description mwjia thermal plant

8/11/2019 Description Mwjia Thermal Plant

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In Thermal Power Station fuel burns & use the resultant to make the steam,

which derives the turbo generator. The Fuel i.e. coal is burnt in pulverized

from. The pressure energ of the steam produce is converted into

mechanical energ with the help of turbine. The mechanical energ is fed

to the generator where the magnet rotate inside a set of stator winding &

thus electricit is produced in India !"# of total power is generated b

thermal power stations. To understand the working of the Thermal Power

Station plant, we can divide the whole process into following parts.

$. %'( F() * In coal fired plants, raw material are air & water in

PTPS, coal is transported through +ailwa wagons from or-ora coal fields

and +anigan- and is kept reserved on a buffer stock. The brought out to the

station is unloaded with the help of wagon tippler. 'fter unloading, the coal

is sent to crusher house with the help of conveor belts. The coal which is

now reduced to ver small pieces is sent to the coal bunkers with the help

of conveor belt. The raw coal is fed to coal mills through raw coal feeders

raw coal feeders basicall regulate raw coal to pulverized coal pipes. '

position of the primar air is heated utilizing the heat of the fuel gases &

then mied with the cold air as per re/uirement b the pulverized coal.

0ormall the temperature is maintained at !1 to 21 degrees. The coal is

now burnt in the furnace using oil in the beginning showered through the



nozzles at different elevations in the furnace. To provide air for

combustion, the heat of the flue gases also heat it the heat produced due to

combustion is utilized for the conversion of water into steam. This water is

stored in the boiler drum. There are two sets of pipes attached to the drum,

one called riser & other known as down corner through which the watercomes to the ring header & steam moves up due to the densit difference of

water & steam. Its steam is super heated using super heaters & meanwhile

the flue gases are through out in the atmosphere through chimne.

3. ST4'5 F()* The super heated steam is sent to the turbine through

pipelines there are three turbines in the units, using this steam at different

temperature & pressures. 'fter passing through high pressure turbine the

steam is send to the reheater for rising the temperature of the steam. 'fter

reheating the steam is sent to the intermediate pressure turbine through

reheated line. 6ere it losses most of its temperature & pressure & finall

sent to low pressure turbine. The uses of three different turbines help in

increasing the efficienc of the plant. The turbine in turn connecting with a

generator produces electricit. Then this electricit is stepped upto 331 78



with the help of step up transformer & supplied to various sub9stations

grids. 5eanwhile, the steam through low pressure :(.P.; Turbine is

condensed and the condensed water is stored in hot well.

<. )'T4+ F()* The condensed water is etracted from the hot wellthrough condensate etraction pumps & sent to the boiler drum with the

help of I(4+ F44= P>5P :.F.P.; before passing through low pressure

heater and dearater. )hile loss in water is make up from %.S. Tank, which

have =.5. 5oor in it. The %.S. Tank is directl connected to hot well. The

water used in condenser is sent to cooling tower for cooling. 'fter cooling

this water is again sent to condenser with the help of circulating water

pump. The loss is making from raw water pump house through clarifier pump house.

%5P040TS =4S%+IPTI0

$. )'?0 TIPP(4+* It is the machine which is used to tip the coal from

the wagon. The coal tipped is directl feed to conveor belt. Its capacit is

$3 wagon per hour.

3. %+>S64+* It crushes the coal into small pieces.

<. %'( 5I((S* In it small pieces of coal are converted into pulverized

from. The are ! in number.

@. F>+0'%4* It is the chamber in which fuel burns & fire blows.

". I(4+ =+>5* It contains water for boiling.

!. 4(4%T+ST'TI% P+4%IPIT'T+* In this we have electrodes which

attract fl ash and etract it from flue gases so that it cannot enter

atmosphere.

2. %6I540A* It is used to release flue gases into the atmosphere.



B. T>+I04* Turbine is the part which revolves due to steam pressure. It

is of three tpes. a; 6igh pressure turbine. b; Intermediate pressure turbine.

c; (ow pressure turbine.

C. T>+ ?404+'T+* It is the main machine which produces 3"1 5)electricit .It is :6 3 ; water and 6 3 :6drogen;

gas cooled therefore it is contained in clindrical chamber.

$1. %0=40S4+* It condenses steam coming from low pressure turbine

:(.P.T.; to hot water. removing air and other non9condensable gases

from steam while passing through them.

$$. %(I0? )'T4+ :%.).; P>5P* This pump send water from cooling

tower to condenser.

$3. %(I0? T)4+*

It is used to coal the water its height is near about $@<." mtrs. The hot water is led to the tower top and falls down through the tower and is broken into

small particles while passing over the baffing devices. 'ir enters the tower

from the bottom and flow upwards. The air vaporizes a small percentage of

water, thereb cooling water falls down into tank below the tower from

where it is pumped to the condenser and ccle is repeated.

$<. +') )'T4+ P>5P 6>S4* It supplies raw water to the boiler.

$@. %('+IFI4+ P>5P 6>S4* The water from raw is clear at clarifier

b putting alum in it & filtering it & then supplied to the condenser.

$". %0=40S'T4 4DT+'%TI0 P>5P* %.4.P. pump is used to etract

the condense water from the hot well and suppl to the deaerator after

passing through (.P. heater & 4conomizer, so that high pressure steam in

the clinder can be created.



$!. () P+4SS>+4 64'T4+* It is used to increase the temperature of

water, in this wa efficienc of sstem increases.

$2. =4'+4'T4+* It is used to remove air from water which is entrapped

in the water molecules. It is ver important part because the entrapped aireffect air drum badl.

$B. I(4+ F44= P>5 :.F.P.;* It is the heaviest drive in the plant &

suppl water to boiler drum from dearator.

$C. 6I?6 P+4SS>+4 64'T4+ :6.P.;* In this temperature of water

increases. Thus efficienc further increases.

31. 4%05IS4+* In this flue gases echange heat to the water to

increase sstem efficienc, causes saving in fuel consumption :" to $1#;.

4conomizer tubes are made up of steel either smooth or covered with fins

to increase the heat transfer surface area.

%>++40T T+'0SF+54+ :%.T.; SP4%IFI%'TI0S +4E>I+4=F+ %TS '+4* $. %lass 3. urden <. +atio @. +esistance ". 7nee Point

8oltageSaturation %Ts can be broadl divided into two categories*9

$. 5etering %Ts* 5etering %Ts are of ver accurate for design of material

having nickel iron allos to reduce losses and having low flu densit. The

accurac class of these %Ts is 13#, 1."# & $.1#. Saturation level of

these %Ts is low i.e. $11 to $31# of I n :%+ secondar current; are thermal protection of meters. 5etering %Ts are also known as 5easuring %Ts.

These %Ts cannot be used for protection purpose. The can onl measure the

current.

3. Protection %Ts* Protection %Ts are less accurate than metering %Ts

having high flu densit. Saturation level of these %Ts is high. Protection

%Ts are of two class i.e. "P $1 & PS. "P $1 is generall used for simple oc

& 4F protection where PS :purchaser specified; class is used for

differential protection. These %Ts are also used to measure current but up to



specified limit.

?404+'T+ SP4%IFI%'TI0 F ?404+'T+ 7)5) 93$1 P.F.

91.B" 78' 93@2111 78 9$".2"

'mp 9C1"1 +otor volt 9<$1

4P5 9<111 +otor amp 93!11

6z 9"1 Phas 9< %onnection9star star ?as pressure 9<."kg

cm 3

Insulation class9 SP4%9 I4 %<@.$B<@.< IS."@3< =I8I096'+I=)'+

%oolants9 6 3 & 6 3

Supervision of cooling gas & cooling water temperature The ad-ustment or

setting of cooling water flow should be made onl b changing the

ad-ustment f cooling water outlet valves. The inlet valve at header or valveat individual cooler section should be opened full. The pressure of water at

inlet of each cooler should be so ad-usted as to keep the

valve to 1.3kgcm 3 below the pressure of 6 3 in casing.

perating value 5'D. S4TTI0? $. Stator water inlet temp. to stator

winding 3. Stator water outlet temp. from stator winding.

@1 o %

!1 o %

@" o %

B" o %



'('+5 6I?6 84+A 6I?6 <. %old gas temp. 3" o 9@@ o % @@ o % "" o %

@. 6ot gas temp. @1 o 92" o % 2" o % 9

". Stator core temp. GC" o % C" o % 9

!. Stator slot temp. "1 o 92" o % 2" o % 9

2. +otor winding temp. G$$" o % $$" o % $31 o % B. 6umidit monitor

dew point G$1 o % G9< o % 9

6A=+?40 %(4+S

The turbo generator has been provided with four nos. gas coolers mounted

longitudinall in side stator bod for cooling of hot gas, thus taking awa

the heat losses generated b rotor winding, stator core and windage losses.

The gas cooler is a shell and tube heat echanger consisting of cooling

tubes with coiled copper wire around them to increase the surface area of

cooling. %ooling water flow through the tubes while hdrogen flowingacross coolers comes into contact with eternal surface of cooling tubes.

6eat remove from hdrogen is dissipated through cooling water.

n both ends of coolers, water chambers are bolted to two plates. The

outside flange of water chamber on slipping end is elasticall fied to stator

bod with the help of molded rubber gasket to allow free epansion of

cooler where as on the turbine endH it is fied rigidl to the stator. 'tturbine end, inlet and outlet water pipes are connected to water flange. In

order to remove air from cooler while filling them with water, wind pipes

are provided on slip ring end. Shut off valves are installed in pipe line at

inlet and outlet of each cooler.

?404+'T+ =4S%+IPTI0

3$1 5) ?404+'T+ The generator is two pole tpe with clindrical

rotor using direct water cooling of stator winding, including phase



connecting bus bar, terminal bushing and direct hdrogen cooling of rotor

winding using gap pick up method. The losses in other parts of generator

such as stator iron losses, friction & windage losses are removed b

hdrogen circulating in casing.

The generator stator frame is of pressure9resistant and gas tight

construction with four horizontal coolers in the frame itself forming part of

ventilation and closed cooling circuit.

The generator consists of the following components as shown*

ST'T+* Stator bod Stator core Stator winding ?as coolers

+T+* +otor shaft +otor winding +otor retaining ring and other

fittings Field connections.

4'+I0? )IT6 +>S6 ?4'+* earing rush gear

ST'T+ $. Stator od* The stator bod is a totall enclosed gas tightfabricated structure, suitable ribbed internall to ensure high rigidit, it is

designed mechanicall to withstand internal pressure and forces as a result

of unlikel event o eplosion of hdrogen, air miture without an residual

deformation, 6drogen gas coolers are housed longitudinall inside the

stator bod.



3. Stator %ore* The stator core is made up of segmental, varnish insulated

punching of 4lectro technical sheet with low loss factor. The stampings are

assembled in an inter leaved manner on dove tailed core bars in order to

damp out the oscillations so that magnetic vibration of stator core are not

transferred to foundation through stator frame.

<. Stator )inding* Stator bars, bus bars and terminal bushing are designed

for direct water cooling. In order to minimize the edd losses, the bars are

composed of separatel insulated strands, solid as well as hollow. The high

voltage insulation is provided b thermosetting insulation using epo mica

paper tape. )ith this insulating sstem, several laers of this tape are

applied to the formed bars continuousl and half over lapped. The no. of

laers i.e. the thickness of the insulation depends on the machine voltage.

The insulation is also water proof and oil resistant. For protection of the

stator winding against the effects of current forces in slots sides fillers,

bottom spacers and top spacer below the slot wedge ensure permanentl

firm seating of the bars in the slot during operation. The water headers are

insulated from stator bod which permits measurement of insulation

resistance of winding. The stator winding is connected inside the machine

b connecting bus bars and brought out to nine bushing located in bo of

welded non magnetic steel below the generator at eciter end.

+T+ $. +T+ S6'FT* The rotor shaft is single piece forging

manufacturing from ingots which are cast b vacuum degassing process.



The longitudinal slots for insertion of the field winding are milled into

barrel portion. The slots are distributed over the circumstances so that two

solid poles are obtained. To ensure that onl high /ualit and defect free

forging is used, strength test, chemical analsis and ultra sonic test are

carried out during manufacturing. 'fter completion of rotor assembl therotor is balanced at different sped in various planes and then sub-ected to an

over sped test at $31# of rated sped for 3 minutes.

3. +T+ )I0=I0? '0= +4T'I0I0? +I0?S* The rotor winding

comprises several coils which are inserted into the slots and series

connected in such a wa that two coil groups enclosing one pole each are

obtained. The centrifugal forces of the rotor overhauling windings are takenup b single piece retaining ring. The retaining rings consist of non9

magnetic high strength steel in order to reduce stra losses.

<. FI4(= (4'= %004%TI0S* a;. S(IP +I0?S* The slip rings consist

of helicall grooved allo steel ring shrunk on the rotor bod shaft and

insulated from it. For convenience in assembl both rings are mounted on a

single common steel bush which has an insulating -acket premoulded on it.The complete bush with slip rings is shrunk on the rotor shaft. The slip

rings are provided with inclined holes for self ventilation. The helical

grooves cut on the outer surface of the slip rings improve brush

performance b breaking the pressurized air pockets that would otherwise

get formed between the brush and slip ring surface.

b;. FI4(= (4'=* The slip rings are connected to the field winding throughsemi fleible copper leads and current carring bolts placed radiall in the

shaft semi9fleible o thin copper sheets silver plated and copper leads are

made up insulated b glass cloth impregnated with epo resin for low

resistance and ease of assembl. The connection between current carring

bolt and field winding is done b a filed lead bar which has similar

construction as that of semi9fleible copper lead.

4'+I0? '0= +>S6 ?4'+



a;. 4'+I0?* The generator bearings are pedestal tpe with spherical

seating to allow self alignment and are supported on a separate pedestal on

slip ring side and in (.P. casing on the turbine side. The bearings have

provision of hdraulic shaft lifting during start up and turning gear

operation. To eliminate shaft current, eciter side bearing and its pipes areinsulated from earth. The bearing temperature detectors embedded in the

abbitt of lower half bearing liner. 8ibrations on the bearing in horizontal

and vertical direction are measured b vibration pickups mounted on the

bearing pedestal.

b;. +>S6 ?4'+* The rotor winding is solidit connected to slip rings b

means of field lead bars, current carring bolts, field lead core bars andfleible leads. The filed current to the rotor winding is provided through the

brush gear. The current carring brush gear assembl is rigidl fied on the

etended part of bearing pedestal on the eciter side. There are two brush

gear stands, each made up of two smmetrical silicon brass casting half

rings, which are bottled at the top to make one stand assembl, kept

verticall. These rings stands are designed as helical from one end to the

other to achieve uniform wear of slip rings as well as carbon brushes andsmooth removal of carbon dust all along the width of slip rings. The design

of brush gear permits replacement of the brushes during normal operation

condition. This complete gear stand assembl is rigid fitted in position on

brush gear support which as a whole unit is to be fied on to bearing.

c;. S6'FT S4'(S* In order to prevent the escape of hdrogen from the

generator casing along the rotor shaft, shaft seals supplied with oil under pressure are used. To ensure perfect sealing, the oil pressure in the annular

gap is maintained at a higher level than the gas pressure in the generator

casBng. 's long as the seal oil pressure in the annular gap between the shaft

seal and the rotor eceeds the gas pressure in the generator. 0o hdrogen

will escape from the generator housing. The shaft seal is provided with seal

oil b a separate closed circuit sstem. For the operation of generator the

following auiliar sstem are re/uired.

d;. ST'T+ )'T4+ %(I0? SAST45* $. Seal oil suppl sstem 3.



?as sstem. <. Stator water cooling sstem.

S4'( I( S>PP(A SAST45* The shaft seals are supplied with seal oil

from a separate circuit which consists of following principle components*

8acuum tank '% seal oil pump $ & 3 =% seal oil pump 8acuum

pump il coolers SeaH oil filters Intermediate oil tank %onstant

pressure regulating valve9$ %onstant pressure regulating valve93

%onstant pressure regulating valve9< =ifferential pressure regulating

valve9$ =ifferential pressure regulating valve93

?'S S>PP(A SAST45* The gas sstem essentiall consists of thefollowing e/uipment* 63 & %3 clinders Pressure reducers %3

vaporizer Pressure gauges ?as drier 6umidit monitors Purit

measuring instruments

ST'T+ )'T4+ %(I0? SAST45* The stator water suppl sstem

essentiall comprises the following components* 4pansion tank

Stator water pump9' Stator water pump9 Stator water cooler9' Stator water cooler9 Stator water filter9' Stator water filter9

?404+'T+ P+T4%TI0*

'n '% generator forms the electromechanical stage of an overall energ

conversion process that results in the production of electrical power. '

reciprocating engine, or one of man forms of turbine, acts as a primemover to provide the rotar mechanical input to the alternator. There are

man forms of generating plant that utilize a variet of sources of energ

available, e.g. %ombustion of fossil fuels, hdro dams and nuclear fission.

?eneration schemes ma be provided for base9load production, peak

lopping or for providing standb power. 4lectrical protection shall /uickl

detect the initiate shut down for ma-or electrical faults associated with the

generating plant and less urgentl to abnormal operating conditions which

ma lead to plant damage. 'bnormal electrical conditions can arise as a

result of a failure within the generating plant itself, but can also be



etremel imposed on the generator. %ommon categories of faults and

abnormal conditions which can be detected electricall are listed as

follows*

5a-or 4lectrical Faults* Insulation failure of stator windings orconnections.

Secondar 4lectrical Faults* Insulation failure of ecitation sstem.

Failure of ecitation sstem. >nsnchronized over voltage

'bnormal Prime 5over or %ontrol %onditions* Failure of prime mover

ver fre/uenc ver fluing =ead machine energisation reakerflashover

Sstem +elated* Feeding on un9cleared fault Prolonged or heav

unbalance loading Prolonged or heav overload (oss of snchronism

ver fre/uenc >nder fre/uenc Snchronized over voltage ver

fluing >nder voltage In addition various tpes of mechanical protection

ma be necessar, such as vibration detection lubricant and coolantmonitoring, temperature detection etc. The action re/uired following

response of an electrical or mechanical protection is often categorized as

follows*

>rgent shutdown 0on9urgent shutdown 'larm onl

'n urgent shutdown would be re/uired for eample, if a phase to phaseJFault occurred within the generator electrical connection. ' non9urgent

shutdown right to se/uential, where the prime mover ma be shutdown

prior to electricall unloading the generator, in order to avoid over sped. '

non9urgent shutdown ma be initiated in the case of continuedK unbalanced

loading. In this case, it is desirable that an alarm should be given before

shutdown becomes necessar, in order to allow for operator intervention to

remed the situation. For urgent tripping, it ma be desirable to electricall

maintain the shutdown condition with latching protection output contact,

which would re/uire manual resetting. For a non9urgent shutdown, it ma



be re/uired that the output contacts are self9reset, so that production of

power can be re9started as soon as possible. 'ccordingl generator

protection has been divided into three categories*

$. %lass ' Protection*9>rgent shutdown tripping without an time delacausing tripping of ?%, F, >'Ts & Turbine simultaneousl. :a; ?T +4F

Protection :!@ ?T; :b; ?enerator differential Protection :B2?; :c;

?enerator interterm fault :B2?$; :d; ?enerator +eserve Power Protection

:<3?$; (oss of 4citation :@1?; :e; ackup impedance Protection 3$?; :f;

>'T =ifferential protection :B2>'T; :g; ?T>'T Turbine :!< PTD?, !<

PTD; :h; ver voltage protection :"C ?$; :i; ?enerator $11# stator 4F

Protection :!@?<; :-; ver fluing Protection :CC?; :k; '8+ Trouble :l;ver %urrent for >'T :"1>'+; :m; +otor 4F Protection :!@F; :n; ?T

verall differential protection :B2 '; :o; ?enerator C"# stator 4F

Protection :!@?3;

3. %lass Protection*9>rgent shutdown tripping dela of turbine first and

tripping of ?%, F, >'Ts & Turbine on class ' protection through

(4P+P protection :1."# to 3$1 5);. :a; >nder Fre/uenc Protection :B$?; :b; ?T Trouble relas :c; (oss of 4citation Protection :@1 ?$; :d;

0egative Phase flow se/. Protection :@! ?; :e; Stator water flow low and

%onductivit high :f; >'T Trouble

<. %lass % Protection*9nl ?% trip and unit can be run on house load. :a;

?enerator ackup impedance protection :3$ ?; :b; ?T ver current

Protection :"$ ?T; :c; 0egative Phase se/, Protection. :d; >F Protection:e; ?T 0atural % Protection :"$ 0?T; :f; ?enerator Pole Slipping

Protection :2B ?;

?enerator =ifferential Protection Failure of stator windings, or connection

isolation, can result in severe damage to the windings and state core. The

etent of the damage will depend, upon the fault current level and the

duration of the fault. Protection should be applied to limit the degree of

damage in order to disconnection of the plant from the power generating

plant, high Lspeed maintain sstem stabilit.



Two methods are commonl used. ' biasing techni/ue, where the rela

setting is raised as through current increases. 'lternativel, a high

impedance techni/ue, where the rela impedance is such element is

insufficient for the rela to operate.

iased differential protection In a biased differential rela, through the

current is used to increase the setting of the differential element or heav

through faults, it is unlikel that the %T outputs at each zone end will be

identical, due to the effects of %T saturation. In this case a differential

current can be produced. 6owever, the biasing will increase the rela

setting, such that the differential spill current is insufficient to operate therela. Through the current is calculated as the average of the scalar sum of

the current entering and leaving the zone of protection. This calculated

through current is then used to appl percentage bias to increase the

differential setting.

Setting ?uidelines For iased =ifferential Protection The differential

current setting, should be set to a low setting to protect as much of themachine winding as possible. ' setting of "# of rated current of machine is

generall considered to be ade/uate. The threshold, above which the

second bias setting is applied, should be set to $31# of the machine rated

current. The initial bias slope setting should be set to 1# to provide

optimum sensitivit for internal faults. The second bias slope ma tpicall

be set to $"1# to provide ade/uate stabilit for eternal faults. These

settings ma be increased where low accurac class %Ts is used to protection.

6igh impedance differential Protection The high impedance principle is

best eplained b considering a differential scheme where one %T is

saturated for an eternal fault, as shown in Figurer. If the rela circuit is

considered to be ver high impedance, the secondar current produced b

the health %T will flow through the saturated %T. If the magnetizing

impedance of the saturated %T is considered to be negligible, the maimum

voltage across the rela circuit will be e/ual to the secondar fault current



multiplied b the connected impedance, :+(<M+ $@ M+% T$3 ;

The rela can be made stable for this maimum applied voltage b

increasing the overall impedance of the rela circuit, such that the resulting

current through the rela is less than its current setting. 's the impedanceof the rela input alone is relativel low, a series connected eternal resistor

is re/uired. The value of this resistor, +S T , is calculated b the formula

shown in Figure <. 'n additional on linear resistor, 5etrosil, ma be

re/uired to limit the peak secondar circuit voltage during internal fault

conditions.

To ensure that the protection will operate /uickl during an internal fault

the %Ts used to operate the protection must have a knee point voltage of at

least @8s.

>S4 F J54T+SI(K 00 L(I04'+ +4SIST+S 5etrosils are used

to limit the peak voltage developed b the below the insulation level of the

current transformer, rela and interconnecting leads, which are normallable* to withstand <11118 peak. The following formulae should be used to

estimate the peak transient voltage that could be produced for an internal

fault will be a function of the current transformer knee point voltage and

the prospective voltage that would be produced for an internal fault if

current transformer saturation did not occur. This prospective voltage will

be a function of maimum internal fault secondar current, the current

transformer ratio, the current transformer led resistance to the common point, the rela lead resistance and the stabilizing resistor value. 8 p N383

8k :8f98k;

8,N$:+ %T M3+ ( M+ ST ;where

8 p Npeak voltage developed b the

%T under internal fault conditions. 8kN current transformer knee9point

voltage 8N 5aimum voltage that would be produced if %T saturation did



not occur.

Setting guidelines for Stator earth fault protection function :"$0; O %urrent

operated from a %T in the neutral earth path. O Two independent tripping

stages. O First stage tripping can incorporate either a definite time orstandard inverse tpe I=5T dela. O Second stage tripping can be

instantaneous or definite time delaed. O Immune to third harmonics.

'pplied to directl connection generators. The protection must be time

graded with other earth fault protectionH the setting emploed should be

less than <<# of the earth fault level. In case of direct generator

connection, it is common that onl one generator of a parallel set is earthedat an one time, with the arth connections of other machines left open, if

the generating plant can also be run directl in parallel with a medium

voltage public suppl, it is a common re/uirement that all generator earth

connections are left open during parallel operation. In such circumstances,

the main earth fault protection element :le; will onl be operational for an

earthed machine, It will provide primar earth fault protection for other

machines and the rest of the power sstem and thermal protection for theearthing resistor. For indirectl connected applications, the time9delaed

earth fault protection function ma be emploed in one of two was. $. To

measure earth fault current indirectl, via a %T in the secondar circuit of a

distribution transformer earthing arrangement. 3. To measure earth fault

directl, via a %T in the generator winding earth connection. )ith the first

mode of application, the current operated protection function :"$0; ma be

used in con-unction with voltage operated protection function :"C0;,measuring the distribution transformer secondar voltage. This is a

complementar arrangement, where the voltage operated protection

function :"C0; is able to operate in the event of an open Lcircuited loading

resistor and the current operated protection function :"$0; is able to

operate in the event of a short circuited resistor.

The second mode of application would be used for cases of direct resistive

earthing. For distribution transformer earthing, this mode offers the

advantage of being able to respond to an earth fault condition that leads to a



flashover of the distribution transformer primar connections. Such a

primar short circuit would render protection on the secondar side of the

transformer inoperative and it would also result in a ver high and

damaging primar earth fault current.

In either mode of application, the main stator earth fault current operated

protection element :le; should be sent to have a primar sensitivit of

around "# of the maimum earth fault current as limited b the earthing

impedance. Such a setting would provide protection for upto C"# of the

generator stator windings. The probabilit of an earth fault occurring in the

lower "# of the generator Qwinding would be etremel low, due to the

fact that the winding voltage with respect to earth is low in this region.

The time characteristic and setting of the main current operated protection

element :le; should be set to prevent false operation during 68 sstem

earth fault clearance, where a transient generator earth connection current

ma appear as Lresult of the inter9winding capacitance of the generator

step9up transformer. The protection element should also co1operate with

operation of generator 8T primar fuses, for a 8T primar earth fault, andwith 8T secondar fuses for a secondar earth fault on a 8T that has its

primar windings earthed. =epending on the 8T fuse characteristics, and

on 68 sstem earth fault protection clearance times, a definite time dela

anwhere between 1."s and <.1s would be appropriate. In machines with

comple winding

g g p g g g p g p p g p g p g g p p g g p g g p p g g p , , g, p p , g g - p g g g gg pp g p g g g p g g g g p p p p p g g g g p g p

g g p p p g g g p g p g pp g p p g p p g g g g p g g g g , p p p g

gp p p p gp g g g g g , - p p p gp g p p g p p p g p R R R R pg g p -

p g - g g p g p g p p p p g g p g g p g, g, g p

gg

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description mwjia thermal plant

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