Lecture 11 Pp

8/9/2019 Lecture 11 Pp

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Lecture contents

Discrimination of fault


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Discrimination with time andCurrent

Each of the two methods descried so farhas a fundamental disadvanta!e

"n the case of discrimination y time alone#

the disadvanta!e is due to the fact that themore severe faults are cleared in thelon!est o$eratin! time

%n the other hand# discrimination ycurrent can e a$$lied only where there isa$$reciale im$edance etween the twocircuit rea&ers concerned


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"t is ecause of the limitations im$osed ythe inde$endent use of either time orcurrent co-ordination that the inverse time

overcurrent relay characteristic hasevolved

'ith this characteristic# the time ofo$eration is inversely $ro$ortional to thefault current level and the actualcharacteristic is a function of oth (time)and *current* settin!s.


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The advantage of this method of relay Co-

ordination may be best illustrated by the

system shown in (Fig.a)

In order to carry out a system analysis, before a

relay co-ordination study of the system shown

in (Fig. a), it is necessary to refer all the system

impedances to a common base and thus, using! "#$ as the reference base, we have%


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Assi!nment + Solution

In order to carry out a system analysis, before a relay co-ordinationstudy of the system shown in (Fig. ), it is necessary to refer all thesystem impedances to a common base and thus, using 10 M! asthe reference base, we ha"e# $M! transformer percentageimpedance on 10M! base%&' (10$) %1&.*

11 + cable between B and A percentage impedance on10 M!

base% (0.0$ ' 100 ' 10) 11% 0.*

11 + cable between / and percentage impedance on 10 M!base

% (0.$ ' 100 '10) 11 %1.2 *

0 M! transformer percentage impedance on 10 M! base

%. ' 10 0 %&. *

1 + o"erhead line percentage impedance on10 M! base

% (3.4100410) 1 %0.3*

1 + source percentage impedance on 10 M! base % (100 410) 00 %0.*


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,he !ra$h in i!./ illustrates the use of *discrimination curves*# which are an im$ortant aid tosatisfactory $rotection co-ordination

"n this e0am$le# a volta!e ase of .&2 has eenchosen and the 3rst curve $lotted is that of the455 A fuse# which is assumed to $rotect the lar!estout!oin! .&2 circuit

%nce the o$eratin! characteristic of the hi!hest

rated .&2 fuse has een $lotted# the !radin! ofthe over current relays at the various su-stationsof the radial system is carried out as follows6


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5ubstation

C, ratio 475+7A 8elay over current characteristicassumed to e e0tremely inverse# as for the ty$eCD9 1: relay. ,his relay must discriminate with the

455A fuse at fault levels u$ to615 0 155/ +1;.7


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5ubstation /

C, ratio 755+7A 8elay over current characteristic assumedto e e0tremely inverse# as for the ty$e CD9 1: relay. ,hisrelay must discriminate with the relay in sustation

B at fault levels u$ to6

15 155/ + 1.=> 5 A at 11 &2

,he o$eratin! characteristics of the CD9 1: relay showthat at a $lu! settin! of 155# that is# 755 A and =.74M2A at 11 &2# and at a time multi$lier settin! of 5.;#suitale discrimination with the relay at sustation isachieved.


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Sustation D

C, ratio 175+1A 8elay over current characteristicassumed to e e0tremely inverse# as for the ty$e CD9 1:relay

,his relay must discriminate with the relay in sustationC at fault levels u$ to

15 155/ + ;.7 < 5.? < 5.4=/ @ 14 M2A

,hat is# 41#755 A at .&2 or 7> A at 14 &2. ,heo$eratin! characteristics of the CD9 1: relay show thatat a $lu! settin! of 155# that is# 175 A and :.4 M2A at14 &2 and at a time multi$lier settin! of 5.47# suitalediscrimination with the relay at sustation C is achieved.


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5ubstation 6

C, ratio 755+1 A 8elay over currentcharacteristic assumed to ee0tremely inverse# as for the ty$eCD9 1: relay

,his relay must discriminate with the

relay in sustation D at fault levelsu$ to6

15 x 155/ + 5.?


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,o 3nalie the co-ordination study itis instructive to assess the avera!eo$eratin! time for each e0tremely

inverse over current relay at itsma0imum and minimum fault levels#and to com$are these with the

o$eratin! time for the de3nite timeover current relay


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7o 8nali9e the co-ordinationstudy it is instructi"e to assessthe a"erage operating time for

each e4tremely in"erse o"ercurrent relay at its ma4imum andminimum fault le"els, and to

compare these with theoperating time for the de8nitetime o"er current relay.


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%vercurrent relay 9eneralPrinci$le/

,he overcurrent ty$e of current-alance relayhas one overcurrent element arran!ed to$roduce torFue in o$$osition to anotherovercurrent element# oth elements actin! onthe same movin! structure

i!ure shows schematically an electroma!netic-attraction Galanced-eamH ty$e of structure

Another commonly used structure is aninduction-ty$e relay havin! two overcurrentelements actin! in o$$osition on a rotor.


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%$eratin! Characteristics


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,he eIect of the control s$rin! is toreFuire a certain minimum value of"1 for $ic&u$ when "4 is ero# ut the

s$rin! eIect ecomes less and lessnoticeale at the hi!her values ofcurrent

,he relay will $ic& u$ for ratios of "1to "4 re$resented y $oints aove theo$eratin! characteristic


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Such an o$eratin! characteristic is s$eci3ed ye0$ressin! in $ercent the ratio of "1 to "4 reFuiredfor $ic&u$ when the relay is o$eratin! on thestrai!ht $art of the characteristic# and y !ivin! the

minimum $ic&u$ value of "1 when "4 is ero "1 is called the Go$eratin!H current since it

$roduces $ositive## or $ic&u$# torFueJ "4 is calledthe Grestrainin!H current.

y $ro$ortionin! the numer of turns on theo$eratin! and the restrainin! coils# one can otainany desired G$ercent slo$eH as it is sometimescalled.


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Should it e desired to close an GaHcontact circuit when either of twocurrents e0ceeds the other y a

!iven $ercenta!e# two elements areused# as illustrated schematically ini!.

or some a$$lications# the contactsof the two elements may earran!ed to tri$ diIerent circuit

rea&ers# de$endin! on which


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y these means# the currents in the diIerent$hases of a circuit# in diIerent circuitranches of the same $hase# or etweencorres$ondin! $hases of diIerent circuits# cane com$ared

'hen a$$lied etween circuits where the ratioof one of the currents to the other never

e0ceeds a certain amount e0ce$t when ashort circuit occurs in one of the circuits# acurrent-alance relay $rovides inherentlyselective $rotection


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Althou!h the torFue eFuations werewritten on the assum$tion that the$hase an!le etween the two

alanced Fuantities had no eIect#the characteristics of such relaysmay e somewhat aIected y the

$hase an!le "n other words# the actual torFue

relation may e6)cos(&'

&

&&

&

+= IIKIKIKT


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where the eIect of the control s$rin!is ne!lected# and where and arede3ned as for directional relays

,he constant K is small# the$roduction of directional torFue ythe interaction etween the induced

currents and stray u0es of the twoelements ein! incidental and often$ur$osely minimied y desi!n


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8elays are availale havin! hi!h-s$eed characteristics or inverse-timecharacteristics with or without an

adNustale time dial A ty$ical set of time curves is shown

in i!.# where the eIect of diIerent

values of restrainin! currents on thesha$e of the time curve is shown forone time adNustment


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Such curves cannot e $lotted on a multi$leasis ecause the $ic&u$ is diIerent for eachvalue of restrainin! current

"t will e noted that each curve is asym$toticto the $ic&u$ current for the !iven value ofrestrainin! current

Oi!h-s$eed relays may o$erate undesiraly ontransient unalances if the $ercent slo$e is toonearly 155 and for this reason such relaysmay reFuire hi!her $ercent-slo$echaracteristics than inverse-time relays.


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Directional ,y$e

,he directional ty$e of current-alancerelay uses a current-current directionalelement in which the $olariin! Fuantity is

the vector diIerence of two currents# andthe actuatin! Fuantity is the vector sum ofthe two currents

"f we assume that the currents are in

$hase# and ne!lect the eIect of the controls$rin!# the torFue is6

,@K1"1


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"f the two currents are 1>5 out of $hase# thedirection of torFue for a !iven unalancewill e the same as when the currents are in

$hase# as can e seen y chan!in! the si!nof either current in the torFue eFuation

,his ty$e of relay may have doule-throwcontacts oth of which are normally o$en#

the control s$rin! ein! arran!ed to$roduce restraint a!ainst movement ineither direction from the mid $osition


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DiIerential 8elay

DiIerential relays ta&e a variety of forms# de$endin!on the eFui$ment they $rotect

,he de3nition of such a relay is one that o$erateswhen the vector diIerence of two or more similar

electrical Fuantities e0ceeds a $redeterminedamount

"t will e seen later that almost any ty$e of relay#when connected in a certain way# can e made to

o$erate as a diIerential relay "n other words# it is not so much the relay

construction as the way the relay is connected in acircuit that ma&es it a diIerential relay


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e0t Lecture

Directional relay continued

Presentation onassi!nment+discussion

Lecture 11 Pp

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