Tharmal Traning reports

CHAPTER 1

1.1 INTRODUCTION

Guru Nanak Dev Thermal Power Plant is a coal-based plant. The requirement of coal

for four units based on specific fuel consumption of 0.60 kg / kWh. The conveying and

crushing system will have the same capacity as that of the unloading system. The coal

comes in as large pieces. This coal is fed to primary crushers, which reduce the size of

coal pieces from 400mm to 150mm. Then the coal is sent to secondary crusher through

forward conveyors where it is crushed from 150mm to 200mm as required at the mills.

Then the coal is sent to boilers with the help of primary fans. The coal is burnt in the

boiler. Boiler includes the pipes carrying water through them; heat produced from the

combustion of coal is used to convert water in pipes into steam. This steam generated is

used to run the turbine. When turbine rotates, the shaft of generator, which is

mechanically coupled to the shaft of turbine, gets rotated so, three phase electric supply is

produced.

The basic requirements are:-

♣ Fuel (coal)

♣ Boiler

♣ Steam turbine

♣ Generator

♣ Ash handling system

♣ Unit auxiliaries

MANPREET KAUR [21302101] Page 1

CHAPTER 2

2.1WORKING OF THERMAL PLANT

Coal received from collieries in the rail wagon is mechanically unloaded by Wagon

Tippler and carried by belt Conveyor System Boiler Raw Coal Bunkers after crushing in

the coal crusher. The crushed coal when not required for Raw Coal Bunker is carried to

the coal storage area through belt conveyor. The raw coal feeder regulates the quantity of

coal from coal bunker to the coal mill, where the coal is pulverized to a fine powder. The

pulverized coal is then sucked by the vapour fan and finally stored in pulverized coal

bunkers.

The pulverized coal is then pushed to boiler furnace with the help of hot air steam

supplied by primary air fan. The coal being in pulverized state gets burnt immediately in


the boiler furnace, which is comprised of water tube wall all around through which water

circulates. The water gets converted into steam by heat released by the combustion of

fuel in the furnace. The air required for the combustion if coal is supplied by forced

draught fan. This air is however heated by the outgoing flue gases in the air heaters

before entering the furnace.

.The steam after doing the useful work in turbine is condensed to water in the

condenser for recycling in the boiler. The water is pumped to deaerator from the

condenser by the condensate extraction pumps after being heated in the low pressure

heater (L.P.H) from the deaerator, a hot water storage tank. The boiler feed pump

discharge feed water to boiler at the economizer by the hot flue gases leaving the boiler,

before entering the boiler drum to which the water walls and super heater of boiler are

connected.

2.1.1 COAL MILLING PLANT

Since G.N.D.T.P. units are primarily coal fired units so each boiler is provided with

closed milling circuits to pulverize the raw coal which is received from coal conveying

system after coal crushes before it is fired in the furnace. The necessity of pulverizing the

coal is to be ensuring its maximum possible combustion in the furnace. The coal data for

units are: -

COAL DATA UNITS 1 & 2 UNITS 3 & 4

Type of Coal

Net Calorific Value

Moisture

Ash Content

Bituminous

4300 Kcal/kg

10 %

30 %

Bituminous

4727 Kcal/Kg

7.5 %

32 %

2.2 SWITCHGEAR


2.2.1 INTRODUCTION

The apparatus including its associated auxiliaries employed for switching, controlling and

protecting the electrical circuits and equipments is known as switchgear.

A tumbler switch, which is an ordinary fuse, is the simplest form of switchgear and is

generally used to control and protect the domestic and commercial appliances and

equipments. For high rating circuits, a high rupturing capacity (H.R.C.) fuse in

conduction with switch may serve the purpose. However, such switchgear cannot be

applied on power system operating at high voltages, i.e. more than 11 KV because of the

following reasons: -

1. When fuse blows, it takes some time to replace it and consequently there is

interruption of power supply.

2. On high voltage system, a fuse cannot successfully interrupt large fault currents.

3. When fault occurs, fault takes some time to blow. During this time the costly

equipments e.g. generators, transformers etc. may be damaged.

Therefore in order to protect lines, generators, transformers and other electrical

equipments from damage, an automatic protective device or switchgear are required.

Automatic protective switchgear mainly consists of the relays and circuit breakers. A

circuit breaker is switchgear, which can be open or close the circuit after an operation.

Therefore, a circuit breaker is rather preferred even in the instance when a fuse is

adequate

Switch: -

It makes and breaks the circuit under full load or no load condition but cannot be

operated under fault conditions. It is generally operated manually.

.


2.2.2 Isolator: -

It is only operated under no load conditions. Its main purpose is to isolate a portion of

the circuit from the other. Isolators are generally place on the both sides of a circuit

breaker from the other in order to make repairs and maintenance on the circuit breaker

without any danger. There are two types of isolators: -

TYPES OF ISOLATER:-

Single pole Isolator

Double pole Isolator


CHAPTER 33.1 D.C. SUPPLY SYSTEM

D.C Supply is the brain of the plant. Each unit has its own 220volts D.C system located

in the electrical bay. 48 battery of requisite rating are also being provided. Each D.C

system comprises of the following:-

1. Storage battery

2 Battery Charges

3. Distribution & sub-distribution boards

The batteries are of lead acid type. Battery cells have high discharge performance cell

types each of 2volts. Battery charge are static type & capable of trickle charging & boost

charging. Adequate standby provision is also made for the outage of the charger in the

form of installation of standby charges.

The D.C distribution & sub-station boards are compartmentalized; draw out type,

construction, housing switches & fuses for various feeders are as per the requirements for

the plant.

The battery rooms are well ventilated & well lighted & there is adequate provision for

expelling acid fumes & fumes h2 gas out from the battery room.

1. Dry cell batteries

2. Lead acid batteries

3. Alkaline cell batteries.

Lead acid cells are of further two types:-

1) Automobile battery

2) Stationary lead acid battery.

POWER REQUIREMENT

A DC source capable of delivering current as specified. The voltage required will be two

times the No. of cells in battery. The initial charging of the battery will takes approximate

55 to 90 hours.


CHAPTER 4

4.1 BOILER SECTION

The steam generating unit is designed to meet the nominal requirements of 110MW turbo

generator set. The unit is designed for a maximum continuous rating of 375 tones/hr. at a

pressure of 139kg/cm2 and a steam temperature of 5400C. the reheated steam flows at

MCR 32H tones/hr. at the feed water temp at MCR is 2400C. The unit is a balance

draught dry bottom; single drum natural circulation, vertical water tube type, construction

with skin casing and a single reheat system. The furnace is arranged for dry ash discharge

and is fitted with burners located at the four corners. Each corner burner comprises coal,

vapour oil and secondary air compartments. The unit is provided with three ball mills and

arranged to operate with intermediate cool powder bunker. The steam super heater


consists of 4 stages Viz. Ceiling, convection, platen and final superheated. The ceiling

super heated forms the roof of the furnace and horizontal pass and finishes as the rear

wall of the second pass. The convection super heated is made up of horizontal banks

located in the second pass. While the platens are located at the furnace exit, the portion

above the furnace nose encloses the final superheated reheated are in two stages, first

stage is the trifle heat exchangers located in the second pass, which absorbs heat from

superheated steam as well as from the flue gases. The second stage is exit reheated

located in the horizontal pass as pendant tubular loops.

(a) The flue gas for drying the cool in the mills is tapped off after the trifle heat

exchangers. The damper located in the hot flue gases pipe leading to mill controls the

quantity. Control the circulating vapour of the mill entry effect temperature control.

Immediately after the trifle heat exchanger, the air heaters and economizers are located.

The air heater is in 2 stages.

(b) The hot air for combustion from air heater stage 2 is led into the common wind box

located on the sided of the furnace. 4 cool air mixed pipes from pulverized coal bounders

are connected to 4 cool burners’ nozzle at the corners. There will be totally 16 coal

nozzles. 4 located in each corner. Oil guns will be located in the secondary air nozzle for

coal burning. The turn down ratio of the guns will be so selected that it will be possible to

use them also for pulverized fuels flame stabilization while operating under load below

the control point.

(c) Take into consideration the high % age of ash and the relatively poor quality of coal

due regards has been paid to wide pitching the tubes and to the gas velocity across the

heating surface areas. In order to insure reliable and continuous operation sample sot

blowing equipment is provided. There are short retractable steam root blowers provide at

the top of furnace fully retractable rotary type blowers are located for cleaning of the

secondary super heater and final heater partly retractable steam blowers are arranged for

the horizontal reheated and super heaters in the second pass. The steam root blowers are

electrically operated.

(d) Root blowing nozzles using blow down from boilers drum are provide for the

cleaning of areas around the burners nozzles zone for dislodging of slag boulder if any in

the bottom ash hopper in the furnace.


(e) Two FD fans are provided per boiler. The FD fans are of the axial type driven by

constant speed motor. The regulation of quantity and pressure is done by inlet vane

control. The flue gases are sucked through the mechanical and electrostatic precipitators

by I.D. fans and delivered into the chimney. Two I.D. fans are provided for each boiler

and they are of the axial type driven by constant speed motors. Inlet vane control effects

the capacity change with reference to load. Both the I.D. and FD fans have been

dimensioned taking into account the minimum margins of 15% on volume and 32% on

pressure.

Specification

Manufacturer B.H.E.L

Maximum continuous rating 375tones/hr.

Super heater outlet pressure 139kg /cm2

Reheated outlet pressure 33.8 kg/cm2

Final super heater temperature 540 dig’s

Feed water temperature 240deg.c

Efficiency 86% (stage-1)

87% (stage-2)

Coal consumption per day 1500 tones


CHAPTER 5

5.1TURBINE SECTION

The turbine is the prime mover for the generator in the power plant Different types of

steam turbines used in thermal power plants, but the ones. Which are used at G.N.D.T. P.

are categorized as follows

S.No. Type of Turbine Turbine at G.N.D.T.P.

1. Horizontal/vertical Horizontal

2. Single/multi cylinder Multi cylinder

3. Condensing/non condensing condensing

4. Reheat/ non-reheat Reheat

5. Regenerative/non regenerative Regenerative

6. With by pas/without bypass with by pass (stage-1)

Without bypass (stage-2)

5.2 BASIC WORKING OF TURBINE

First of all the turbine is run on gear motor with the help of exciter. At that time steam is

kept on recalculating with the help of bypass valve. When the pressure of steam is

increased to an optimum level and turbine acquires a particular rpm then steam is

introduced in the H.P. (high-pressure) cylinder first. The temperature of steam at entrance

is 540C and pressure is about 139 Kg/cm2. After doing its work on the H.P. Turbine, the

steam is taken out for reheating rated temperature of steam at reheated inlet is 360C. The

temperature of steam is increased upto 535C in the boiler shell and steam is again

introduced in M.P (Medium pressure) turbine. After M.P.turbine, the steam is passed on

to L.P. (Low-pressure) turbine. This process helps the turbine to reach the speed of 3000

rpm. After L.P. turbine, the steam is condensed in condenser, build below the turbine

unit. The condenser contains a number of brass tubes through which cooling out from

L.P. turbine it comes in contact with colder brass tubes then steam get transformed into

water. This water get collected in HOT WELL just below the condenser. From here the

hot water is again pumped with the help of condensate pumps. The cooling water is used


to condense steam gets heated up and is cooled by falling from cooling tower. This

completes the processing of steam through turbine and condenser.

5.3 ROTOR OF TURBINE

All the rotors are manually by means of rigid coupling, including the rotors of the

generator. The speed of whole system of rotor lies in the following ranges of the speed at

the operating conditions: -

1900 to 2000 rpm Best noticed on the M.P. and L.P. rotors and generators.

2350 rpm Best noticed on H.P. rotor.

1. BEARING OF ROTORS

The axial load of the entire system of rotors is taken up by a double-sided axial bearing

located in the bearing stand between the H.P. and M.P casing. These are two protections

mounted near the axial bearing one hydro chemical and one electromagnetic, which fouls

the turboset during the non-permissible movement of the rotor.

The rotors are placed on radial bearing which are machined to elliptical shape. Further

scrapping operations or change top and side clearances and change in temperature of oil,

influence the oil wedge and the position of the journal bearing to maintain the same

condition as existed during the initial assembly.In the lower half of bearing a hollow

groove is provided in the babbit metal through which oil the supplied through a drilled

hole through H.P. jacking oil pumps.

The high-pressure oil rotors are lifted in the bearing so that any scrapping of the bearing

is prevented.

2. TURBINE CASING

The high-pressure part of turbine is consisted of two-concentric horizontal casing. Inner

casing is connected in such a way to the other casing that it enables to expand in all

direction. The nozzles are attached to the inner casing. The steam pipe is connected to the

condensers and the condensers are supported by springs. The casings are inter connected

by the system of guide keys through bearing pedestals in such a way that thermal

expansion of casing does not destroy the various parts of turbine.


The displacement-bearing pedestal between M.P. and H.P parts is measured by the

electromagnetic pick up. This valve is about of 15 mm to prevent deformation at the

casing. It is very important that sliding part clean lubricated and free from hazard for

connecting parts bolts elements. The heating of bolts before tightening up and before the

locking presents. The flanges of M.P. and H.P. casings are designed to heat up by steam

during the starting up of turbo boost by which the difference in temperature between the

cylindrical position of the casing flanges and connected bolts is reduced to limited

deformation. The thermo couples are used for measuring temperatures. The thermo

couple is partially connected to the indicated apparatus. Cooling fluid is generally used

for reducing the temperature of various parts.

3. REGULATION AND SAFETY EQUIPMENT FOR TURBINE GOVERNING


The quality of steam entering in the turbine is regulated by the four governing valves on

the inlet to the M.P. part. The amount of opening at any instant of these valves is

controlled by the pressure of secondary oil, which is indirectly depending on the primary

oil pressure and directly depending upon the spring force in the transformer during the

stand still and during starting of the turbo set. The pressure of primary oil is directly

depending on the speed of the set through the speed-sensing element. Operating the speed

changer or the normal speed changer can vary the tension in the spring in the transformer.

Thus make it possible to vary the speed before synchronizing. In case break down of any

equipment of the block the quick closing devices are provided in the regulation system of

the turbo set. H.P. quick closing valves (H.P.Q.C) M.P. quick closing valves (M.P.Q.C)

at return flap valves are operated by either directly by the tripling lever or through the

relay magnet on the main relay which creates instantaneously loss of pressure of the

quick closing oil by the change of flow of oil inside the relay.

Distribution is used for checking the function of H.P.Q.C and M.P.Q.C valves. The H.P.

and M.P quick closing valves, the non return flaps and non return extraction valves

during normal operating condition have only two positions one is fully opened another

fully closed.

4. STEAM CYCLE

The design of the power cycle based on the modern concept, where a unit consists of a

steam generator with its independent firing system tied to the steam generation. The

steam generator is designed for maximum continuous rating of 375-tonnes/hr. and steam

Pressure of 139-kg/cm2 at temperature of 540C respectively. The steam generator is

designed to supply to a single reheat type condensing steam turbine with a 8 non

regulated extraction points of steam for heading the condensate and feed water. The

steam cycle can be classified into the following three divisions: -

(a) Main stream (b) Reheat steam (c) extraction steam

(a) MAIN STEAM


Saturated steam from the steam generator drum is led to the super heater bank to heat if

4up to 540C saturated steam from the drum is led to the ceiling super hearter (between

SHH1 and SHH2) from ceiling super steam goes to convection


Super heater (between SHH2 and SHH3) the first regulated infection for at temperature

takes place after convection super heater (between SHH9 and SHH10). Before entry to

final super heater the steam is again at temperature by regulated injection. The steam is

coming out from the final super heater normally at a pressure of 139 kg/cm2 at a

temperature of 540oC. This steam is feed to the control valve. In each of the two live

steam lines there is one turbine side main stream stop valve and one high pressure quick

closing valve along with two control valves.

(b) EXTRACTION STEAM

Steam for heating of the condenser and the feed steam is extracted from 8 non regulated

extraction points from the turbine. Heating is carried out in five stages of L.P. heaters,

one deareating heater and in two H.P. heaters extraction 1, 2, 3, is taken from L.P.

turbine. Extraction 4, 5, 6 and 7 are taken from M.P. turbine. Extraction 8 is obtained

from C.R.H. line first and second stage of heating is done by two sets of twin low-

pressure heaters mounted directly in the L.P. casing of the turbine. Extraction 3, 4 and 5

are connected to the deaereating heater placed above feed water storage tank 7th and 8th

extraction steam is fed to the vertical H.P. heaters respectively.

(C) REHEAT STEAM

Exit steam from the H.P. turbine is taken back to the reheater section of the steam

generating unit. Reheating is done in two stages both by flue gas and by super heated

steam. The steam to be reheated is first pass through the triple-heated exchanger, where

super heated steam is used as the heating media. The steam is finally reheated in final

reheaters (RHH3) RHH4 and RHH5) suspended in the horizontal pass of the furnace.

Reheat steam at a normal pressure of 36.4 kg/cm2 at a temperature of 540C respectively

is fed to the M.P. cylinder by two hot reheat steam pipes through strainers and combined

stop and interceptor valves. In each of the cold reheat steam lines from H.P. cylinder a

non-return valve is operated by oil pressure is provided.

5. Turbine Accessories and Auxiliaries

I. Surface condenser.

II. Steam jet air ejector

III. LP and HP heaters.


IV. Chimney steam condenser.

V. Gland steam condenser.

VI. Oil purifier or centrifuge.

VII. Clean oil pump with clean oil tank

VIII. Dirty oil pump with clean oil tank.

IX. Auxiliary oil pump with auxiliary oil tank

X. Starting oil pump.

XI. Emergency oil pump.

(I) SURFACE CONDENSER

Two surface condensers are used for condensing the steam which has worked in the

turbine. The coolant for condensing the steam is circulating water which is inside the

condenser brass tubes and steam is outside.

Technical data of Condenser

Cooling Area 3300 msq.

Number of brass tubes 6000

Circulating water required 7500 tonnes/hr.

Vacuum in the condenser 0.90 kg/cm sq.

(II) STEAM JET AIR EJECTOR

Starting ejector is used for quick evacuation of the turbo set during starting whereas main

steam jet air ejector is used to maintain Vacuum in the condenser. It works on the

principle of ‘VENTURI’ with steam working media to eject air from the condenser.

(III) LP AND HP HEATERS

In regenerative system there is a steam of 5 LP heaters, one Deaereator, 2 HP heaters. All

LP and HP heaters are of surface type i.e. condensate or feed water is inside the heater

tubes in the heater shells. L.P. heaters are of single flow whereas HP heaters are of

double flow type. Deaereator is contact type heater in which steam and condensate come

in direct contact.

(IV, V) CHIMNEY STEAM AND GLAND STEAM CODENSER: - There are

additional two heating stages provided in the regeneration system of the turbine for


heating the condense flowing through it steam leaks off from the turbine glands is used

for heating the condensate in these heaters.

(VI, VII, VIII, IX, X) VARIOUS OIL PUMPS

Centrifuge is an oil purifier used to remove moisture and other impurities from the

turbine oil. Maximum allowable moisture content in the turbine oil is 0.2%. In case the

oil level of the main oil tank is to be made up then either oil can transferred from clean

oil tank to main oil tank with centrifuge or from dirty oil tank to main oil tank with

centrifuge.

(XI) STARTING OIL PUMPS AND EMERGENCY OIL PUMPS

Starting oil pumps supply the necessary turbine oil during starting of the turbine and upto

turbine speed of 2930 rpm till the main oil pump mounted on the turbine rotor at the HP

extension takes manually in order to provide lubrication oil for the turbo set. Emergency

oil pumps are meant to start on auto, when turbine trips and lubrication oil pressure falls

in order to provide lubrication to the turbine and generator bearings.

5.4 MAIN TECHNICAL DATA ABOUT TURBINE

a) The Basic Parameters

Rated output measured at terminal of the generator. 110,000KW

Economical output. 95,000KW

Rated speed 3,000 RPM

Rated temp. Of steam just before the stop valve. 535C

Max temp. Of steam before the stop valve 545C

Rated pressure of steam before the MP casing 31.63 ata

Max. Pressure of steam before the MP casing 35 ata

Rated temp. Of steam before the MP casing 535C

Max. Temp. of steam before the MP casing 545C

(a) System of turbine:

Governing valves 2 interceptor valves

HP cylinder 2 Row Curtis wheel +8 moving wheels.

Wt. of HP rotor is approx. 5,5000kg.


MP cylinder 12 moving wheels.

Wt. Of MP rotor is approx. 11,000kg

LP cylinder 4 Moving wheels of double flow design.

Wt. of MP rotor is approx. 24,000.

Direction of the turbine rotation is to the right when looking at the turbine from the front

bearing pedestal.

5.5 RELAY SECTION

Protective relay is a device that detects the fault and initiates the operation of the circuit

breaker to isolate the defective section from the rest of the system.

We have seen that whenever fault occurs on the power system, the relay detects the fault

and closes the trip coil circuit. This results in the opening of circuit breaker, which

disconnects the faulty section. Thus a relay ensures the safety of the circuit equipments

from damage which may be causes by the faulty current.

ESSENTIAL ELEMENTS OF A RELAY

All the relays have the following three essential fundamental elements as shown in block

diagram see fig.

(a) Sensing element: - Sensing or measuring element is the element which responds to

the change in magnitude or phase of the actuating quantity e.g. current in the over current

relay.

(b) Comparing element: - It is the element which compares the action of the actuation

quantity of the relay with pre-designed relay setting. The relays only pick up if the

actuating quantity is more than the relay setting.

(c) Control Element: - When a relay picks up it accomplishes a sudden change in the

controlled quantity such as closing of trip coil circuit.

1. TYPES OF RELAYS


There are many kinds of relays applied in the power system. The relays can be designed

and constructed. To, operate in response to one or more electrical quantities such as

voltage, current, phase angle etc. The relays are classified in different ways.

According to construction and principle of operation

(i) Thermal relays: - The heating effect of electric current is used for the operation

of these relays.

(ii) Electromagnetic attraction relays: - The operation of these relays depends upon

the movement of an armature under the influence of attractive forces due to the magnetic

field set up by current flowing through the relay coil.

(iii) Induction Relays: - Electromagnetic induction phenomenon is used for the

operation of these relays by induction, eddy currents are induced in the aluminum disc,

free to rotate, which exerts torque on it.

2. ACCORDING TO APPLICATION

(i) Over current, over voltage or over power relays:-These relays operate when the

current, voltage or power rises beyond a specific value.

(ii) Directional or reverse current relays: - These relays operate when the applied

current assumes a specified phase displacement with respect to the applied voltage and

the relay is compensated for fall in voltage.

(iii) Under current, under voltage or under power relays: - These relays operated

when the current, voltage or power falls below a specific value.

(IV) Directional or reverse power relays: - These relays operate when the applied

voltage and current assumes a specified phase. Displacement and no compensation is

allowed for fall in voltage.

(V) Distance relays: - The operation of these relays depends upon the ratio of the voltage

to the current.

(Vi) Differential relays: - The operation of these relays takes place at some specific

phase difference or magnitude difference between two or more electrical quantities.

3. ACCORDING TO THE TIME OF OPERATION:-


(i) Instantaneous relays:-In these relays, complete operation takes place

instantaneously i.e., the operation is complete in a negligibly small interval of time from

the incidence of the actuating quantity.

(ii) Definite time lag Relays: - In these relays operation takes place after definite

time lag which is independent of the magnitude of actuating quantity.

(iii) Inverse time lag Relays:–In these relays the time of operation is inversely

proportional to the magnitude of actuating quantity.

Inverse Definite Minimum Time Lag Relays: - In these relays, the time of operation is

approximately inversely proportional to the actuating quantity, but is never less than a

definite minimum time for which relay is set.

THERMAL RELAYS

A relay in which heating effect of electric current is used for its operation is known as

thermal relays. These relays may be actuated by a.c. or d.c.

CONSTRUCTION

The schematic diagram of an indirectly heated general purpose thermal relay is shown in

fig. It has a bimetallic strip which is heated by heating element which gets supply from a

current transformer. An insulated contact arm carrying a moving contact is pivoted and is

held by a spring. The other contact of trip circuit is a fixed contact. The spring tension

can be varied by changing the position of contact arm with the help of sector plate.


WORKING

Under normal conditions, the current flowing through the heating element is proportional

to the normal full load current of the circuit. The heat produced by the heating element,

under this condition is not sufficient to bend the bimetallic strip. However, when fault

occurs current flowing through the heating element increases which produces heat

sufficient to bend the bimetallic strip. This releases the contact arm and because of the

spring tension the relays contacts are closed which closes the trip coil circuit or the alarm

circuit once the alarm circuit or the trip coil circuit is closed; it operates the alarm circuit

or the circuit breaker to open the circuit respectively.

These over current tripping relays are use mostly for motor controls. The heating

elements of such relays are designed to with stand short time overload up to 7 times the

normal full load.

5.6 WATERTREATMENT PLANT

WATER TREATMENT PLANT

The basic requirement of Water Treatment in the thermal power station is to provide

suitable water for the boiler i.e. the water, which is free from dissolved, suspended, and


any other type of impurities. If the water is taken as such without any treatment then this

will result in scale and sludge formation, caustic embrittlement and corrosion in the boiler

and the pipes. Water Treatment at GNDTP, Bhatinda may be broadly divided into

A) External Water Treatment.

B) Internal Water Treatment.


CHAPTER 66.1SWITCHYAED

132/220KV SWITCH YARD

The electricity generated at 11 K.V. by the turbo generator sets is step up by power

transformers up to 132 KV in case of stage-1 and 220 KV in case of stage-2. For further

transmission of power from power station to grid is controlled through 7 no’s 220 KV

and 15 no’s 132 KV air blast circuit breakers along with their associated protective

system.

According to design substation are classified:-

1. Indoor substation.

2. Outdoor substation.

1. INDOOR SUBSTATION

In these substations the equipments are installed within the building of the substation and

hence the name indoor substation. Such substations are usually designed for 11 KV but


can be erected for 33 KV or 66 KV, if the surrounding atmosphere is containing

impurities, which may damage the equipments.

2. OUTDOOR SUBSTATION

In these substations, the equipments are installed open and hence the name outdoor

substation. Such substation can be designed to handle low, high and extra high voltages.

The outdoor substations may be further classified as:

(I) Pole mounted substation

(II) Foundation mounted substation

(I) POLE MOUNTED SUBSTATION

Such substations are designed for monthly distribution transformers of capacity unto 300

KVA. Such substations are cheapest simple and smallest of other substations. All the

equipment’s are of outdoor type and mounted on the supporting structure of HT

distribution line. Tripple pole mechanically operated switch is used for switching on and

off of H.T transmission line. To control L.T. side, iron clad low-tension switch of suitable

capacity with fuses is installed. Lighting arrestors are installed over the HT line to protect

the installation from the surges. Substations are earthed at two or more places. Generally

transformers of capacity upto 125 KVA are mounted on double pole structure and for

transformers of capacity above 125 KVA 4 pole structure with suitable platform is used

such substations are situated in very thickly populated location.


(II) FOUNDATION MOUNTED SUBSTATION

These substations are built entirely in the open and in such substations all the equipments

are assembled into one unit usually enclosed by a fence from the point of view of safety.

Substations for primary and secondary transmission and for secondary distribution are of

this type.

The major equipment’s installed at G.N.D.T.P. substations are

1. Transformers.

2. Circuit breakers.

3. Isolators.


4. Bus Bars.

5. Lightning arrestors.

6. Current Transformer

7. Potential Transformer.

8. Insulator.

9. Wave traps.


CHAPTER 7

TRANSFORMER7.1 TRANSFORMER: - It is a static device which transfers a.c. electrical power

from one circuit to the other at same frequency. It is used to step up or step down the

voltage. In all the substations except the generating station transformers are employed.

These are of following types: -

(A) POWER TRANSFORMER:-These are provided for stepping up the voltage. For

units 1&2 the power transformers step up the voltage from 11 KV to 132 KV and for

units 3&4 the power transformers step up the voltage from 11 KV to 220 KV. All the

four transformers have a rated capacity of 125 MVA.

Specification of power transformer

Sr. No. 6002737

Electrical specific No. 600250

Manufacturer Heavy electrical Ltd. (Bhopal)

Year of manufacturing 1972

Insulation level :


H.V. 650 KV peak

H.V. neutral 38 KV rms

L.V. 75 KV peak

Weight of core and winding 94000 kg

Weight of oil 37200 kg.

Total weight 188880 kg

Oil quantity 43890 liters

Oil circulation litre/min. 2 * 2724

Air circulation cubic/metre 10 * 385

(B) Inter bus linking (auto) transformer: - The auto transformers are used to balance

the load between 132 kv bus bars and 220 Kv bus bars. These transformers have a

capacity of 100 MVA each.

Specification of Inter bus linking transformer

S.No. 4137621

Electrical specification No. 410216

Manufacturer Heavy Electrical Ltd. (Bhopal)

Year of manufacturing May, 1972

Weight of core and winding 75200 kg.

Total weight 151104 kg.

Rated capacity 100 MVA


(C) Unit auxiliary transformer: There is one unit auxiliary transformer provided on

each unit to step down the voltage from 11 Kv to 6.6 Kv, which is required to run the

major plant auxiliaries.

Specification

S.No. 902016

Manufacturer Heavy Electrical Ltd. (Baroda)


Rated Capacity 15 MVA

Weight of core of winding 11685 Kg.

Total weight 22188 kg.

Oil in main tank 12200 litre.

Oil in cooler including pipes 2300 litre.

Total oil quantity 15620 litre.

Oil in OLTC 1120 litre.


(D) Station Transformer: There are two station transformers one for each unit is

provided to step down the voltage from 132 Kv to 6.6 Kv. These transformers have

capacity of 22.5 MVA. They serve as a stand by source of supply to auxiliaries.

Specification

S. No. 23600

Total weight 70 tones

Manufacturer Made in Bombay Martin Burn Ltd.


Oil in main tank 14940 lt.

Oil in cooler including pipe work 4500 lt.

Oil in OLTC 1760 lt.

Total oil 21200 lt.

7.2 CIRCUIT BREAKER

Circuit breaker is a device, which may be operated under different conditions i.e. no load,

full load fault conditions. However the major duty of a circuit breaker is to operate the


circuit breaker under fault conditions under short circuit conditions, a circuit breaker

required to perform three major duties:-

(i) It must be capable of opening the circuit under abnormal conditions.

(ii) It must be capable of classing the circuit on to a fault.

(iii) It must be capable of carrying a fault current safely for a short time while another

circuit breaker is clearing the fault

.

OPERATING PRINCIPLE OF A CIRCUIT BREAKER

A circuit breaker is a device which.

(i) Makes or breaks a circuit either manually or by remote control under normal

conditions.

(ii) Breaks a circuit automatically under abnormal conditions.

(iii) Makes a circuit manually or by remote control under abnormal conditions.

Thus, circuit breaker is just a switch which can be opened under normal abnormal

conditions both manually and automatically.

To perform the above operations, a circuit breaker essentially consists of fixed and

moving contacts, called electrodes, under normal operating conditions; these contacts

remain closed until and unless these are not operated manually or by remote control.

However when a fault occurs on the power system, the trip coil of the circuit breaker is

energized, which pulls apart the moving contracts from the fixed contacts thus opens the

circuit.

When the moving contacts are separated from the fixed contacts, an arc is struck

between them. The production of are not only delays the current interruption proxies but

is also generates enormous heat which causes damage to the equipment of the power

system or to the breaker itself. Therefore every effort is made to extinguish the arc

produced in the circuit breaker as quickly as possible.

CIRCUIT BREAKER RATINGS

According to the duties to be performed by a circuit breaker they have the following three

ratings:-


(i) Breaking capacity

(ii) Making capacity.

(iii) Short time capacity.

(i)Breaking capacity: - when fault occurs the contacts are separated in the circuit

breaker. The r.m.s. value of current that a circuit breaker is capable of breaking at a given

recovery voltage and under specified conditions is known as breaking capacity of the

circuit breaker.If the current is symmetrical, breaking capacity is referred to is

symmetrical breaking capacity. Whereas if the current is asymmetrical it is referred as

asymmetrical breaking capacity. Symmetrical breaking current is the r.m.s. value of the

a.c. component of the short circuit current at the instant when contacts are separated.

Whereas the asymmetrical breaking current is the r.m.s. value of the total current which

compresses of a.c. and d.c. component of current at the instant when contacts are

separated.The breaking capacity rating of a circuit breaker is generally expresses in terms

of MVA.In case of 3-phase circuit breakers breaking capacity in MVA = 3* rated

voltage * rated breaking current * 10-6.

(ii)Making capacity: - There is every possibility of closing the circuit breaker under

deed short circuit conditions. The capacity of a circuit breaker to close the circuit under

short circuit conditions depends upon its ability to with stand the effects of

electromagnetic forces. These forces are proportional to the square of the peak value of

the current on closing.

The peak value of the current during the first cycle of current wave after the circuit the

first cycle of current wave after the circuit is closed by the circuit breaker is called

making capacity. The making capacity is stated in terms of a peak value of current

instead of and r.m.s. value.

To find out the making capacity the symmetrical breaking current is multiplied by 2 to

convert the r.m.s value to peak value and then it is multiplied by 1.8 to include the

doubling effect of maximum asymmetry. Making capacity = 2 * 1.8 * symmetrical

breaking capacity = 2.55* symmetrical breaking capacity.

(iii)Short time capacity :- Sometimes fault occurs in the power system in such a way

that one circuit breaker is clearing the fault at that time the other circuit breaker


connected in series must carry the fault current safely for a short period. Moreover

sometimes the fault on the power system is of very temporary nature and presets for a

small period after which the fault is automatically cleared. In the interest of continuity of

supply the circuit breaker should not be allowed to trip in such situations and should be

capable of carrying the fault current safely for a short period.

The period for which the circuit breaker is able to carry the fault current while remaining

closed is called short time capacity of the circuit breaker.

At G.N.D.T.P switch yard air blast circuit breakers and SF6 circuit breakers are

employed instead of oil Circuit breakers due to following reasons:-

(i) There is no risk of fire hazard and explosion.

(ii) Due to less arc duration in it as compared to that in oil circuit breakers, burning of

contact is less.

(iii) It requires less maintenance

(iv) They provide facility of high speed reclosure.

AXIAL-BLAST AIR CIRCUIT BREAKERS

A schematic arrangement of an axial-blast air circuit breaker is as shown in figure. The

arcing potions of the fixed and moving contacts are coated with silver tungsten alloy. The

moving contacts are coated to a piston and shaft of the contact is guided by guide spring.

Opening the lower air valve closes the circuit breaker and under normal conditions the

valve remains open. Whenever a fault occurs, the upper valve is opened and the lower

valve is closed by the mechanism not shown in figure. Air enters the upper vessel at a

high pressure, which separates the moving contacts from the fixed. An arc is struck

between the contacts, which is extinguished by the axial blast of cold air and current is

interrupted. Once the arc is extinguished, the upper valve is closed and the lower valve is

opened to close the circuit.


SF6 CIRCUIT BREAKERS

Sulpher hexafluoride CB is shown in fig. In this the movable cylinder is coupled with the

moving contacts, whereas the piston is fixed. When fault occurs, the moving contacts are

separated from the fixed contact. Since the movable cylinder is attached with the moving

contacts, it moves against the fixed piston. Thus the gas filled in the cylinder is

compressed and released through the nozzle as shown in fig. The gas moves along the arc

and reduces its diameter by axial convection and radial dissipation. At zero current, the

diameter becomes too small and the arc gets extinguished. The gas is not exhausted to the

atmosphere; it is rather again used for arc extinction.

Advantages:

They are smaller in size because of high dielectric strength of SF6 gas.

No danger of or explosion.

They require minimum maintenance.

Since same gas is recycled, a small quantity of SF6 gas is required for long run.

They give silent operation; they do not make any sound like A.C.B. during

operation.

It requires less maintenance.

Specification

S.No. 729/13

Rated voltage 132 Kv

Rated frequency 50 Hz

Rated gas pressure 18 kg/cm2

Replaced with SF6* 7 kg/cm2


Rated current 1600 Amp.

Rated making capacity 17850 MVA

Rated breaking capacity 7000 MVA

(3) Isolator: - These are knife switches which are operated only at no-load. Their

main function is to isolate a portion of the circuit from the other. These are generally

placed on both sides of a circuit breaker in order to do repair and maintenance on the

circuit breaker without any danger. For maintenance first of all circuit breakers are

opened then isolators are opened and properly earthed. Only then maintenance is

done. Isolators do not have the arc control devices and therefore cannot be used to

interrupt current at which an arc will be drawn across the contacts. The open arc that

would be drawn in such a case is very dangerous in that it will not only damage the

isolator and the equipment surrounding it, but will also as a rule will cause flash over

between phases. In other words results in short circuit in the installation. That is why

isolators are used only for disconnecting and connecting parts or units after first de-

energizing them by opening their circuits with respective circuit breakers.

Specifications

S.No. 09

Manufacturer Hi-Velm industries Pvt. Ltd., Madras

Unit type THP

Isolator 541/3

Auxiliary switch no. of pairs 12

Pressure 16 kg/cm2


Isolator kV 132 kV

Control voltage 220 V

Isolator current 800 Amp.

(4)Bus Bars: - The thick conductors run on the towers at the generating stations, grid

stations or sub-stations operating at constant voltage required to connect a number of

generators or feeders operating at the same voltage are called bus-bars.

The bus bars are arranged in different manner. The main aim of any particular

arrangement of bus bars is to achieve adequate operating flexibility, sufficient reliability

and minimum cost. At G.N.D.T.P. there are two no’s 132 kV Bus bars are made up of

aluminum. The bus coupler number two connects the 132 kV bus bars and 220 kV bus

bars with each other.

(5)Lightning Arrestor: - A lightning arrestor or surge diverter is a device which

provides an easy conducting path or relatively low impedance path for the flow of current

when the system voltage increases more than the designed value and regains its original

properties of an insulator at normal voltage.These are the arc apparatus devices designed

to protect insulators of power lines and electrical installation from lighting surges by

diverting the surge to earth and instantly restoring the circuit insulation to its normal


strength with respect to earth. These are connected between earth and line. Their purpose

is to protect the t/f winding against over voltages.

Necessity of lightning arrestors

The ground wires and earth screens do not provide protection against the high voltage

waves reaching at the terminals of costly equipment such as transformers. These high

voltage waves may cause the following damages.

(i) The waves may cause flash over in the internal windings of transformer and

spoils the winding insulation.

(ii) It may cause internal flash over between turns of the same winding of

transformer.

(iii) It may cause external flash over between the terminals of electrical equipment,

which may cause damage to insulator.

(iv) It may cause internal or external flash over causing building up of the oscillations

in the electrical apparatus. Hence it is absolutely necessary to divert this high

voltage wave to earth before it reaches at the terminals of the equipments. This is

achieved by connecting a lighting arrestor between line and earth.

(6)Current Transformers: - The current transformers are basically step up transformers.

The connections of an ammeter when used in conjunction with a current transformer for

measurement of current are shown in fig.

The primary winding having one or a few turns of thick wire is connected in series with

the line, whose current is to be measured. The secondary winding having large number of


turns of fine wire carries the instrument directly connected across it. The working of

current transformer is slightly different to that of on ordinary power transformer. In case

Of current transformer, the load impedance or burden on the secondary is very small,

therefore it is considered to be short circuited. Hence current transformer works under

short circuit conditions. Moreover the current in secondary winding is not governed by its

load impedance rather it depends upon the current flowing through the primary.

(7)Potential Transformer:-The potential transformers are basically step down

transformers. The connection of a voltmeter is used in conjunction with a potential

transformer for measurement of high ac voltages. The voltage to be measured is applied

across primary winding, which has a large no. of turns. The secondary side, which has

much smaller number of turns, is coupled magnetically to the primary winding. The turn

ratio is so adjusted that the secondary voltage is 110V, when full rated primary voltage is

applied to primary. Potential transformers are used to operate voltmeter, the potential

coils of wattmeter and relays from high voltage lines. The design of potential

transformers in quite similar to that of a power transformer, but the loading of a potential

transformer is very small in comparison to that of a power transformer. The loading of

potential transformer sometimes is only a few volt amperes. These transformers are made

shell type because this condition develops a higher degree of accuracy. For medium

voltage i.e. unto 6.6 KV to 11 kV they may be either dry or oil immersed but for voltage

more than 11 kV they are always oil immersed type. An out door type oil immersed

voltage transformer having rating 66000/3 or 110/ 3. The working of potential

transformer is essentially the same as that of a power transformer, the main point of

difference is that the power loading of a potential transformer is very small and

consequently the exciting current is almost of the same order as that of secondary current.

Whereas in power transformers exciting current is very small fraction of secondary load

current.


(8)Insulators: - They are used to prevent the flow of current from bare conductors to

earth through line support; the conductors are secured to insulators. They provide

insulation between the conductors and earthed steel towers. The insulators are usually

placed on the cross arms which is clamped on line support. Thus the successful operation

of transmission system depends to a great extent on the quality and maintenance of line

insulators. Generally suspension and strain type insulators are employed at the

substations.

(9)Wave traps: - These are used in carrier communication circuits and are mounted on

lines. Wave trap or line trap contains a main coil, lightning arrestor and a tuning device.

All are connected in parallel as shown in fig..


The main coil has an inductance of 0.2 MH to 2.0 MH. This inductance offers high

impedance to the frequency (50 kHz to 500 kHz) carrier signals and blocks them here. It

does not allow them to enter the power system equipment. However it offers very low

impedance to the power frequency signal (i.e. power system voltage and current). Hence

it acts as an insulator for high frequency carrier signals and a conductor for the low.

Since the main coil is connected in series with the line, it has to carry the line current

even under fault conditions. There fore it is designed from the current rating point of

view. The current rating of the main coil may vary from 400 A to 4000 A.

The lighting arrestor is used to protect the main coil from high voltage surge whereas the

tuning device is used to block the signals of narrow band carrier frequency.

All these components of the wave trap are hanged in air through a string of insulators as

shown in fig.

Sometimes these components of wave trap are immersed in oil and are enclosed in a

drum type porcelain container having sufficient mechanical strength.


CONCLUSIONSpending my six /four weeks of training in Guru Nanak Dev Thermal Plant, Bathinda, I

concluded that this is a very excellent industry of its own type. They have achieved

milestones in the field of power generation. They guide well to every person in the

industry i.e. trainees or any worker. I had an opportunity to work in various sections

namely switch gear, Boiler section, Turbine section, De-mineralized water plant, E.S.P,

EM-2 CELL etc. while attending various equipments and machines. I had got an

endeverous knowledge about the handling of coal, various processes involved like

unloading, belting, crushing and firing of coal. The other machines related to my field

that I got familiar with boiler, turbine, compressors, condenser etc. I found that there

existed a big gap between the working in an institute workshop and that in the industry.

Above all the knowledge about the production of electricity from steam helped me a lot

to discover and sort out my problems in my mind related to the steam turbine, their

manufacture, their capacity, their angle of blades and their manufacturing. The training

that I had undergone in this industry will definitely help me to apply theoretical

knowledge to the practical situation with confidence.


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