Zeeshan's Report

7/27/2019 Zeeshan's Report

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AES Thermal Power Plant,Lalpir, District Muzaffargarh

Prepared by:Syed Zeeshan Haider

BSc Electrical EnggUET, Lahore

Submitted to:Abdul Quddus Abbasi

Teamleader (Turbine)

AES Thermal Power Plant.

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Rankine Cycle

These machines (steam turbines) are widely used in a

number of different cycles, such as:

Rankine Cycle

Reheat cycle

Regenerative cycle Combined cycle

The cycle on which this power plant is working is the

Rankine Cycle. The Rankine cycle is the standard for

steam power plants that are built around the world.

The basic Rankine cycle consists of four main

components:

Steam Turbine

Steam Condenser

Boiler

Feed Pump

AES power plant uses a slight modification in this

Rankine cycle but these four components remain theinevitable in all power plants including this. Rankine cycle

normally has water as a working fluid. Common heat

sources for power plants using the Rankine cycle are coal,

natural gas, oil, and nuclear. This cycle consists of four

steps.

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Step 1-2: The dry saturated steam expands through a

turbine, generating power. This decreases the

temperature and pressure of the steam, and some

condensation may occur.

Step 2-3: The wet steam then enters a condenser

where it is cooled at a constant pressure andtemperature to become a saturated liquid. The

pressure and temperature of the condenser is fixed by

the temperature of the cooling water as the fluid is

undergoing a phase-change.

Step 3-4: The saturated liquid is pumped from low to

high pressure, as the fluid is a liquid at this stage the

pump requires little input energy.

Step 4-1: The high pressure liquid enters a boiler

where it is heated at constant pressure by an external

heat source to become a dry saturated steam once

again.

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Steam Turbine

A Steam Turbine is a device which converts the

thermal energy stored in pressurized steam into rotational

mechanical energy. Because the turbine generates rotary

motion, it is particularly suited to be used as prime mover

for an electrical generator. About 80% of all the electric

generation in the world is done by use of steam turbines.

This is because it doesn't require any other mechanism,such as crankshaft, to convert rotary motion to

reciprocating motion as used by steam engines.

A steam turbine can either be of condensing or non-

condensing type. Non-condensing turbines are most widely

used for process steam applications. These are commonly

found at refineries, district heating units, pulp and paper

plants, and desalination facilities where large amount of

low pressure process steam is available. Condensing

turbines are most commonly found in electrical power

plants. These turbines exhaust steam in a partially

condensed state, typically of a quality near 90%, at a

pressure well below atmospheric to a condenser.

To increase the efficiency of the turbine, the processof reheating is used. In a reheat turbine, steam flow exits

from a high pressure section of the turbine and is returned

to the boiler where additional superheat is added. The

steam then goes back into an intermediate pressure section

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of the turbine and continues its expansion. Extraction type

turbines are also used to increase the overall efficiency. In

an extracting type turbine, steam is released from various

stages of the turbine, and used for industrial process needsor sent to boiler feed water heaters to improve overall

cycle efficiency.

To maximize turbine efficiency, the steam is

expanded, generating work, in a number of stages. These

stages are characterized by how the energy is extracted

from them and are known as impulse orreaction turbines.

Most modern steam turbines are a combination of thereaction and impulse design. Typically, higher pressure

sections are impulse type and lower pressure stages are

reaction type.

Here is a brief description of the turbine used in this

power plant.

Turbine Specifications

Type Single reheat condensing

tandem two cylinder double

flow exhaust

MCR 362 MW, 365 MW

Speed 3000 rpmDirection of rotation clockwise (from GV end)

Inlet pressure 169kg/cm2

Inlet temperature 538 oC (Main steam

and reheat)

Exhaust pressure 692 mmHg

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No of Extractions 08

General Description:

The steam for driving the turbine comes from the

boiler and is contains in the two steam chests located on

each side of the HP-IP turbine. Each steam chest has a

valve known as main steam valve to allow the steam into

the steam chest. The outlets of these steam chests are

connected to turbine through four governor valves of two

are in the base and two in the cover. These governor valves

are used to control the steam inlet into the turbine. The

steam passes via the governor valves into the HP turbine

where its mechanical energy is transferred to the kinetic

energy of the rotor by hitting on the blades. After doing

work in the HP turbine, it goes into the boiler for reheating

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and then back into the IP turbine making it rotate at a much

greater speed. Then the steam enters into the double-flow

LP turbine via an overhead pipe and does work in it. By

that time the rotor has begun to rotate at a speed of 3000rpm. The steam then flows to either ways towards the

exhaust opening and into the condenser located below it.

Along the entire length of the turbine, there are 8

extractions one from each stage. Some part of the steam is

taken out of each stage. The purpose of these extractions is

to provide heating media for the heaters which preheat the

condensating water before it enters into the boiler.

Following are some main components of the turbine

system in the power plant and their brief descriptions.

Steam Chest:- Temporarily stores the main steam

coming from the boiler

Main Steam Valve:- Controls the steam entering into

the steam chest Governor Valves:- Controls the steam inlet into the

HP turbine. Indirectly controls the speed of the

turbine before synchronization and controls the load

after synchronization

Interceptor Valves:- Controls the reheated steam

before it enters into the IP turbine

Nozzles:- Converts the heat energy of steam intokinetic energy

Turning Gear:- Keeps the turbine in rotation at 3~5

rpm when the unit is shut down to prevent sagging

and hogging

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Glands:- Prevents the leakage of main steam from the

turbine the also the seeping of air into the turbine

Bearings:- Hold the weight of the shaft and also

prevents the sideways motion of the shaft Lube Oil System:- Lubricates the moving parts by

forming a thin film between the shaft and the bearing

and also scavenges heat from the moving parts

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Steam Condenser

A condenser is a heat exchanger which converts

steam from its gaseous to its liquid state at a pressure

below atmospheric pressure. In thermal power plants, the

primary purpose of a surface condenser is to condense the

exhaust steam from a steam turbine to obtain maximum

efficiency and also to convert the turbine exhaust steam

into pure water (referred to as steam condensate) so that it

may be reused in the steam generator or boiler as boilerfeed water.

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The medium used to condense the steam is

demineralized water that comes from the cooling tower. It

extracts the latent heat of vaporization from the steam so

that steam is converted to water (also known ascondensate). It is necessary to convert steam into water

because this makes it easy to pump liquid condensate into

the boiler - a work that cannot be done with steam. This is

because steam is a gas so its volume decreases by

compressing it instead of being pumped. So this increases

the efficiency of the power plant.

A vacuum is created inside the condenser to regulate

the flow of steam from the LP turbine towards the

condenser.

There are two types of condensers:

Direct Contact Condensers:- In this type of

condensers, cooling water is directly sprayed inthe steam. This type is used when water and

condensate have the same chemistry.

Surface Condensers:- In this type of

condensers, steam is condensed by the direct

contact with the surfaces of the tubes in which

the cooling water is flowing.

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Steam Generator

(Boiler)

A steam generator or a boiler is used in all the

conventional power plants to create steam by applying heatenergy to water. A boiler incorporates a firebox or furnace

in order to burn the fuel and generate heat; the heat is

initially transferred to water to make steam; this produces

saturated steam at ebullition temperature (saturated steam

which can vary according to the pressure above the boiling

water. The higher the furnace temperature, the faster the

steam production. Any remaining heat in the combustion

gases can then either be evacuated or made to pass throughan economizer the role of which is to warm the feed water

before it reaches the boiler.

There are basically two types of boiler:

1. Fire tube Boiler

2. Water tube boiler

In a fire tube boiler the burning media is present

inside the tubes which are immersed in the water.

In a water tube boiler, water is present in the tubes

while the burning media is outside on the shell side.

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The boiler used here is a water tube boiler. It is of the

forced circulation radiant reheat type. It is designed to

generate 1200tons/hr steam at 541 oC at the superheater

outlet and 947.56tons/hr steam at 541 oC at the reheateroutlet. The steam generator (boiler) incorporates all the

piping and equipment between the economizer inlet and

the superheater outlet.

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Super Heaters:- Converts steam to superheated

steam to be used in the HP turbine

Reheaters:- Raises the temperature of steam to be

used later in the IP turbine Gas Recirculation Fan:- Re-circulates the flue gases

within the boiler

Air Heater:- Mixes flue gases with the air

Steam Air Heater:- Preheats the fresh air before it

enters into the boiler using steam

Forced Draft Fan:- Sucks fresh air from the

atmosphere and forces in towards the steam air

heaters

Fuel Injectors:- Injects fuel into the boiler in such a

way that a fireball is formed inside

Water Drum: - Serves as a distribution heater for the

water wall tubes.

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AC Generator

A.C. generators or alternators (as they are usuallycalled) operate on the fundamental principles of

electromagnetic induction. An ac 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, the sun or solar energy, compressed air orany other source of mechanical energy.

Alternating voltage may be generated by rotating a

coil in the magnetic field or by rotating a magnetic field

within a stationary coil. The value of the voltage generated

depends on-

the number of turns in the coil.strength of the field.

the speed at which the coil or magnetic field

rotates.

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Typically, a rotating magnet called the rotor turns within a

stationary set of conductors wound in coils on an iron core,

called the stator. The field cuts across the conductors,

generating an electrical current, as the mechanical input

causes the rotor to turn.

The ac generator used here in this power plant is a

synchronous generator also called alternator. It is of the

liquid/hydrogen cooled type, excited from a static system.

Its stator is constructed in two parts- inner frame and the

outer casing (no physical distinction between the two). The

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inner frame supports the core and the windings which are

internally cooled by demineralised water.

The demineralised water is circulated through theconductors from the exciter end to the turbine end of the

generator. Coolers are also incorporated in the system

which extract heat from the stator coolant (demin water)

after it has passed through the windings.

The stator core and the rotor are cooled by pressurized

hydrogen at 3 bars circulated within the casing by twocentrifugal fans mounted one at each end of the rotor.

Hydrogen seals of the journal type prevent the

leakage of hydrogen at the points where the rotor emerges

from the stator casing. Oil is supplied to these seals with a

differential pressure of 0.8 bars.

The generator uses field coils rather than permanent

magnets to create the main magnetic field in the generator.

For this purpose, dc winding has been done on the rotor

surface to create the field. The supply to these windings is

given through dc excitation via carbon brushes. The

excitation supply comes from the excitation transformerafter it has been converted to dc by using thyristor

converters.

The output frequency of an alternator depends on the

number of poles and the rotational speed. The speed

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corresponding to a particular frequency is called the

synchronous speed for that frequency. This table gives

some examples:

The generator used here is designed for two poles, so

it has to be run at 3000 rpm in order to generate a

frequency of 50Hz.

Since the generator uses electromagnet to create a

magnetic field, it needs to have a constant dc supply for its

operation. This supply is given from the excitation

transformer with thyristor converters in between to convert

ac to dc. However during the start up the supply from the

transformer is not available, the dc supply is given from an

alternative 380V ac supply through the converters. This

supply remains in operation only for 10 seconds.

After the generator has been synchronized with the

bus bar system:

Poles RPM at 50Hz RPM at 60Hz

2 3000 3600

4 1500 1800

6 1000 1200

8 750 900

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i. the system controls the terminal voltage and the

frequency of the generator

ii. the governor valves control the real power developed

by the generator (aka load)iii. the excitation system controls the reactive power

developed by the generator and thus its power factor

But before the synchronization has been done, the

frequency of the generator is controlled by the governor

valves and the terminal voltage is controlled by the AVR

(automatic voltage regulator).

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Transformers

In AES Lalpir, we have four main large transformersas:

1. GSU (Generator Step Up Transformer)

2. UAT (Unit Auxiliary Transformer)

3. SUT (Start Up Transformer)

4. Excitation Transformer.

These are described as under.

1. GSU (Generator Step Up Transformer):

This transformer steps up 24kV (from the generator

output) to 220kV. It is 180/320/430MVA transformer and

is the largest one used here. The output of this transformer

is passed to the switch yard which is further connected to

the 220kV bus bar.

2. UAT (Unit Auxiliary Transformer):

The unit auxiliary transformer is responsible for

driving the unit itself. It is a step down transformer which

steps down 24kV to 11kV for the plant usage. Then there

are other small transformers which steps down the voltage

according to the machine requirement. UAT is 30/40MVAtransformer.

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3. SUT (Start Up Transformer):

Unit Start Up Transformer, as the name suggests, is

used to start up the unit. If the unit is stopped or turned off,

then there is input of 220kV from WAPDA through thebus bar. This transformer steps down 220kV to 11kV to

run or start up the unit. It is a 33/44MVA transformer.

4. Excitation Transformer:

The Excitation transformer is used to excite the

generator. It is also a step down transformer and converts

24kV to 489V which is then used to excite the generator

windings. This excitation is given to the generator by

converting into DC using the converters. This 24KV is

taken from the generator. This transformer has the apparent

power of 2546KVA.

Cooling of the Transformers:

As very high currents passes through thetransformers windings, it produces a large amount of heat.

There must be a technique to reduce this temperature.

Generally, oil is used as a coolant for the transformers.

There are three methods used for keeping the

transformers temperature as low as possible and to

prevent them from heating too much. These are:

i. OFAF (Oil Forced, Air Forced)

ii. ONAF (Oil Natural, Air Forced)

iii. ONAN (Oil Natural, Air Natural)

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i. OFAF:

This technique is used in large transformers such as

GSU (Generator Step Up) Transformer when running at itsmaximum load. At lower loads, it uses other techniques

such as ONAF and ONAN for this purpose.

In this technique, oil as well as air is forced to

circulate within the transformer to maintain the

temperature. Oil is made to circulate using pumps. The air

is forced to move in and out of the transformers with the

help of coolant fans.

ii. ONAF:

This is another method for maintaining the

temperature of the transformer at the desired level. As the

temperature of the oil rises, it expands and moves upward

through the small piping configuration to the oil storage

tank, which is placed above the transformer. As it cools, itcomes down again. While the air forced through the

transformers using fans.

In AES Lalpir, UAT and SUT are cooled on the

principle of ONAF at their full loads.

iii. ONAN:This is the simplest method used traditionally for

keeping the transformers temperature up to minimum

level. Oil and air, both work as coolants. Small

transformers always work on the same principle as do the

Excitation Transformer in AES Lalpir.

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The direction of flue gases can be controlled using

dampers. The dampers direct the flow of these gases to

either the large orthodox stack or the wet stack after

passing through the FGD unit. These gases are forced intothe absorber section in an upward direction where slurry is

sprayed in fine particles in the form of rain from the top.

Limestone is first brought to the site and is crushed

into powdered form. It is moved to the ball mill where it is

mixed with water and then pumped to the cyclone

classifier. In cyclone classifier, the larger lumps are forced

back to the ball mill and the proper suspension of lime

stone (slurry) is dumped into the slurry tank. The slurry

tank is constantly agitated by a stirrer to avoid

sedimentation. A pump is located the tank which sucks

slurry from it and pumps it into the absorber section.

The slurry is then taken from the base of the absorber

section and pumped to the top from where it is allowed tofall in the form of fine particles. The slurry reacts with

sulphur dioxide SO2 on its way down according to the

following reaction:CaCO3 + SO2 + H2O CaSO3 .H2O + CO2

Calcium sulphite (CaSO3 .H2O) is a useless, wastematerial. Instead, it is oxidized by using air from the

compressor. The reaction for this process is as under:

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