2 rpm sources and sinks mnm

8/10/2019 2 Rpm Sources and Sinks Mnm

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CHAPTER - 1REACTIVE POWER FUNDAMENTALS

--M.N.Murthy, Director, PSTI, Bangalore

1.0 Introduction

Voltage is proportional to the magnetic flux in the po er system element. Most of the Po er System elements are reacti!e in nature. They a"sor" # generate reacti!e po er $epen$ing on systemloa$ing con$itions. The "alance in reacti!e po er a!aila"ility an$ re%uirement at a no$e in$icates stea$y!oltage. Dra al of reacti!e po er lea$s to re$uction in !oltage an$ supply of reacti!e po er lea$s toincrease in !oltage at the no$e. I$eally, the reacti!e po er "alance shoul$ "e effecte$ ithin each region,

ithin each $istri"ution system.&xcess of MV'r high !oltageDeficit of MV'r (o VoltageMV') "alance *oo$ !oltage lo system losses

' great many loa$s consume not only acti!e "ut also reacti!e po er. The electric net or+ itself "othconsumes an$ pro$uces reacti!e po er. Transmission an$ $istri"ution of electric po er in!ol!e reacti!e

po er losses $ue to the series in$uctance of transformers, o!erhea$ lines an$ un$ergroun$ ca"les. (inesan$ ca"les also generate reacti!e po er $ue to their shunt capacitance this generation of reacti!e po er is, ho e!er, only of significance at high system !oltages.

During the stea$y-state operation of an ' po er system the acti!e po er pro$uction must match theconsumption plus the losses, since other ise the fre%uency ill change. There is an e%ually strongrelationship "et een the reacti!e po er "alance of a po er system an$ the !oltages. In itself, a reacti!e

po er "alance ill al ays inherently "e present, "ut ith unaccepta"le !oltages if the "alance is not a proper one. 'n excess of reacti!e po er in an area means high !oltages a $eficit means lo !oltages.The reacti!e po er "alance of a po er system also influences the acti!e losses of the net or+, theheating of components an$, in some cases, the po er system sta"ility.

ontrary to the acti!e po er "alance, hich has to "e effecte$ "y means of the generators alone, a proper reacti!e po er "alance can an$ often has to "e effecte$ "oth "y the generators an$ "y $isperse$ specialreacti!e $e!ices, pro$ucing or a"sor"ing reacti!e po er. The use of shunt reacti!e $e!ices. i.e. shuntcompensation, is a straightfor ar$ reacti!e-po er compensation metho$. The use of series capacitors,i.e. series compensation is a line reactance compensation metho$.

No special reacti!e compensation $e!ices ere use$ in the early ' po er systems, "ecause thegenerators ere situate$ close to the loa$s. 's net or+s "ecame more i$esprea$, synchronous motors,small synchronous compensators an$ static shunt capacitors ere a$opte$ for po er-factor correction.

&!er larger synchronous compensators ere installe$ in transmission systems. 'long ith the$e!elopment of more efficient an$ economic capacitors, there has "een a phenomenal gro th in the useof shunt capacitors as a means of furnishing reacti!e po er, particularly ithin $istri"ution systems./ith the intro$uction of extra-high-!oltage lines, shunt reactors an$ series capacitors "ecame importantcompensation $e!ices. The latest $e!elopment is the Thyristor-controlle$ static !ar compensator, hichis no ell esta"lishe$ not only in high- po er in$ustrial net or+s "ut also in transmission systems.

In the follo ing a $istinction is ma$e "et een transmission an$ $istri"ution systems an$ also "et een$ifferent !oltage ranges in terms of 0V, &0V, etc. It shoul$ therefore "e appropriate to explain "rieflythese terms.

1


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C !""i#ic!tion o# S$"t%& Vo t!'%"

Vo t!'% L%(% in )V C!t%'or$ o# Vo t!'%233 +V Distri"ution System

33 +V to134 +V Su". Transmission System435 +V to 655 +V 0V Transmission System785 +V an$ a"o!e 90V System

Transmission systems form those parts of po er systems con!eying comparati!ely large amounts of electrical po er. They lin+ the generating sources ith the $istri"ution systems an$ interconnect parts of the po er system or a$:acent po er systems. Distri"ution systems form the continue$ lin+s to theconsumers. The "oun$ary "et een transmission an$ $istri"ution systems is not !ery ell $efine$.Systems for !oltages higher than 134 ;V are usually calle$ transmission systems. Systems for !oltageslo er than 33 ;V are usually calle$ $istri"ution systems. Systems in the range 33 to 134 +V are calle$

$istri"ution, su" transmission systems.'ll the figures gi!en in this intro$uction refer to the highest !oltage for e%uipment.

1.1 N%%d #or &!n!'%&%nt o# r%!cti(% *o+%rIn an integrate$ po er system, efficient management of acti!e an$ reacti!e po er flo s is !eryimportant. an$ M/ $eman$ impinge$ on the system, the !oltage is in$icati!e of reacti!e po er flo s.

In a po er system, the ac generators an$ &0V an$ 90V transmission lines generate reacti!e po er.In$ustrial installations hether small or large as also the irrigation pump motors, ater supply systems

$ra su"stantial reacti!e po er from the po er gri$.The generators ha!e limite$ $efine$ capa"ility to generate reacti!e po er- this is more so in respect of large si?e generating units of 415 M/#855 M/ capacity. *eneration of higher reacti!e po er correspon$ingly re$uces a!aila"ility of useful po er from the generators. During light loa$ con$itions,there is excess reacti!e po er a!aila"le in the system since the transmission lines continue to generatethe reacti!e po er there"y raising the system !oltage an$ this causes reacti!e po er flo s to thegenerators.

Particularly in In$ia, the loa$ cur!es sho i$e fluctuations at !arious hours of the $ay an$ in !ariousseasons of the year. /hen loa$ $eman$ is hea!y, there is lo !oltage, hich is harmful to the consumersas ell as utility@s installations. Burning of motors occur. /hen loa$ $eman$ is !ery lo , high !oltageoccurs in the system an$ this has harmful effect on insulation of po er transformers. Aailure of po er transformers occur.

Aor "etter efficiency, it is necessary to re$uce an$ minimi?e reacti!e po er flo s in the system.Besi$es harmful effects, the reacti!e po er flo s also affect the economy a$!ersely "oth for the utilityan$ the consumer. If reacti!e po er flo s are re$uce$ i ) po er losses as ell as i C losses arere$uce$. The generators can pro$uce a$$itional acti!e po er. If the consumer re$uces reacti!e po er re%uirement his $eman$ ;V' is re$uce$. Aor energy conser!ation also there is nee$ to re$uce reacti!e

po er $eman$ in the system.

It is therefore !ery clear that for efficient management of po er system an$ for impro!ing the %uality of electric supply, it is !ery essential to install reacti!e compensation e%uipment. Such installations arenecessary an$ essential for utility as ell as the consumer. Infact the utility shoul$ "e ma$e responsi"le

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for ma+ing a!aila"le only the acti!e po er to the consumer. 9nfortunately, in In$ia, the responsi"ilitiesof users are not ell $efine$ an$ there is not enough reali?ation in this regar$. 9tilities ha!e nointro$uce$ po er factor clause in the tariff structure. 0o e!er. It oul$ "e orth hile to note that e!ena 5E po er factor loa$ re%uires 63E reacti!e po er from the gri$.

1., !"ic Princi* %"' phasor $escription of !oltage an$ current, the reacti!e po er supplie$ to an ' circuit is the pro$uct of the !oltage an$ the reacti!e = att-less> component of the current, this reacti!e current component "eingin %ua$rature ith the !oltage.

' single-phase circuit accor$ing to Aigure 1.1 the reacti!e po er < is gi!en "y

9nit is !olt-ampere reacti!e =V')> The sign of < is a matter of con!ention, it $epen$s on the $efinitionof the $irection of . 'ccor$ing to the I& the sign shall "e such that the net reacti!e po er supplie$ toan in$ucti!e element is positi!e. onse%uently, the net reacti!e po er supplie$ to capaciti!e element isnegati!e. In the past the opposite sign con!ention has also "een use$. /ith the sign con!ention as "ase,reacti!e po er is sai$ to "e pro$uce$#generate$ "y o!erexcite$ synchronous machines an$ capacitors,an$ consume$ or a"sor"e$ "y un$er excite$ synchronous machines, in$uctors, etc.

)eacti!e po er can "e consi$ere$ as a con!enient e!aluation %uantity, gi!ing information a"out the att-less current, hich greatly influences !oltages, acti!e losses.

1./ Sourc%" !nd "in)" o# R%!cti(% *o+%r

S.No. Sourc%" - 2%n%r!tion3 Sin)" 4 A5"or*tion31 *en. G!er excite$ *en. 9n$er excite$4 Transmission (ines - charging Transmission (ines - series reactance $rop3 Shunt apacitors Shunt )eactors6 Static Var ompensators =< Hgen mo$e> Static Var ompensators =< H a"sor"

mo$e>

8 Series apacitors = se> -

Synchronous on$enser o!er excite$ Synchronous on$enser un$er excite$

7 (oa$s - apaciti!e (oa$s - In$ucti!e

1.6 Po+%r tr!n"&i""ion in ! Tr!n"&i""ion in%

3

* M

VsM

Vr 5I

r

Sr

: C

Aig. 1.4 Simple Transmission System


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(etVs FSen$ing en$ !oltageVr F)ecei!ing en$ !oltageSr F )ecei!ing en$ complex po er Pr F )ecei!ing en$ acti!e po er


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V1 lagging V4 P is 4 14. Magnitu$es of V1 an$ V4 $o not $etermine the M/ flo $irection3. Though P1FP4,

< loss FI 4C F X V

Q P .

4

44 + =L>

0ence in or$er to minimise losses e ha!e to minimise the transfer of


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&Th FV5 J: C I : V J : C

KV

jQ P r r

F V J V XP

jV

XQ r r

+ = >

The !oltage rise term in phase ith V $epen$s on Varying the turns ratio of transformers "y G(T an$ =c> Varying shunt compensation.

Shunt compensation is $ra ing or in:ection of reacti!e po er at a no$e. )eactor a"sor"s reacti!e po eran$ so re$uces system !oltage. apacitor in:ects reacti!e po er an$ so increases system !oltage.

1.; S=If > p.u. F I f p.u. F

Th X

1 =11>

V5- FThe prefault !oltage in p.u. F 1.5 p.u.

C Th F The!inin impe$ance F Dri!ing point impe$ance of the net or+.The change in !oltage hen certain %uantity of reacti!e po er is supplie$ to the system is gi!en "y

. puS

QV

SC

=

/here< F hange in < in:ectionSsc FShort circuit capacityV F hange in !oltage in per unit

1.= R%!cti(% *o+%r - *


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1.> Po+%r tr!n"#%r co&*on%nt"Transformers, o!erhea$ lines an$ un$ergroun$ ca"les ma+e up the ma:or ' po er transfer componentsan$ are $iscusse$ in this su"section.

1.>.1 Tr!n"#or&%r"

Aigure 1.8 sho s a simple e%ui!alent circuit of a t o- in$ing transformer. The series reactance C is of main interest, usually lying ithin the range 5.58 to 5.18 p.u. "ase$ on the transformer po er rating, ithlo !alues for small an$ high !alues for large transformers. The resistance is usually negligi"le. The totalreacti!e po er losses $ue to the magneti?ing shunt reactance Cm of many small transformers ithin a$istri"ution system can, ho e!er, "e of some importance. The magneti?ing reacti!e po er may alsoincrease rapi$ly ith the !oltage le!el, $ue to core Saturation.1.>., O(%r P 5 or natural loa$ of an uncompensate$ line is a con!enient !alue for reference

purposes. It is gi!en approximately "y

MW x

bV P o

4= ------------------------------------------------=14>

hereV F !oltage, line-line +V " F susceptance mho#+m

x F reactance ohm#+m

7

Aig 1.8 &%ui!alent ircuit of Transformer


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' loss less line =a reasona"le approximation of an &0V line> transferring an acti!e po er P 5 an$ ithe%ual !oltages at the line en$s has reacti!e po er "alance. The reacti!e po er loss $ue to the linein$uctance is e%ual to the reacti!e po er generate$ "y the line capacitance.

Gperating !oltage+V

SI(M/

(ine chargingM!ar#+m

CGhm#+m

C#)

5.615

135445655855785

--

85135885

154455

--

5.585.165.1.54.3

5.655.655.655.655.335.355.4L

5.85.83

18135

Ta"le 1. Typical !alues of o!erhea$ line characteristics at 850?.

Ta"le 1 gi!es typical !alues of o!erhea$ line characteristics at 850?. 't 5 0? the SI( !alues are thesame hile the line charging, C an$ C#) !alues are 45 per cent higher. The SI( is usually much lo er than the thermal rating. Belo +V the line charging is usually negligi"le hile it is a significantsource of reacti!e po er for long lines of higher system !oltages.

Para$oxically, the series reactance is fairly in$epen$ent of the system !oltage, assuming a singlecon$uctor. The lo er !alues at 655 +V, 855 +V an$ 785 +V illustrate the effect of the necessary use of

"un$le con$uctors for these system !oltages. In reality there is a great sprea$ in the C#) !alues, for asystem !oltage un$er consi$eration, in particular at lo system !oltages. The figures are ho e!er,inclu$e$ in or$er to illustrate that the C#) ratio increases rapi$ly ith the system !oltage.1.>./. Und%r'round c!5 %"Ta"le 4 gi!es sample !alues of un$ergroun$ ca"le characteristics. The sprea$ in parameter !alues for a

system !oltage un$er consi$eration is !ery much larger than for o!erhea$ lines, $epen$ing on the ca"letype, si?e an$ con$uctor geometry an$ spacings. &xcept for lo !oltage ca"les, the SI( is usually muchlarger than the thermal rating. The line charging of polyethylene insulate$ ca"les, no "eing intro$uce$at e!er higher system !oltaes, is much lo er, e.g. 85 per cent of that of paper-insulate$ ca"les.

Gperating !oltage+V

SI(M/

(ine chargingM!ar#+m

CGhm#+m

C#)

5.615

135445655

-3

85515553455

-5.51

46

13

5.575.155.185.1L5.4

5.35.64

Ta"le.4 Sample !alues of un$ergroun$ ca"le characteristics at 85 0?. 5.6. 15 +V PV , 134,655+V paper-insulate$ ca"les.

L


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1.10 Lo!d"' great many loa$s consume not only acti!e "ut also reacti!e po er.The In$ustry ise po er factor is generally o"ser!e$ to "e as follo s

Some typical !alues of reacti!e po er consumption of in$i!i$ual loa$s are gi!en "elo

In$uction motors 5.8 to 1.1 +!ar#+/, at rate$ output. 9ncontrolle$ rectifiers 5.3 +!ar#+/. ontrolle$ rectifiers usually consume much more +!ar#+/ than uncontrolle$ ones an$ ith

$epen$ence on the rectifier $elay angle.

'rc furnaces aroun$ 1 +!ar#+/.Both controlle$ rectifiers an$ arc furnaces of steel mills ha!e a reacti!e po er consumption characteri?e$

"y a high a!erage !alue an$ fast !ariations. Purely resisti!e loa$s, li+e filament lamps an$ electricheaters, $o not, of course, consume reacti!e po er.

The synchronous motor is the only type of in$i!i$ual loa$,hich can pro$uce reacti!e po er. it consumes reacti!e po er hen un$er excite$ an$ pro$uces reacti!e po er hen

o!erexcite$. Synchronous motors are usually operate$o!erexcite$ an$ thus usually pro$uce reacti!e po er.In$i!i$ual loa$s may, of course, !ary ithin short or long timeranges. The composite loa$s of a po er system. &ach one

"eing the total loa$ of a certain area, usually !ary ith thetime of the $ay, the $ay of the ee+ an$ the season of the year an$ may also gro from year to year. The consumer $eman$for reacti!e po er !aries in a some hat similar ay to the$eman$ for acti!e po er. Aigure 1.7 illustrates ho the acti!e

Aig.1.7 &xamples of loa$

INDUSTR? POWER FACTOR

Textiles 5. 8#5.78hemical 5.78#5.L8

Machine shop 5.6 # 5. 8'rc /el$ing 5.38# 5.6'rc Aurnaces 5.7 # 5.

oreless in$uction furnaces an$ heaters 5.18#5.6ement plants 5.7L#5.L

*arment factories 5.38#5.Bre eries 5.78#5.LSteel Plants 5. # 5.L8

ollieries 5. 8#5.L8Bric+ /or+s 5. # 5.78

ol$ Storage 5.7 # 5.LAoun$ries 5.8 # 5.7Plastic moul$ing plants 5. # 5.78Printing 5.88#5.7 5.3 # 5.78


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an$ the reacti!e po er supplie$ from a transmission su"station into a loa$ area, ith mixe$ in$ustrialan$ $omestic loa$s, may !ary $uring a Sun$ay an$ a Mon$ay.The resultant acti!e po er $eman$ of a po er system !aries roughly as the !ariation of total toa$. Theresultant reacti!e po er $eman$ may !ary consi$era"ly more $ue to the changing series reacti!e po er losses in the net or+s.

1.11. R% !tion"

The Phasor $iagram of Aigure 1.L, for a case ith lagging po er factor, sho s that it can "e approximately expresse$ "y the follo ing e%uations

VF)I cos JCI sin --------------- ----- ------------------=16> VF =)PJC # V4 --------------------------------------- =18>

The accuracy of the e%uations =16> an$ =18> is "etter, the less the !oltage-angle $ifference is. Thee%uations are usually sufficiently accurate for calculations concerning a single lin+ ith lagging po er factor. The e%uations are less accurate an$ shoul$ not "e use$ in calculations for -lea$ing po er factor.Precise calculations concerning a complete net or+ are, no a$ays, performe$ "y means of computer

po er flo programs.

The e%uation =18> is, ho e!er, generally useful for %ualitati!e $iscussions of !oltage !ersus reacti!e po er. Aor transformers, ) can al ays "e $isregar$e$. Aor transmission =not $istri"ution> lines an$ca"les. C is usually much larger than ). Aor all these many lin+s, here C is -much larger than ), there

ill e!i$ently "e a much greater influence on V per +!ar of reacti!e po er than per +/ of acti!e po er transmitte$.

/hen po er is supplie$ through a single lin+, Aigure 1.L, assuming V1 constant, V4 !aries ith changesin P an$


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pro$uction "y means of a shunt capacitor, effects !oltage rise. The e%uation =18> an$ Aigure 1.L shoho shunt compensation influences the !oltage. The !oltage-change $irections mentione$ arise "ecausethe net or+ e%ui!alent impe$ance has an in$ucti!e character at the fun$amental fre%uency. The shuntcompensation may "e fixe$, s itcha"le in steps or continuously controlla"le. 'roun$ the nominal!oltage, the !oltage change V, hen the shunt compensation is change$ in step, is approximatelyexpresse$ "y

V F scS Q

------------------=1 >

/here


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In PSS TP('N, PV cur!es are pro!i$e$ as a $istinct 'nalytical &ngine. 's such it is pro!i$e$ ith po erful features

&asy setup omprehensi!e results '$apti!e step si?e. ou $efine a range for the transfer increment, an$ PSS TP('N ill select a

step si?e hich ill maintain the accuracy of the simulation at minimum loss of resolution. Non-$i!ergent po er flo . The last point on the cur!e is al ays accurately $etermine$ "y a

special algorithm hich can i$entify $i!ergence.

1.1/ N%%d to o*ti&i@% r%!cti(% *o+%r r%"ourc%"The nee$ to optimi?e reacti!e po er sources is essential to

apacity utili?ation of existing transmission facilities for po er transfer.Maximi?e the existing reacti!e po er resources to minimi?e in!estment in a$$itionalfacilities.Minimi?e transmission lossesImpro!e system securityMaintain po er supply %uality "y maintaining "us !oltages close to nominal !alue.

1.16. R%&!r)"'cti!e po er must, of course, "e transmitte$ from the generators to the loa$s. )eacti!e po er nee$ not,an$ ith regar$ to !oltage $ifferences, losses an$ thermal loa$ing as $iscusse$ in the prece$ingsu"sections, shoul$ not "e unnecessarily transferre$. I$eally, a reacti!e po er "alance shoul$ "e effecte$

ithin each region of a po er system, ithin each transmission system an$ ithin each $istri"ution

system. In practice, ho e!er, this principle is not al ays follo e$ for one reason or another. The su":ectof reacti!e po er compensation is easy to un$erstan$ if e consi$er a single lin+ of a po er system, "ut%uite complex hen e consi$er an entire po er system ith its $ifferent con$itions an$ "eha!iors.

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CHAPTER - , REACTIVE POWER SOURCES AND SIN S

,.0 IntroductionSources of reacti!e po er are

*enerating units Synchronous con$enser Gn-loa$ tap changers an$ phase-shifting transformers. apacitors an$ reactors Static compensators.

Po+%r "$"t%& co&*on%nt c

pro$uces reacti!e po er hen o!erexcite$. The reacti!e po er output is continuously controlla"lethrough !arying the excitation current. The allo a"le reacti!e po er a"sorption or pro$uction is$epen$ent on the acti!e po er output as illustrate$ "y the po er charts of Aigures 4.1 an$ 4.4. Aor short-term operation the thermal limits are usually allo e$ to "e o!erri$$en.

The step-response time in !oltage control is from se!eral tenths of a secon$ an$ up ar$s. The rate$ po er factor of generators usually lies ithin the range 5.L5 to 5. 8. *enerators installe$ remotely fromloa$ centers usually ha!e a high rate$ po er factor this is often the case ith large hy$ro-tur"ine

generators. *enerators installe$ close to loa$ centers usually ha!e a lo er rate$ po er factor. In somecases of large steam-tur"ine generators the rate$ po er factor may ha!e "een selecte$ at the lo er en$ of the a"o!e range in or$er to ensure reacti!e po er reser!e for se!ere force$ outage con$itions of the

po er system.

Aig 4.1 Typical Po er chart for large steam tur"ine an$ gas tur"ine generatorshere

a O Tur"ine po er limit " O Stator in$ing thermal limitc O Aiel$ in$ing thermal limit$ O Stea$y-slate sta"ility limit ith proper 'V) e O 'ssume$ inter!ention cur!e of un$er excitation limiter

13


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Aig.4.4. Typical po er chart for large hy$ro-tur"ine generators =salient-pole machines>

(arge generators are usually connecte$ $irect to transmission net or+s !ia step-up transformers. Theterminal !oltage of a large generator is usually allo e$ to "e controlle$ ithin a 8E range aroun$ thenominal !oltage, at rate$ loa$. In most countries the generator step-up transformers are usually note%uippe$ ith on-loa$ tap changers.

EBcit!tion Contro The MV') output of a generator is $epen$ent on its excitation. The MV') isgenerate$ $uring o!er excitation an$ is a"sor"e$ $uring un$er excitation. The rotor current $epen$s onthe excitation. The rotor in$ing temperature, the air gap temperature an$ the machine temperatureincrease $uring o!er excitation. The in$ing temperature is limite$ to a"out 5 o $uring normalloa$ing. It increases to 155 H 158 o $uring o!er loa$ing. The machine hich is alrea$y o!er heate$ $ueto MV') generation can not ta+e M/ loa$ to its full capacity. 0ence M/ loa$ is to "e compromise$

hen the unit is excite$ "eyon$ its normal limits.

/hen the unit generates MV') an$ supplies to the system, the system !oltage profile aroun$ thegenerating station increases. This increase in !oltage is more in first neigh"ourhoo$. The loa$ en$

!oltages hich are "eyon$, say secon$ neigh"ourhoo$ ill not get effecte$ "ecause of this unitexcitation. 0ence the influence of a unit on !oltage profile in the system is local in nature. The loa$ en$!oltages can not "e controlle$ "y the generating units.

0o e!er $epen$ing on the capa"ility cur!e of the generating unit an$ as long as margin is a!aila"le inthe unit, it can "e use$ to control the system !oltages in its !icinity.

The change in the !oltage V in the first neigh"ourhoo$ of the generating station $epen$s on the relationV F


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Shunt reactors are necessarily installe$ to suppress high !oltage $uring light loa$ con$itions. Aor 655+V an$ 90V lines, shunt reactors are $irectly connecte$ on line. This is for the purpose of compensating lea$ing charging MV') release$ "y the line. Shunt reactors are also connecte$ on tertiary$elta in$ings of autotransformers so that these can "e s itche$ on $uring light loa$ perio$s.

R%!ctor O*%r!tion The shunt reactor is a coil connecte$ to the system !oltage an$ groun$e$ at theother en$. It $ra s the magneti?ing current, hich is purely in$ucti!e, from the system an$ hence formsan in$ucti!e loa$ at the point of connection. 0ence the reactor a"sor"s reacti!e po er from the systemas long as it is connecte$ to the system. 0ence it is complimentary to a capacitor "an+ in its function.The re$uction in !oltage at the point of connection is gi!en "y V F The "us reactor, hich is connecte$ to the "us through a circuit "rea+er an$ hence can "es itche$ as an$ hen re%uire$.

B> The line reactor hich is connecte$ to the line through only an isolator an$ hence can "eremo!e$ from the system only hen the line is s itche$ off.

The functions of "oth "us reactor as ell as line reactor are same. They a"sor" the reacti!e po er fromthe system $epen$ing upon their capacity.The "us reactors are s itcha"le an$ hence are cut-in hene!er the system !oltage is higher an$ can "ecut-off from the system hene!er the system !oltage re$uces.

The line reactors are permanently connecte$ to the lines an$ hence the system. Their role is to

a> )e$uce the effect of line charging "> Pro!i$e a least impe$ance path for the s itching o!er !oltages generate$ in the system $ue to

in$ucti!e loa$ currents@ s itching. The s itching o!er !oltages are of po er fre%uency an$e%ual to 1.8 to 4.8 p.u. in magnitu$e.

c> /hen the &0V lines ha!e single phase s itching facility an$ auto reclose protection scheme isimplemente$, the a"normal !oltages $e!elope$ across the circuit "rea+er can "e containe$ only

ith a line reactor on the line si$e.$> The line reactors pro!i$e a least impe$ance path for lo fre%uency =po er fre%uency> s itching

o!er !oltages. 0ence they act as surge $i!erters for po er fre%uency o!er !oltages. Thelightning o!er !oltages cannot pass through the line reactor "ecause of their high fre%uency.

,./ S are to "e foun$ in 0VD terminal stations.Shunt capacitors in use range in si?e from a single unit rate$ a fe +!ar at lo !oltage up to a "an+ of units, rate$ hun$re$s of M!ar.

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C!*!citor O*%r!tion The capacitor "an+s are reacti!e po er sources. They pro$uce reacti!e po er e%ual to their rating hen connecte$ to the "us. In or$er to +eep the insulation costs less, they areconnecte$ to the system at $istri"ution !oltage le!els, e.g. 5.6 +V, 11 +V, 33 +V etc.The output of a capacitor "an+ is < c F V 4 c

/here < c F output in MV') V F the system !oltage in +.V.

F in fara$s

0ence the output is proportional to the s%uare of the !oltage. If the system !oltage to hich the capacitor "an+ is connecte$ re$uces to 5. p.u. the MV') generate$ "y the capacitor re$uces to 5.L1 p.u. 0encethe performance of a capacitor "an+ ill "e poor un$er lo !oltage con$itions, at hich time it isre%uire$ most.

The influence of a capacitor "an+ on the system !oltage is again local li+e in case of a generator. It is

most pre $ominent at the "us to hich it is connecte$. Its effect gets re$uce$ as e go to nextneigh"ourhoo$. The change in !oltage at the point of connection is go!erne$ "y the relation V F

"uc+ing tap at, say -8, for e.g. at 5. pu the 0V "us ill get a net reacti!e po er in-flo of say 455

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MV') through its &0V transmission net or+. The same reacti!e po er flo s to ar$s the (V "us. The(V "us !oltage no increases. This is illustrate$ in Aig 4.3.

If the transformer tap is raise$ to say 8, it is no "oosting the 0V !oltage to say, 1.54 pu. No the

reacti!e po er in-flo re$uces to 0V "us, to say 45 MV'). This re$uce$ MV') is flo ing to (V "us.0ence the (V "us !oltage re$uces. This is illustrate$ in Aig 4.6. 0ence the transformer tap only altersthe num"er of turns in the 0V in$ing there "y altering the 0V !oltage. If this 0V !oltage is less thanthe neigh"ourhoo$ !oltage it recei!es MV'), if it is more, then it pumps MV') to its neigh"ourhoo$.The (V "us !oltage is maintaine$ only as a conse%uence of MV') inflo or outflo to it from the 0V

"us.

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Both in$oor an$ out$oor installations exist. Synchronous compensators "elo , say, 85 MV' are usuallyair-coole$, hile those a"o!e are usually hy$rogen-coole$. Mo$ern synchronous compensators areusually e%uippe$ ith a fast excitation system ith a potential-source rectifier exciter. Various startingmetho$s are use$ the mo$ern one is in!erter starting.

The si?e of a synchronous compensator is referre$ to the ontinuous MV' rating far the generation of reacti!e po er. In the generating mo$e of operation it usually has a rather high short-time o!erloa$capa"ility. The a"sorption capa"ility is normally of the or$er of 5 per cent of the MV' rating, hichmeans that the control range is usually 1 5 per cent of the MV' rating. The reacti!e po er output iscontinuously controlla"le. The step-response time ith close$-loop !oltage control is from a fe tenths of a secon$, an$ up. The losses of hy$rogen-coole$ synchronous compensators are of the or$er of 15 /#+!ar at rate$ output. The losses of small air-coole$ machines are of the or$er of 45 /#+!ar at rate$ output.

In recent years the synchronous compensator has "een practically rule$ out "y the SV , in the case of ne installations, $ue to "enefits in cost performance an$ relia"ility of the latter. Gne exception is 0VDin!erter stations, in cases here the short-circuit capacity has to "e increase$. The synchronouscompensators can $o this, "ut not the SV .

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capacitors

. They a$$ to the short circuit current of a system

an$ therefore increase the si?e of =11+V etc.> "rea+ers in the neigh"our-hoo$.

The capacitors $o not increase the short

circuit capacity of the system, as their output is proportional to V 4

15. This is a rotating $e!ice. 0ence the GQM pro"lems are more

These are static an$ simple $e!ices. 0enceGQM pro"lems are negligi"le

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4.8.1.4. R%!cti(% Po+%r Contro #or Indu"tri! o!d" SV can "e use$ to compensate the reacti!e po er to the loa$s, li+e furnaces, roller mills. The loa$

po er factor is measure$ from !oltage an$ current signals, compare$ ith a reference signal. &rror signalcontrols the firing angle of T ) or s itching of TS to generate the re%uire$ reacti!e po er.

4.8.1.3. Lo!d ! !ncin' #or un5! !nc%d "$"t%&" 9n"alance$ loa$s are create$ in traction loa$s, electric arc furnaces. The SV regulator consists of separate reacti!e po er measurement control an$ firing pulse generation circuits for each phase to ena"lein$i!i$ual phase control. The firing angle for each phase ill "e $ifferent $epen$ing on its loa$con$itions thus effecting un"alance$ control

,.7.1.6. F ic)%r contro #or % %ctric !rc #urn!c%"'rc furnaces use$ to melt scrap in steel mills represent highly un"alance$ an$ rapi$ly fluctuating loa$s.They pro$uce the follo ing types of $istur"ances.- )api$ open#short circuit con$itions $uring arc initiation in the furnace- /i$e an$ rapi$ current fluctuations ith un"alance "et een phases- Aluctuations in the reacti!e current resulting in !oltage !ariation hich causes flic+er.These loa$s cause flic+er in lamps, interference in TV reception an$ other electronic loa$sTo control flic+er, furnace !oltage an$ current are measure$ an$ reacti!e po er re%uirement calculate$.

ontrol of firing angle is $one "y open loop to get !ery fast response.

The follo ing su"sections 4.8.4 to 4.8.8 apply in the first place to transmission system SV s. In$ustrialsystem SV s in con:unction ith arc furnaces usually $iffer in some respects No SV transformer, fixe$capacitor =filter>#Thyristor-controlle$ reactor main circuit arrangement only, open-loop reacti!e-po er compensation control instea$ of close$-loop !oltage control.

Princi* %" o# o*%r!tionT o types of Thyristor-controlle$ elements are use$ in SV s

1. TS O Thyristor-s itche$ capacitor 4. T ) O Thyristor- controlle$ reactor

Arom a po er-fre%uency point of !ie they can "oth "e consi$ere$ as a !aria"le reactance, capaciti!e or in$ucti!e, respecti!ely.

,.7., T henthe "ranch !oltage has its maximum !alue an$ the same polarity as the capacitor !oltage. This ensuresthat the s itching on ta+es place ith practically no transient.S itching off a capacitor is accomplishe$ "y suppression of the firing pulses to the Thyristor so that theThyristor ill "loc+ as soon as the current "ecomes ?ero =t4>. In principle, the capacitor ill then remaincharge$ to the positi!e or negati!e pea+ !oltage an$ "e prepare$ for a ne s itching on.

The TS is characteri?e$ "y Step ise control

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'!erage one half- ycle =maximum one cycle> $elay for executing a comman$ from the regulator,as seen for a single phase

S itching transients are negligi"le.

No generation of harmonics

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Aig. 4. sho s the "asic $iagram of a T ). The "ranch sho n inclu$es an in$uctor ( an$ a "i-$irectional Thyristor s itch T . The current an$ there "y also the po er fre%uency component of thecurrent are controlle$ "y $elaying the closing of the thyristor s itch ith respect to the natural ?ero

passages.

T

,.7.6 St!tic V!r Co&*%n"!tor

It is configure$ as A J T ) or TS J T ).The T ) an$ TS are connecte$ in $elta for trapping harmonic currents of ?ero se%uence =3 r$, th etc.>Aig 4.L illustrates the operating performance of the compensator accor$ing to fig 4.7 =">Most transmission applications re%uire close$-loop "us !oltage control "y an 'V). Aor a rapi$ change of the control or$er the change from full lagging current to full lea$ing current ta+es

place ithin a maximum of one cycle of the net or+ !oltage.

44

Aig 4.7 =a> SV of the A # T ) type ="> SV of the TS # T ) type


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Aig 4.L Gperating principle of a SV of type TS J T ) for a slo change of control or$er

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H!r&onic" in SVC' TS $oes not pro$uce harmonic currents, "ut a T ) $oes. 'll SV s ith continuous reacti!e po er control inclu$e one T ) or more thus they pro$uce harmonic currents. The harmonics of ?ero se%uencecharacter =eg. 3 r$, th etc.> are eliminate$ "y some $elta connection. The 8 th an$ 7 th harmonics are in somecases eliminate$ "y 14 pulse arrangement. 's a last resort a filter is inclu$e$. The allo a"le amount of harmonic currents into the Po er System expresse$ in terms of !oltage $istortion at the point of SV

connection are The allo e$ !oltage $istortion cause$ "y a single harmonic current F1.5E The allo e$ total !oltage $istortion cause$ "y all harmonic currentsF1.8E

D$n!&ic P%r#or&!nc%The small-signal performance of an SV ith close$-loop !oltage control may "e characteri?e$ "y itsstep-response time. It is $efine$ here as the time re%uire$ to achie!e 5E of the calle$-for change in!oltage, for a step change in the reference !oltage. The step change must "e small enough for the SVnot to reach a limit. The step-response time $epen$s on the po er-system e%ui!alent impe$ance at theSV point of connection. It is typically less than a fe cycles of the po er-fre%uency !oltage at theminimum short-circuit MV' le!el consi$ere$ hen choosing the !oltage regulator gain.If there is a ris+ that the short-circuit MV' le!el can "e e!en lo er an$ there"y cause SV !oltagecontrol insta"ility, this can "e cure$ "y a gain super!isor automatically re$ucing the gain in case of insta"ility.

If there are fre%uent i$e !ariations in the short-circuit MV' le!el an$ if it is :u$ge$ important to get asfast small-signal !oltage control as possi"le for all operating con$itions, this can "e achie!e$ "y a gainoptimi?er, automatically an$ repeate$ly a$:usting the gain up or $o n !ersus the short-circuit MV' le!el.

The a"o!e $iscussion is primarily referre$ to continuously acting SV s, "ut $oes in principle also applyto $iscrete acting SV s =SV s of TS , TS) or TS #TS) type in a "inary arrangement>.The large-signal performance is essentially characteri?e$ "y the actuating time of the SV triggering an$main circuits only. Aor a large !oltage $e!iation, the SV response time is typically of the or$er of one

po er-fre%uency cycle, consi$ering the po er-fre%uency !oltage component only.

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Fi' . 4.11 Illustrates the $ynamic performance of an SV for a large step change in the reference !oltageIT, I an$ IB mean total, capacitor an$ reactor current respecti!ely.

,.8 S%ri%" C!*!citorIt is a "an+ of capacitor units inserte$ in a line for the purpose of canceling a part of the line in$ucti!ereactance an$ so re$ucing the transfer impe$ance.The reacti!e po er generate$ in a series capacitor is proportional to I ( 4 an$ so increases ith increasingtransmitte$ po er an$ thus influences the reacti!e po er "alance of the system.The typical uses are

To increase the transmission loa$ing capa"ility as $etermine$ "y Transient sta"ility limits To o"tain a $esire$ stea$y state acti!e po er $i!ision among parallel circuits in or$er to re$uce

o!erall losses To control transmission !oltages an$ reacti!e po er "alance To pre!ent !oltage collapse in hea!ily loa$e$ systems To $amp the po er oscillations in association ith Thyristor control

The $egree of compensation is 45 to 75E of line in$ucti!e reactance. The series capacitor = se> can "elocate$ at the en$s of a long Transmission line or in a s itching station in the mi$$le of it.

onsi$erations are !oltage profiles, efficiency of compensation, losses, fault currents, o!er !oltages, proximity to atten$e$ stations etc.

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its reactance are constant.

4. The !oltage across the shunt capacitor is

su"stantially constant as it is e%ual to thesystem !oltage an$ generally ithincertain limits of say 5. to 1.1 pu.

The !oltage across the series capacitor

changes instantaneously as it $epen$s onthe loa$ current through it, hich !ariesfrom 5 to I (max

3. The po er $e!elope$ across the shuntcapacitor is

sh ;V') FCshcSH x

vv

x

v 4. =

The po er $e!elope$ across the seriescapacitor is

se ;V') F =I ( C se> =I( >F I( 4 C se

6. The shunt capacitor supplies laggingreacti!e po er to the system. 0ence$irectly compensating the lagging ;V') loa$. It impro!es the loa$ po er factor su"stantially. 0ence its main purpose is tocompensate the loa$ Po er factor

The series capacitor re$uces the linereactance as it intro$uces lea$ingreactance in series of the line. Thus seriescapacitor at rate$ fre%uency ompensatesfor the $rop, through in$ucti!e reactanceof the fee$er. 0ence it is use$ to increasethe line transmission capacity.

8. The si?e an$ capacity of shunt capacitor isgenerally higher for the same !oltageregulation

The si?e an$ capacity of a series capacitor is relati!ely lesser for the same !oltageregulation

. Not suita"le for transient !oltage $ropscause$ "y say, fre%uent motor starting,electric el$ing etc.

The !oltage regulation $ue to seriescapacitor is proportional to the I ( 4 hence itmeets the re%uirements of transient!oltage changes

7. Performance is $epen$ent on terminal!oltage. 0ence not effecti!e in fluctuating!oltage con$itions.

The performance $oes not $epen$ on thesystem !oltage !ariations. But $epen$son system loa$ current. 0ence gi!es fulloutput un$er lo !oltage an$ hea!ilyloa$e$ con$itions

L. The shunt capacitor nee$ not "e on thesource si$e. But closer to the loa$ point

The series capacitor shoul$ al ays "e onthe source si$e of the loa$.

. The rating is "ase$ on;V') sh F ;/=Tan 1 - Tan 4> here1 is the po er factor angle "eforecorrection, 4 is the pf angle after correction

The rating is "ase$ on percentagecompensation of the line reactance.*enerally C se F 5.3 to 5.6 of Cline &x' 445;V, 5.6 #+m, 155+m line, 65E, C(F 5.6 C 155 F 65 , Ccse F 5.6x 65 F 1 F 1#4f se se F

F F x

x 455

1I316

151 I

15. The Aerranti effect is aggra!ate$ "y shunt The Aerranti effect is re$uce$ "y the series

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compensation capacitor

11. Po er transferre$ through a line

PF Sin X V V r s

ith shunt capacitor, Vr increases Pincreases

/ith se, Vr increases an$ C $ecreases

hence P increases much more.

14. The shunt compensation $oes not re%uirespecial protection arrangements as theterminal !oltage of the capacitor "an+ fallsun$er fault con$itions

The !oltage across series capacitor a"normally rises $ue to flo of faultcurrent through it. 0ence it re%uiresspecial protection schemes.

The fig. 4.14 Sho s the "ypass arrangement series capacitor = se> in case of faults as large !oltage

$e!elops across the series capacitor. But the transient sta"ility arrants reinsertion of se into the systemat the earliest. This is achie!e$ "y the Rinc Gxi$e =Rno> !aristor. It pro!i$es instantaneous capacitor reinsertion after fault clearing. ' triggere$ spar+ gap is pro!i$e$ to ta+e care of excess energy a"sor"e$

"y Rno. Damping circuit =D> limits the $ischarge current.Rno arrestor is highly non linear. It is connecte$ across the series capacitor in a$$ition to the triggere$gap an$ "y pass s itch. The !aristor clamps the capacitor !oltage "elo its short time o!er !oltagerating $uring the fault. The re-insertion is almost instantaneous. Thus "oth capacitor protection an$system sta"ility aspects are ta+en care of.

S%ri%" C!*!citor in r!di! di"tri5ution "$"t%&"' Series apacitor is "ecoming popular in ra$ial $istri"ution systems "ecause

se is a cost effecti!e $e!ice of re$ucing !oltage $rops cause$ "y stea$y loa$s on a 11 or 33 ;V ra$ial line ith loa$ Po er factor of say 5.7 to 5.

To ta+e care of starting of a large motor an$ conse%uential !oltage fluctuations To $ecrease line losses $ue to the lo er current To increase loa$ a"ility of the fee$er Simple an$ relia"le "ypass systems are a!aila"le '$!ance$ resonance $etectors are a!aila"le.

47

Aig.4.14 Series apacitor ith Rinc-oxi$e !aristor "y-pass system.

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)ef 1> Po er capacitor han$ "oo+ -T (onglan$, T / 0unt, / ' Brec+nell Butter orths H 1 L6

4> )eacti!e Po er ompensation- Tore Peterson, 'BB Po er systems, S/&D&N H 1 3

3> Procee$ings of Seminar on 'P' ITG)S $uring 1L H 1 Uanuary 4551.- ' BIP an$ MP&B pu"lication H 4551.

2 rpm sources and sinks mnm

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