kinetic energy and then from kinetic energy to mechanical energy as rotation of the shaft.

 There are two types of blade, fixed and moving blade. Moving blade is also two types.

One is impulse blade and another reaction blade.

Fixed blade:

A fixed blade assembly is very important for turbine blading. It is also known as diaphragm. The shape of the blade is the key to the energy conversion process. Since the fixed blades have a conversing nozzle shape, it is also called nozzles. When steam is passed over the fixed blades, they increase the velocity of steam as an operation of nozzles. Here blades are converted the thermal energy of steam into kinetic energy by causing the steam to speed up and gain velocity.

                                                                                          

Moving blade:

Moving blade can be shaped in either of two ways: reaction shaped or impulse shaped. The shape of the blade determines how the energy is actually converted. Either type of moving blades or a combination of both can be attached to the shaft of the rotor on dices, called wheels as shown in the figure. Along the outer rim of the blades is a metal band, called shrouding which ties the blades together. The moving blades convert the kinetic energy in the moving speed into the mechanical energy as rotor rotation.

                      

Steam turbine:

Steam turbine is such type of turbine where steam is used as working fluid. When steam is injected over the blades it rotates at a certain speed. Since steam is used for rotation it is called steam turbine. Generally it is used at steam turbine power station.

In most power station, steam turbines are used for steam turbine units. All the turbines are manufactured by BBC (Germany).

                                                 Figure: Steam turbine (case opened).

Main parts of steam turbine:                

Ø      Rotor

Ø      Blades (fixed and moving)

Ø      Bearings (thrust and journal)

Ø      Turbine casing

Ø      Valves (main stop valve, control valve etc).

  

Turbine:

A turbine is a rotary engine that extracts energy from a fluid flow and converts it into useful work. The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they move and impart rotational energy to the rotor.

Types of turbine:

1.      Steam turbine

2.      Gas turbine

3.      Water turbine

4.      Wind turbine

5.      Transonic turbine

6.      Ceramic turbine