LECT_1_E_1435_POWER_SYSTEM_PROTECTION.ppt

power system

1. Prof. Mahmoud El Bahy
Professor of High Voltage Engineereing
LECTUERE1
E 1435 POWER SYSTEM PROTECTION
BENHA UNIVERSITY
FACULTY OF ENGINEERING AT BENHA
DEPARTMENT OF ELECTRICAL ENGINEERING

2. PRINCIPLES OF PROTECTION
BY RELAYS

3. 1. INTRODUCTION
The power system is divided into protection
zones defined by the equipment and the
available circuit breakers. Six categories of
protection zones are possible in each power
system:
(1) generators and generator–transformer
units, (2) transformers,
(3) buses,
(4) lines (transmission, subtransmission, and
distribution),
(5) utilization equipment (motors, static loads,
or other), and
(6) capacitor or reactor banks (when separately
protected).

4. 2-Zones and General Principles of Protection
Figure 1 (a) : Primary relay protection zones.

5. Figure 1 (b) : Primary
relay protection zones.

6. InFigure1,theprotectionzonesofasimplepowersystemare
shown.Eachzoneprotectsasingleelementofthepowersystem.
Theprotectionzonesoverlaparoundcircuitbreakers.The
purposeistoprotectallsectionsofthepowersystem.Typical
zonesofprotectionwithtransmissionlines,buses,and
transformers,eachresideinitsownzone.Closedzonesinwhich
allpowerapparatusenteringthezoneismonitored.
ProtectionZones
General Principles of Protection
Primaryrelaysarerelayswithinagivenprotectionzone
thatshouldoperateforprescribedabnormalitieswithin
thatzone.InFigure1,forexample,considerafaultat
lineJK.Forthiscondition,relayssupervisingbreakersJ
andKshouldtripbeforeanyothersandtheserelaysare
calledprimaryrelays.
Protection zones (primary protection zones) are regions
of primary sensitivity. Figure 1 shows a small segment of
a power system with protection zones enclosed by
dashed lines.

7. Back-upprotectionisprovidedtoensurethatthefaultedelement
isdisconnectedeveniftheprimaryprotectionfailstoisolatethe
faultedelement.Back-upprotectioncanbeprovidedlocallyor
fromaremotelocation.
Localbackuprelaysareanalternatesetofrelaysina
primaryprotectionzonethatoperateunderprescribed
conditionsinthatprotectionzone.Oftensuchlocalbackup
relaysareaduplicatesetofprimaryrelayssettooperate
independentlyforthesameconditionsastheprimaryset.
Thisconstitutesaneffectivesafeguardagainstrelay
failures.
Remoteback-upprotectionisprovidedbyequipmentthatis
physicallylocatedatsubstationsawayfromthelocationwhere
equipmentforprimaryprotectionislocated.Forexample,
supposeafaultatlineJKofFigure1cannotbeclearedby
breakerJduetorelayorbreakerJmalfunction.Backup
relaysatlocationsIandMshouldbesettooperateforthe
faultatlineJK,butonlyafterasuitabledelaythatwould
allowbreakerJtoopenfirst,ifpossible.

8. Reliabilityofaprotectivesystemisdefinedasthe
probabilitythatthesystemwillfunctioncorrectlywhen
requiredtoact.
Sensitivityinprotectivesystemsistheabilityofthe
systemtoidentifyanabnormalconditionthatexceedsa
nominal"pickup"
Selectivity in a protective system refers to the overall
design of protective strategy wherein only those
protective devices closest to a fault will operate to remove
the faulted component.
Thesecuritypropertyisdefinedintermsofregionsofapower
systemcalledzonesofprotection.Arelaywillbeconsidered
secureifitrespondsonlytofaultswithinitszoneofprotection.
Undesiredtripping(falsetripping)resultswhena
relaytripsunnecessarilyforafaultoutsideits
protectionzoneorwhenthereisnofaultatall.This
canoccurwhentheprotectivesystemissetwithtoo
highasensitivity.

9. Basic parts of protective relaying mechanism:
(i) First part is the primary winding of a current transformer
(C.T.) which is connected in series with the line to be protected.
(ii) Second part consists of secondary winding of C.T. and the
relay operating coil. The relay coil makes the trip circuit when it
is energized sufficiently high to drag the arm to close the contacts
of the trip circuit.
(iii)Third part is the tripping circuit which may be either a.c. or
d.c.It consists of a source of supply, the trip coil of the circuit
breaker and the relay stationary contacts.

10. Figure 2 Basic protective relaying mechanism

12. 3-ClassificationandFunctionof
Relays
Aprotectionrelayisadevicethatsensesany
changeinthesignalitisreceiving,usuallyfrom
acurrentandorvoltage,ifthemagnitudeofthe
incomingsignalisoutsideapre-setvaluethe
relaywillcarryoutaspecificoperation,generally
tocloseoropenelectricalcontactstoinitiate
somefurtheroperation,forexamplethetripping
ofacircuitbreaker,figure2.

13. A-Classification based on the construction and
principle of operation:
1.Electromagnetic relays. They are activated by
A.C. or D.C. quantities.
2. Electro-thermal relays. Thermal protection
using Bi-metallic strip.
3. Physico-electric relays. Change in the physical
parameters (Buchholz relay).
4. Static relays. use solid state devices for their
operation.
5. Microprocessor based relays. Use VLSI
technology.
Classification of Relays:

14. 1.Over current relays. Operate when the
activating quantity (current) rises above a
specified value.
2. Under voltage relay. Operates when the
activating quantity (voltage) falls below a
specified value.
3. Distance relays. Its operation depends upon
ratio.
4. Differential relays. Its operation depends upon
comparison of two or more electrical quantities.
5. Directional relays.
B-Classification based on its application:

15. 1.Instantaneous relays. Operation takes place
after a small interval of time that is
negligible.
2. Definite time relays. Its operation is
independent of magnitude of activating
quantity.
3. Inverse time relays. Their time of operation
is inversely proportional to the magnitude of
the activating quantity.
C-Classification based on time of operation:

16. D-Function E-IncomingSignal

18. Electromechanicalrelaysrepresentamaturetechnologyfor
protectivedevicesthathavebeenwidelyusedformany
yearsandarestillappliedformanypurposes.Thesedevices
havebeenproventobereliableandareoftenfavoredby
protectionengineersformanyapplicationsbecauseoftheir
reliableperformanceandlowcost.Thecharacteristicsof
electromechanicalrelayshavebeenexhaustivelytreatedin
theliterature.
Therelaysareconstructedwithelectrical,magneticand
mechanicalcomponentsandhaveanoperatingcoiland
variouscontacts.Theirconstructioncharacteristicscanbe
classifiedintwogroups,AttractionRelaysandInduction
Relays.
4-ElectromechanicalRelays

19. A-Attraction Relays
Attraction relays operate by the movement of a piece of
metal when it is attracted by the magnetic field produced by
a coil. The attracted relay types are armature and solenoid
relays, figures 3 a and b.
Itcanbeshownthattheforceofattractionisequalto
(K
1I
2
-K
2),whereK
1dependsuponthenumberofturnson
theoperatingcoil,theairgap,theeffectiveareaandthe
reluctanceofthemagneticcircuit,K
2istherestrainingforce
producedbyaspring.Whentherelayisbalanced,the
resultantforceiszeroand(K
1I
2=
K
2),sothatI
2=
K
2/K
1=
constant,Iiscalledpickupcurrent(I
pickup).Attraction
relayhasnotimedelayandforthatreasonitusedwhen
instantaneousoperationisrequired.
( a ) Armature type relay

20. For current above the threshold I
pick upthe force
developed by the solenoid plunger overcomes the force
of gravity and closes the open contacts. The solenoid
relay is often referred to as an "instantaneous relay,"
The speed of this type of relay actually depends on
the magnitude of current flowing in the solenoid, and
if the current is large the relay will trip in about one
cycle.

21. Figure 3 ( a ) Armature type relay

22. Figure 3: ( b ) Solenoid type relay

23. Figure 3: ( C ) Characteristic of Solenoid type relay

24. Inductionrelayscanbegroupedintothreeclasses;
shadedpoletype,wattmetricandcuptyperelays,
figures4to6.Figure4illustratesoneoftheinduction
diskprotectiverelaying(shadedpolerelay).Thediskcan
becausedtorotateduetoeddycurrentsthatflowinthe
disk,thecurrentsbeinginducedduetothefields
establishedbythepoles.Therearemanyingenious
formsofinductiondiskrelays.Thisonemeasuresonly
thecurrent,buttheshapeoftherelaytime-current
characteristiccanbechangedtorepresentthevarious
generictypesdescribedinFigure3.Thetimerequiredto
tripforagivencurrentdependsontheangleofrotation
requiredtocausethemovablecontacttoreachthefixed
contact.Thisangle,andhencethetimetotrip,is
adjustablebythe"timelever"ordialsetting,wherebythe
fixedcontactcanbeadjustedtoadesiredangular
displacement.Thissimplefeaturemakestherelayvery
flexibleinitsapplicationandprovidesavaluable
B-Induction Relays

25. Figure 4 (a) Shaded pole relay

26. Figure 4 (b) Shaded pole relay

27. Figure 4 ( c ) Principle of construction of an induction
disc relay. Shaded poles and damping magnets are
omitted for clarity

28. Figure 4

30. Inductionrelayconsistsofanelectromagnetic
systemwhichoperatesonamovingconductor,
generallyintheformofadiscorcup,andfunctions
throughtheinteractionofelectromagneticfluxeswith
theeddycurrentsthatareinducedintherotorby
thesefluxes.Thesetwofluxeswhicharemutually
displacedbothinangleandinposition,producea
torquethatcanbeexpressedby

31. Typicalovercurrentrelaycharacteristicsforafamilyof
similarrelaysofthesamemanufacturerareshownin
Figure,whichdistinguishesqualitativelybetween
characteristicshapesfromdefinitetime(bottomcurve)
toextremelyinversetime(topcurve).Mostrelay
manufacturersofferthesevariouscharacteristicsin
electromechanicalrelaysofthesamebasictypeof
inductiondiskdevices.
Theinductiondiskrelaycanbeanalyzedbysumming
thetorquesactingonthedisk.Thecurrentflowingin
thepolesdevelopsafluxthatcreateseddycurrentsin
theinductiondisk.Thesecurrentsinteractwiththeflux
toproducetorquethattendstorotatethedisk.The
springcreatesaretardingtorque.Adampingtorqueis
alsoproducedthatisproportionaltotheangular
velocityofrotation.Wecansummarizethesetorques
asfollows:

32. The driving torque is proportional to the square of the
current in the current coil, the spring torque is a
constant retarding torque, and the damping torque is
proportional to the angular velocity. Therefore, we
may write as follows :
whereappropriateconstantsofproportionality
havebeenintroduced.Wecandeterminethefirst
constantbymeansofasimpleexperiment.Ifthe
diskisatrest,theright-handsideiszero.Ifwe
slowlyincreasethecurrentuntilthediskbeginsto
rotate,thisestablishesthethresholdvalueof
current,whichisusuallycalledthe"pickup"
current.Thuswehavetherelation

33. where I
P= Pickup current
Then,

34. It is reasonable to ignore the initial acceleration of the
disk, since the disk is very light and accelerated quickly
to its final constant velocity. If this simplification is
introduced, we approximate (3.5) as
for any current greater than pickup. As long as this
current continues to flow, the disk rotates at constant
velocity until the contacts close. If we designate the
angle of travel required to make these contacts as Op
we can find the time required for pickup. This time is
given by

35. wheretpisthetimetopickup.Notethatthecoefficient
inthenumeratorontheright-handsidehasthe
dimensionsofsecondsandisthereforerecognizedas
thetimeconstantT
I.Thistimeconstantisarelay
designparameterandwillhavedifferentvalues
dependingontheshapeofrelaycharacteristiccurve
thatisdesired.
Theforegoingignoresthesaturationofthemagnetic
circuit.Largecurrents,correspondingtolargevaluesof
M,causetheelectromagnettosaturate.Thiscausesthe
fluxtoreacha
limitingvalue,whichproducesaconstantoperating
time,whichwedesignateasT2•However,largevalues
ofMcausest
pcalculatedabovetoapproachzero.
Therefore,theeffectofsaturationistoaddaconstant
T2.Lastequationalsofailstoaccountforthefactthat
somerelaydesignsrequiredifferentexponentsofthe
variableM.Wecanaccountfortheseadditional
conceptsasfollows.Let

36. wherewehaveaddedasecondtimeconstantto
accountforsaturationandhavechangedtheexponent
onMtoavariablepthatcanbechangedaccordingto
therelaydesign.Commercially
availableinductiondiskrelayshavevaluesofthe
exponentpthatvaryoveraratherwiderange.This
flexibility,inadditiontobeingabletoselectthetwo-
timeconstantsinmakesitpossibletodeveloprelays
withmanydifferentcharacteristics.

37. FiguregivesasemilogarithmicplotofTCcharacteristics
foraninverserelaytoillustratethemanychoicesoftime
dialsettingsthatareusuallyavailableondevicesofthis
type.AsnotedinFigure,thevarioustimedialorlever
settingsprovideameansofadjustingtheangletraveled
bytherotatingcontactinordertoreachthefixedcontact.
ThevariouscurvesshowninFigurecorrespondto
distinctsettingsofthetimedialorleversetting.Therelay
usedfortheillustrationhasaninversecharacteristicthat
correspondstooneofthesteepercurvesshowninFigure
.Theadjective"inverse"isarelativeterm.Thuswesee
inverse,moderatelyinverse.veryinverse,andmanyother
namesusedtodistinguishthevariouscharacteristics.

38. Typicalovercurrentrelaycharacteristicsforafamilyof
similarrelaysofthesamemanufacturerareshownin
Figure,whichdistinguishesqualitativelybetween
characteristicshapesfromdefinitetime(bottomcurve)
toextremelyinversetime(topcurve).Mostrelay
manufacturersofferthesevariouscharacteristicsin
electromechanicalrelaysofthesamebasic
typeofinductiondiskdevices.

39. Figure 5 (a ) Schematic diagram for wattmetric type relay

40. Figure 5 (b)Schematic diagram for wattmetric type relay

41. THANK
YOU
Prof. Mahmoud El Bahy
Professor of High Voltage Engineereing
For any questions feel free to contact me by
mail
[email protected]