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Applied Protective Relaying
Westinghouse Electric Corporation Relay-lnstrument Division Coral Springs, Florida 33065
TABLE OF CONTENTS
Page
Chapters and Authors 1 -
lntroduction
and General Philosophies
Chapters and Authors IJI. IV. V. V l. Vil. VIII. IX. X. XI. XJ1.
-
J. L. Blackburn
l. lntroduction ll. Classiñcatíon of Relays 111. Protective Relaying Systems and their Design IV. Applying Protectíve Relays V. Relays and Application Data VI. Circuit Breaker Control 2 - Technical Tools of the Relay Engineer · Phasors, Polarity, and Symmetrical Components · J. L. Blackburn l. lntroduction IJ. Phasors fil. Polarity in Relay Circuits IV. Faults on Power Systems V. Symmetrical Components VI. Syrnmetrical Components and Relaying
. .
1-1 1-1
. . . .
1-2 1-5 1-6 1-8
. . . . .
XIII.
2-1 2-1
XIV. XV. XVI.
2-5
2-8 2-10
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
4 - Protection Against Transients · W. A. El more l. Introduction . . . . . . . . . . . . . . . . . . íl. Transients Originating in the High Voltage System . . . . . . . . . . . . . . . . III. Transients Originating in the Low Voltage System . . . . . . . . . . . . . . . . IV. Protective Measures. . . . . . . . . . . . . .
. . . . . .
3-1 3-1 3-6 3-9 3-1 5 3-26
.
4-1
.
4-3
. .
4-4 4-5
5 - lnstrument Transformers for Relaying · W. A. Elmore l. lntroduction . Il. Currcnt Transformers . IIJ. Voltage Transformers and Coupling Capacitance Voltage Transformcrs ..... 6 - Generator Protection · W. A. Elmore l. Introduction JI. Fault Detection
6-3 6-4 6-7 6-8
6-8 6-1 O 6-11 6-12 6-13
6-14 6-1 5 6-16
6-16 6-18
2-32
7 3 - Basic Relay Units. J. V. Kresser and J. L. Blackburn l. Lnlroduction . . . . . . . . . . . . . JI. Electromechanical Units . . . . . III. Sequen ce Networks. . . . . . . . . IV. Solid-State Units . . . . . . . . . . V. Basic Logic Circuits . . . . . . . . VI. lntegrated Circuits . . . . . . . . .
Grourid-Fault Protection . Backup Protection . Overload Protection . Overspeed Protection . Loss-of-Excitation Protection . KLF and KLF-1 Curves . Two-Zone KLF Scheme . Protection Against Generator Motoring Field Ground Protection . Alternating-Current Overvoltage Protection for Hydroelectric Generators . Generator Protection at Reduced Frequencies . Recommended Protection . Out-of-Step Protection . Bus Transfer Systems for Station Auxiliarles .
Page
. .
5-1 5-1 5-11
6-1 6-1
Motor Protection · W. A. Elmore l. Jntroduction . 11. Phase-Fault Protection . IIJ. Ground-Fault Protection . IV. Locked-Rotor Protection . V. Overload Protection . VI. Thermal Relays . VII. COM Relays for Overload Protection .. Vlll. Low Voltage Protection . IX. Phase-Rotation Protection . X. Negative Sequence Voltage Relay . XI. Phase-U n balance Protection . XII. Negative Sequen ce Current Relays . XIII. Out-of-Step Protection . XIV. Loss of Excitation . XV. Typical Application Combinations .
8 - Transformer Protection · H. J. Li l. Introduction . 11. Magnetizing I nrush . III. DifferentiaI Relays for Transf'orrner Protection . IV. General Gu.idelines for Transformer Differential Relaying . V. Sample Checks for Applying Transformer Differential Relays ..... VI. Typical Applications of Transformer Differential Protection .
7-1 7-2
7-2 7-4
7-6 7-7 7-8 7-8
7-9 7-9 7-10 7-11 7-11 7-11 7-1 1
8-1 8-2 8-4
8-8
8-10 8-15
/
ii
Chapler~
and Aulhors
Page
111. System M ap Response rv. Program Application
1 ypical Protective Schcmcs for
VII.
Industrial and Com mcrcial Power Transformers. . . . . . . . . . . . . . . . . .
8-19
Rcmotc Tripping ofTransformer Bank . . . . . . . . . . . . . . . . . . . . . . .
8-22
IX. Protcction of Phase-Angle Regularor and Voltage Regulat ors . . . . . . . . . . .
8-22
X. Zig-Zag Transformer Protection . . . . .
8-24
VIII.
XI. Interconnected, Wye-Delta Transformcr Protcction . . . . . . . . . . . . . . . . . . . XI l. Protcction
9 - Sration-Bus Protection -11. J. Li l. Introduction . . . . . . . . . . . . . . . . . . . 11.
Linear Couplcr Diffcrenlial
IV.
J!igh-Impedance
-
l. lntroduction
111. Protection
-
9-1
. . .
9-7
Ditferent ial Systern . . •
9-8
Protection
VI l. Othcr Bus-Protcctive 10 -
11. Dislribution IV.
Transmission
V. Ground
Circuit Protection •..
Circuit Protection
Fault Protection
VI. Series Compensated Transmission Line Protection . . . . . VI l. Protecting Direct-Current -
.
10-26
.
10-46
.
10-65 10-65
Systerns
System Grounding and Proteclive Relaying. J. L. Blackburn and J. V. Krcsser
l. 1 ntroduction 11. Ungroundcd
. . . . . . .
.
Systerns
.
1 1-1
.
11-2
11-4
Resistan ce Grounding
V. Sensitive
Ground Relaying
VI. Ground Faull Protcction for ThreePhasc, Four-Wire Systerns 12
-
.
11-7
.
11-8
.
1 1-1 1
l. lnlrocluction 11.
. . . . . . . . . . . . . . . . . •
Pr otcctivc Device Coordination Prograrn Structure . . . . . . . . . . . . . .
12-2
13-10
.
14-1 14-1 l4-3 14-5
V. Relay Systern Testmg
.
14-6
VI. Pilot Wire Requirernents
.
14-6
.
14-8
Protectivc
Devices for Pilot Wires
Pilot \Vire Supervision...........
14-10
Pilo! Channels for Protective Relaying · 11. W. Lensner .
15-1
.
15-2
111. Carrier Transmiltcrs & Reccivers
.
15-22
IV. Audio Tone Channels
.
15-30
V. Microwave Channels
.
15-37
Transmission Line Relaying: Pilot Relaying and Tripping Systerns for Circuit Breakers · R. E. Ray
IV.
16-1
.
.
16-1
Relaying Systems
.
16-2
Phasc Cornparison Pilot Relaying Systerns
.
16-7
.
16-14
.
16-16
!l. Classification of Pilot Systerns lll. Dircctional Comparison Pilot
V. Additional Security & Protcction VI. Selccting a Pilot Transmission Rclaying System VII.
12-1
.
.
Relay Selting
Techniques
Application and Setting by Computers · V. F. Wilrckcr and R. /1.. Wilson
13-1
R. L. Ray J. Introduction . 11. IICB and HC'B-1 Pilot Wirc Systerns .. 111. Criteria for Set ting HCB arvd IICB-1
l. l ntroduction
111. Reactance Grounding
JV.
16 -
13-1
.
13-J 5
11. Power-line Carrier Channels
10-14
. Plant
l. lntroduction
10-4
Circuir Protcction ...•..
111. Subtransmission
-
10-2
12-8
Pilot Wire Protection.
VIII. 15
.
12-4
.
/1.utomatic Transfcr Schernes ,
Vil.
9-12
Line and Circuit Protection · J. V. Kresser and J. L. Blackburn l. lntroduction
11
9-11
Schernes . . . . . . .
of Utilily/lnclustrial
IV. Example of IICB-1
9-11
Protccting a Bus that lncludes a Transforrner Bank . . . . . . . . . . . . . .
VI.
.
Relays
V. Sctting Exarnple
for KAB Bus . . . . . . . . . . . . . . . . .
. . . . . . . . .
of Nctwork Systems
Ties IV. 14
.
Protection of Network Systems, Utility/ Industrial Plant Tics, and Automatic Transfer Schemes . J. V. Kresser 11. Protection
8-26
9-1
Syste m . . . .
111. Multi-Rcstraint Diffcrcntial System
13
8-25
of Shunt Reactors . . . . . . .
Page
Chapters and Authors
VIII. IX.
Tripping Systcrns for Circuir
Breakers
Lino
.
16-23
Breaker Pote Disagrcernent Protection
.
16-26
Direct Transfcr-Trip
.
16-27
Systems
üi
Page
Chapters and Authors 17
-
Multi-Terminal, Tapped Lines, and WeakFeed Protection · W. L. llinman
l. 11. III. IV. 18
In troduction .
.
.
Multí-Terrrunal Line Protectíon
.
Tapped Line Protection . . . Protecting Wcak-Fced Tcrrmnals
.
Backup Protection - W. l . Hinman l. lnlroduction . . . . . . . 11. Remole versus Local Backup 111. Breaker-l-arlure/Local Backup A pplication . . IV. Timing Characteristics V. Brcaker-FailurejLocal Backup Re lay Charactcristics
19 - System Stability and Out-of-Step Relaying - W. A. Elmore l. Introduction 11. Steady-State Stability III. Transient Stability TV. Relay Quantities During Swings V. Effects of Out-of-Step (OS) Conditions Vl. Out-of-Step Relaying Vil. Philosophies of Out-of-Step Relaying Vlll. Types of Out-of-Step Schemes IX. Relays for Out-of-Step Systerns X. Selection of an Out-of-Step Relay System 20 -
21 -
Reclosing and Synchronizing W. A. Elmore l. 1 ntroduction JI. Reclosing Systern Considerations 111. Considcrations for Application of lnstantaneous Reclosing IV. Rcclosing Rclays & 1 heir Operation V. Synchrorusm Check VI. Dead Line or Dead Bus Reclosing VII. Sclcctivc lnitiauon VIII. A u toma tic Synchroruzing
17-1 17-2 17-10 17-12
. .
18-1 18-2
. .
18-4
.
18-7
. . . .
18-7
19-1
19-2 19-2
19-3
Chapters and Authors 11. IV. V. VI
System Operating Limits Load-Shedding. . . . . Frequency Relays. . . . . . . . Formulating a l.oad-Shcddmg Scherne VII Spccial Considerations for Industrial Systerns . . . . . VIII. RcstoringScrvice. IX. Other Frequency Relay Applications
l. 11. 111. IV. V. V l. VII. VJII. IX.
Introduction Test lntervals ....•.•..•••..• Test Conccpts I'est Object ives .....•...•.... Acccssories for Testing Typical Relay Tests Typical Relay Test Equipment T.:st Circuits . . . . . . . . Testing Potential Polarized Ground Relays wilh Load Curren!
19-5 19-6
Appendix I
. . .
19-8
Appendix IJ -
.
19-14
. .
20-1 20-1
.
20-3
. .
20-4 20-11 20-12 20-12 20-13
. .
Load-Shedding and Frequency Relaying 1. A. Udrcn l. lntroduction . 11. Ratc of Frcquency Decline .
21-1 21-1
2 l-3 21-4 21-4
.
21-8
.
21-12 21-14 21-15
.
22-1 22-1
22 - Testing and Maintenance of Protective Relnys - L. J. Schulze
. .
19-8 19-1 O
. . .
Bibliography
Formulas Commonly used in Relaying Electrical Power System Device Numbers & Functions
.
:n-1 22-1
. . . .
22-3 22-4 22-5
.
22-7
1-1
Chaptcr
l. INTRODUCTION
I
lntroduction Philosophies Author.
and General
J. L. Blackburn
l. lntroduction
111. Protective
Relaying
Rclays are co mpuct analog networks that are connected or unwanteJ condttions within an assigned arca. lhcy are, in eff'ect , a form of active insurance dcsigned to mamtuin a htgh dcgrec ot service continuity. ami limit equiprnent durnage. They are "Silcnt Scnunels." Whtle prot ect ive relays \\ ill be throughou l the power system Lo dctcct mtolcrable
thc main ernphasis of this book. ot her types of relays. up-
Syvtcms anti t hcir Dcsign
De,11•n Criterra t Reliah11ity 2. Specd 3. Selcctivity versus Fcononucs 4. Sun plicity B. Faciors lnf'luencing Rclay Pcrtorrnunce C'. Zones of Protecuon
phcd on a more J1111ited basis or medas pan of a total protect rve reluy systern will also be covered ,
A
IV. Applying Protecuve Rclays
A. B. C. D. 1 .
Systern C'onfiguration Existing Syst ern Protection and Proccdures Degree of Protection Require d Fault Study Máximum Loads. Transformer Data, and Impedanees
V. Relays and Application Data A. Switchboard Relays B. Rack-Mounted Relays
11. CLASSIFICATJON
OF RELA YS
Relays can he dividcd into í'ive Iunctional ca tegoriev: a. Protcctive Relays, which detect defectivo lines. dcfect ivc appuratus. or oihcr dangerous or intolerable conduions. fhesc relays can euher irnuate or perrrut swilching or simply providc an alarrn. h. Moniloring Relays, which vcrify conditions on l hc power systern or in the protection systern. These relays inelude Iault de tectors. alarm uníts, channel-rnonitoring rclays, synchronism verification, anti network phasing. Power syst crn condit ions that do not involve opening circuir breakers during Iaults can he monitored by veril'icalion relays.
VI. Circuir Brcaker Control c. Programming Relays, which evrablish or detect electrical sequen ces. Programrning relays are used for reclosing and synclu onizing. d. Regularing Relays, which are activuted when an operuting parame tct dcviutcs trom predct ermined linuts Rcg· ulating rclays Iuncuon (hrough supplementary equip1111.'nl io rcstore thc quanuty to thc prcscrihe d lirnus. c. Auxiliary Relays. which operatc 111 response to thc open· ing 01 closing til the operuung crrcuit to supplcment unot her re lay or dcvice
multiphe
i
1 hese include t imers, contad
relayv, vcahng u111h, receivei rcluyx, lock-out
rclays, closing rclays, anti t rrp rclays,
1-2
In addition
to thcsc
Iuncuonal
catcgories,
rclays
may be
clussified by input, operating principie or structure,
:111d
performance charactensnc a.
Input • ( urrent •
Volt age
• Power • Pressu re • 1 rcqucncy • 1 emperature • rlow •
In lhcory, a relay system should be a ble to respond to the iní'init y of abnorrnahucs thal can possihly occur within Lhc power systern. In pracuce, thc rclay cnginecr must urrivc ata comprornise hased on thc tour l'actors that inrluence tJIIY rclay applicution:
Vibration.
h Operaung Prmcrple or Sl ructure • Pcrcentage • Mulu-rest raint
• Solid statc • l- lect rornecharucal • Thermal.
•
Characteristic
Directionul-overcurrent
• • • • •
lnverse time Definite time Undervoltage Ground or phase lligh- or slow-speed • Phase comparison • Dírectional comparison • Scgregated phase. The ahove classifications ami definitions are hased on the ANSI Standard C 37.90 (IE:H 313).
111. PROTECTIVE DESIGN
a. Econormcs
initial, operut ing, and rnaintenance.
h. Available measures oí fault or trouhles
• Product
c. Performance • Distance
diately available lo handle intolerable system conditions and avoid serious outages and damage. Thus, t he true operuting life ol these relays can be on thc order of ,1 few soconds, eve n lhough they are connectcd in a sysle m Ior many years. In practice, t he rclays opcratc fur more during testing and maintenance than in response lo adverse service condiuons,
RELA YING SYSTEMS ANO THEIR
Technicially, most relays are small systems within thernselvcs, Throughout this book , howevcr, thc tcrrn syst ems will he used to indicate a combination of reluys of lile same or different types. Properly speaking, the prol cctive rclaying systern includes circuit breakers as well as rclays. Relays anti circuit brcakers must function together ; thcrc is little 01 no valué in applying one without the ot hcr. Protcctive relays or systerns are not required to Iunction d .rrmg normal power systern operation, hul musl be im me-
tudes and location ol curren! transformers l ransforrners.
faull magniand voltage
c. Operaling practices conformity lo vtandards and accepted practices: ensuring efficient system operation. d. Previous cxperíence-chistory anti anticipation of t he typcs of trouhlc hkely lo he encountcrcd wrthin thc system. The third and Iourth considerations are perhaps better expressed as the "personality of lhe system and lhe relay engineer." Since il is sirnply not feasible to designa protectivc rclaying systern capa ble of handling any potenual problem, compro mises must be made. In general, only those problcrns which, according lo past e xperience , are lil,.. cly lo occur, receive primary considcration. Naturally , thís makes rclay1ng somewhat of an arl. Oíl fercnl relay engineers will, usmg sound logic, dcsign significanlly tlifforcnt protecllvc syslt!ms fr esscntially the sarnc power systcm. As a rcsull, there is liltle standard1¿at1on m protect1ve rclaying. Not only may lhc typc of relay1ng system v.iry, hul so will lhc exlcnt of lhe protect1ve coveragc. roo much protccllon 1s almost as hadas too ltlllt! Nonethcless. proteclive relaying is a highly specializcd tcchnology rcqu1nng an m-depth understand 1ng of the power system as a whole. The relay engineer must know not only thc technology of the abnormal, but have a basic understanding of ali the system componcnts and their o peral ion in the system. Relaymg, then, is a "vertical" specialily requiring a "horiwntal" viewpoint. This horizont:il,
1-3
or total systern, concept of relaying includes Iault protec-
erat ion in response Lo systern trouble, while securíly is the
tion and the performance
ability of the systern lo avoid misoperatton
of the protection
abnorrnal system operation erauon deficiency,
systern during
out-of-step condittons, and so torlh.
though these ureas are vilally importunt shared by his colleagues.
Al-
lo the relay engi-
necr, his concern has not always been fully appreciated
between faulls.
Unfortunutely
such as severe overloads, gen-
or
For this reusen, close and con-
, these two aspects of reliability tend to countcr one another. mcreasrng security tcnds to decrcase dependabilíty
and vice versa.
In general, howcver , modcrn
rcluying systerns are highly rchable and provrde prucucal com prom ise bet ween security and dependubility ,
unucd cornrnunication betwcen the planrung, relay dcsign, and operation of
departments
is essent ial. Frequenl
protcctivc systems should
be
mandatory,
systerns grow and operaling conditions
since
reviews
Protecuve
powcr
verse systcm and environrnental
change.
relay
systerns must
perform
correctly undcr
conditions.
whether other systerns are momentarily
ad-
Rcgardlcss o
í
blinded during this
pcriod, thc relays musí perform accurately and dcpcndably. t\ cornplex relaying systern may resull from poor systern
They rnust euher opcrate 111 response to troublc in thcir as-
design or thc econormc need to use fewer circuit
signcd arca or block correctly
breakcrs,
Considerable savings can be reulized by usmg fewer circuit brcakers anda more complex relay system. usually involve design compromises
atíon if acceptable protection
if the trouble is outside thcrr
designated arca.
Such systems
requiring careful evalu-
is to be maintained.
Dependubility
can be checked relatively easily in the labora-
tory or dunng installation
by simulated tests or staged
faults. Securily, on the other hand, is much more d1fficult 111.A.
A true test of system security would have to
to check.
Design Criteria
measure response to an almost infi111te variety of potential The applicat ion logic of protective relays divides the power
transients and counterleit
systern into severa) zones, each requiring its own group of
system and its environment.
trouble indications in the power A secure system is usually the
relays,
resull of a good background
in design c.:ombined with ex-
In ali cases, the five design criteria listed below are common
confirmed
tensive miniature power system testing, and can only be to any well-designed and efficient protective tem segment a. Reliability
c.
III.A.2. Speed the ability of the relay or relay systern Lo
perform correctly
when needed (dependability)
avoid unnecessary
operation
b. Speed
and lo
(securuy).
maxirnum service continuity
darnage.
wilh mínimum
systern disconnection. d. l conomics e. Simplicity
maximum
Relays that could anltc1pate a fault would be utop1an. even if available, they would doubtlessly out. The development
minimum equiprnent
al rrururnu rn cost. and circuilry.
wanted or unexplained
to fully satísfy all these design criterio
operations.
Time, no matter how
shorl, 1s still the best mcthod of distmguishing bctwcen real trouble.
Applted Lo a n:lay, lugh speed tndicates that the operat1ng
lime usually :------+-BrossSpocers ..,....,__'--__._ ond Moun1,ng Blocks _.~,...___...,.. ,.,_...,.___ -+Pe1monen1 Mognel
.'-!----+--
!ron Pole Shoes
..J......¡~.....::.1,2,;~----¡--
Movmg
eo,1
10-
lb=j../JI1-h/312
(3-4)
Substituting this and the sequence equation (2-29) for la, in cquation (3-3):
Figure 3-7: O'Araonval Type Unít
111.8.
Composite Sequenc Current Networks
The nctwork shown in Figure 3-8 can be adapted for a variety of single-phase outputs. V F is obtained from in pul currents 13, lb, and le, with neutral (310) return. Using Thevenin's Theorem, these t hree-phase networks can be reduced to a simple equivalent circuit , as shown in Figure 3·8b. VF is the open circuit voltagc at the out put , and Z is the impedance looking back into the three-phase net-
(3-5) Varying Xm, R 1, R0, and the connectíons produces diftercnt output churacteristics. In sorne applications, the currents lb and le are interchanged , changing Equation (3-5) to
3·1)
I'abte 3.1 • ·1 ypícal Sequence Networ k Co mbuiarlons
-Net work Type Positive
Sequcnce
Switch r
Switch s
e lo sed
open
xm
Figure 3-8 Notes
=
R 1¡..fi
in terchange
VF reduces from: Equation to Equal (3-6)
2R111
lb ancl le
Ncguuve Sequence
closed
o pen
R ¡1'/3
as shown
(3-5) 2R¡ 12
IICU
open
do:.t:J
R¡/V3
I nterchange
(3-6)
Cornposne
2R111
+(R1
+
3R0)10
lb and le HCB-1
and SKB
du:.1:!J
UP,:11
+ .46212 +
\ 3-5) -0.211
JS shown
l .4óR l 01 .191 ohrns
Composites "
(R¡
+3R0)10
*Data for Tap C of three taps available.
With switch, closcd and switch sopen, (he zero sequeuce response of Equat ions (3-5) and (3-6) is climinatcd. The
xbli-lVbyll lhefilteroulput,Vxy=Yp,1Sllie phasor sum of t hcse t wo voltages. thJtl
zero sequence drop across R1 is 2/3R1 tao - l/3R1 (lbo + lc0) O. The swit ches r and s are used in Figure 3-8 as a
=
~ ....~~~~~~~~~~~~~~~~~~~o
convenicnce.
~-+-~~~~~~~~~
Severa! typical sequence
network
.:urnl>111Jt1011:.
J11;:
g1v.:11
.... ~~~~~b
........ .._~~
in
Table 3-1.
111.C. Sequence
Voltuge
e rworks
Au101,ansfo,me,
l
Se quencc voltage net wor ks provide
J
:.mgh:·phd:,i; out pul
propon ion al to cithcr thc positive or thc negativo sequen, .. c •
voltagc of phase a. A net work in common use is shown in Figure 3-9. Since this net work is delta-connected
to tht
three-phase potenlial source (a, b, e), thcrc is no response zero sequence. The nctwork Figure
is best cx plauted tlu ough t he phasor
3-1 O. By design , t he pitase angle of Z
+R
tu
r---1/F---
Tyµ,, ""'"
~mve SeQuence Negot,ve~
Sw11c1>es r
Sw11cnes
s
1
y AnQle ol Z + R bO• Anc¡leol Z•a2•
Ope,,
Closed
VF=l.135 Voi!-60°
Closed
Opén
VF•l.135 Voz !-60°
Voltage
Network.
J1lG1'•uv Bteolo. 1. , .. ,i;:,
-
- - ---
)
rhree ditpolarities, ln positive logic, inputs and outputs are positive : in negative
ferent logic conventions
In addition,
t here are
for the input/outpur
logic, bot h inputs and out puts are negative , In mixed logíc, inputs are either positive or negative ; outputs are usually positive but could be negative.
Relay systems normally use
posit ive logic, a lt houg h sorne elernents Fo, ,vU1d 61()-N_eg:....o_t,_ve----~--------
04 and 02 are forward-biased,
turn through R2 and R I blockecl by D 1.
Polarizing
current
going up from point A flows backward through 02 ami up through R2 (nel currenl in 02 is forward).
Figure 3-38: Block-To-Btock Type Phase Ar19leComparator Crrcurt
with the re-
age, -IPoLR2,
is reversed.
The output volt·
3-19
-- • n
H
lop
A
Operore
B
~
H
Iop
it
.__,___,
In Phose Relotoonsh,p lop> !POI.
H
(+)
...............
lpoL
-Iop
--
Operole
•
Iop
¡POI.
n
u A
B
~
it
n
.__,___,
In Phose Reloroonshíp I POe lop
--
{a) Operation for In-Phase Conditíon of Operate and Polariza Ouantities
-- • H
H
~
(+l
lpoL
iQP
H ....._........, IPOL
tt
lop
A
Operore
B
~
n
!i
H
.__,___,
tt
(+)
--.........., ¡POI.
IPOL 90° Reloroonshíp Iop> !POI.
T~. l_ . T~. l_. T~·~ Outpul
Output
Oulpul
••
No1e: Curren1s Showno1(+)from010PT1me
(b) Operation for 90º Cond111on Between Operate and Polarize Ouantities
Figure 3-39: Principie of Operetion of Ring Moduletor Type Phese Angla Comparetor.
With reversed
but still smaller-polarizing
lhe polarizing current
current , part of
would flow up through R 1, around
lhrough DI (which is forwar d-biased
by lhe opérate
current)
lf t he polarizing current is the larger, as in the bottom half of Figure 3-39a, DI is Iorward-biased is forward-biased
through R2.
lhrough R 1, and 04
lf RI equals R2, net output
and back. The other part would flow up through R 1, down
from lhe polarlzing
through D3, and back. is reversed,
rent flows through DI, RI, R2, and back through
Again, t he out pul voltage , - IPOL Rl ,
curren! is zero.
The smaller operate cur04. Net
.> ..!O
u111.:111in1)41i, Iur war d because ol the laigo::1 value v1 pv lurizing currcnt. 'l'he net output, then, 1s 'or (RI + R2), v1 21opRI. Rever Mng e IIher l he pu lariling or operat uig curren l wrll re-
verse the output volrage. 'I he out pul wilJ not be ¿c1v us long J~ the smaller cuu ent i:, a hove a threshold or pick-up value. Wl11.:n the sum of the eurrents through a diode is zcro, as through 03 in hgure 3-39a, the output is still 101,R 1. Any tendency of cither current to flow in another pal h, because 03 is not conducting, will result in one of thc I wo cornponents becorning larger ; t hat is, thc zero condiuon no longer exists. Figure 3-39b shows the r ing rnodulator operarion when t he opérate current is larger lhan the polarizmg current and lcads it by 90º. At time zero, half of IPOL flows up lhrough R 1, clown through 03, and returns up the lower half of the transformer lo A. The other half Ilows down lhrough R2, up lhrough 02, and down the upper half of the transforrner to A. The net output is zero as lop is zero, As 'or increases from zero lo equal IPOL (Point P), I0p flows through 02, R2, R 1, and 03, producing an output oí -21opR 1, where R 1, and R2 are equal. When lop equals lpo L• the current in 02 goes to zero. As lop becomes largor than lpo L• DI conducts. The negative lpo L ali Ilows through R 1; half passes through D 1, and the other half continues through 03. t0p Ilows through O I and 03, producing an output of -21POL R 1.
01
l l1111c:
(J wltete lop
Jl;\JIII
equuls IPOL• t he polarrztng cui -
rent is about to become lhc larger curren! and Iorward-bias 1) 1 and 04. The output changes lo +21opRI and decreasing When 101' crosses the zcro axis, the output is zero. 11.s the operating current becomes negative, so does lhe out pul, which reaches a maximum of -210pR. l·urthcr Jnaly~1s shows thut t here is a rnaxirnurn positivo or negative output each time lop = IPOL and altérnate onehalf -cycle penods ( 4.1 7 m ) of posit ive and negative outputs rhese out puts are crosshatched in Figure 3-39b. Similar results are obtained if the polarizing quuntity is greater than the operat mg quantity, but with 'or lcading IPOL by 90°. l·or direcuonal logic applications, thc output of the ring modulator is fed into the logic cir cuit shown in Figure 3-40. Wilh a positive output across R I-R2, transistor Q I turns on, turning Q2 off and charging C2 through R7, R8, and R9. The tirning circuit is adiusted so that if the positivo output exists for 4.17 ms, the uni-junction Q3 conducts, turning on Q4 first and then QS, to produce an output. I'he discharge lime of C3 through R J 2 and R 13 is adjusted to keep Q4 conduct ing for at least 8.S ms. This provides a continuous logic output whenever the ring modulator bridge provides a positive output for 4.1 7 ms or longer. 1 f, in Figure 3-39b, lhc operate current Oop) wave is shifted lo thc righl, representing lead angles of less than 90°, the positive outpul periods will be grealer than 4.17 ms. (The negative output periods will be lcss.)
Pos,11.e RIO RI
TI
kj
e
Ope,01e
R2
k4
O:>
Ou1pu1
~>
Rl6
RI
e~ Polom,ng Figure 3-40: Ring Modulator Type Phase Angle Comparator Circuit
_.~~.-.~+--~_..~--4>--~..-~+--~-e---~~~ @] 'Bo11nd,COIH No,molly ON T,ons,s1or
3-21
Convet sely, if t he o pc i a t c cur rcn t sh1tl~ lv t hc kit, r c pr esenting lead angles of more than 90°, the positive out puts will be less than 4.17 ms. (The negative outputs will be greater.) That is, the ring modulator output will becorne negative before Q3 reaches its firing point. This negative output will turn off Ql and turn on Q2, blocking Q3.
Further analysis for cases where t he v peratuig curreru lags the polarízing curren t prod uces si rnilar fin dings, Thus, t he coincidenl-time phase angle cornparator pro vides a continuous output when the operute and polarizing currents are ±90° or ícss, the watt characteristic for a clirectional unit
V.B. Amplification Units V.8.1.
Breaker Trip Coil lnitiator
The breake, trip ccü initrator crrcurt bot.h pt o vnles po wei
(amplification) tor a trip coil a nd isolutes t he control circuitry from lhe tripping cnergy source (t hc station battcry J A typical circuir is shown in Figure 3-41. ,,,~ í:jo1111, y
Pos,1,ve R~
T
1
Ne9011ve
Figure 3·41:
Lxcept Ioi t he t r ansforrner T2, the devices assocíated with Q4 pr ovide security. Zener ZI clips high voltage transients on the battery leads to one-third of Q4 rat ing. This voltage clipping prevents false operation of Q4 fro m surges and overvoltage. The two-winding reactor LA-LB suppresses any transients that could be transrnittcd t hrough the interwinding capacitance ofTI or between the trip circuir and ot her lcgic circuit wiring (Sec Chapter 4 and Figure 4-l 2). Zcner Z4 prevents shock ex citation from setting up high freque ncy oscillation. which might reverse the current through Q4 and return it to a hlocking statc. l'a¡rn.;1lo1 l'J ,s 111,tially chargecl through R9 and Z3 whcn tlte breaker or switch is closed, bypassing T2 to avoid a false 111dication. When Q4 fires, ('3 dischargcs through Q4, Z2, and R8. This discharge provides a holding currcnl for Q4 fo1 ahout I ms long cnough for thc currcnl I hrough lhe inductivc lrip coil lt> reach thc re4uired holding currcnl for Q4. V.C.
R':I
R6
R4
lnpv1
111 uus way , a repéllllVé u ain uf pulses 1s genci atc d as long as the input signa) exists. These pulses are transformed through TI to fire t he thyristor Q4, perrnitt ing cu rrent to pass through LA, Q4, T2 pr irnary, LB, and Z4 and to trip the circuit breaker. The time delay of this circuit is approximately I ms. TI has two secondaries, t he second of which is connccted lo a similar Q4 cir cuitry for double trip.
R5
23
A11111111cia1or
Ctrcuits
I w0 ly¡.,c~ 0l -.:1rcu1ts
are uscd Lv p1ov1dt: light iltHI alar111111tlications: onc is for circuil breakcr trip opcrations and Lhe oth.::r is for general use.
C2
To Dupncore Cvcuus fo, Second Trip Circvn
8r&ak.,r Tnp Co,i trunauon
V.C. l
Aux1líary U11i1s
"fi1pCu11c11l
lnd,co101
C1rcú1t
QI turns on when the input voltage from the Iault-sc nsmg and data processin g circuit exceeds 2 V. Q I t hen turns on Q2, allo wing C2 to charge through R6. When the voltage across C2 reaches thc "Iir ing voltage" of the uni-junction transistor Q3, the capacitor energy discharges through T l. This discharge reduces t he voltage across the ca pacitor , turuing Q3 off until the charge on C2 builds up again.
A typ1cal breaker trip indicator
and alurm kigtL 1s sl1uwn in Figure 3-42. Transformer T2 is in thc lrip circu1l, as in Figure 3-41. The transformer core uses a square hysten:sis loop material to produce a very small excit111g cuirent and neghgible inductive rcactance when saturated. When lrip current f'lows (,1ftcr Q4 fircs), thc circuil of R 1. l' 1, R2, and R3 strelches a 2-rns pulse at the secondary of T2 into 6 ms at 20 V at the output of Q2. Thc input signal turns on buth QI anct Q2 to charge capacilor C2. When lite voilage builds up to the "intrinsic stancloff ratio" of the uni-junction transistor (Yp of Figure 3-15), Q3 fircs and gates Q4, energizing the indicating light. The conduction of Q4 also gales Q5 through R l O from the drop a cross R l l. QS energizes t he alarm relay. Even if the indicating light circuit is open, QS will still be gated.
3·22
Trip 20\/o •s De Su¡,ply Alo,m
R8
fie1oy
04
03 RI
T2
C4
R2 C2
Cl
01
R9
R10
C3
R3
su lnd,ro,o, ¡,.,.,
B•eoke, Tr1pC01I
Oe101s,n F19ure 3·41 Figure
3,42. Breekor Trlp lnd,cetor end Alarm Circuit
lhc general mdicator circuu rs shown nor
condition is
mal
a
(
1) input, which
III
Figure 3·43
makes
QI
1 he
conduct
For in dicat ion, thc (1) is removed, tui ning Ql off.
Oc Supply R8
R3 01
C4
@] Cl
lnpul
02
R7
Rl
R2
R9
C3
RlO Rll
lnd,co10, Bus Ne,;¡ot,ve
@], Box lnd.::o•cs
Coordinating
and Loop Logic Timers
ing.
I'hcn Cl
chargcs t hrough R3 and R7. Whcn thc voltage across CI rcaches thc trrmg point of Q~. Q2 is turned on, gallng Q4 and Q:i to energizo the indicating light and alarm reluy.
20Vohs
V .C.2.
lrxcd-ume dclay t imcrs are used cxtensivcly in logic cirut ry A typical circuir of t his typc rs shown in Figure 3-44. Wit h an in put , Ql is normally conducting and shorts CI t lu ough R4. Rcrnoving thc input turns Ql off, and perrnits C'I to charge t hrough IU and R4. When t he voltugc across ('I reaches t he Zener voltage of ZI plus thc potcntial hill of DI and Q2, hase currcnt wilt flow, turning on Q2. Turning Q2 on rcrnoves voltage from lhe output. Thc f'ixcd time in· tcrval is bctwecn rcrnoval of input to removal of out pul. Al· t hough normally u sed for short dela ys, judicious sclcction of values for R3, R4. C'I, anti 21 providcs a wulc rango of available time dclays. Similar circuit ry can prevido a dclay bet wecn an O input andan O out put , or othcr variat ions. Also, urncrs can be malle adjustable hy rnaking clerncnts such as R3 adjustable. l
Normal !y 0111 r,ons,s10
SerClear-v
r.o Ourw• ,Jnles~
Arm•lond Se1,1
Clear Outp,n
R4
: VI
--..¡¡.
Uul~ul
4---~~~~~--'l>
20Volls Oc
F·F
R5
Mom~rnor y kt-~er-
Outpurs Re1u,n os rv, ~,.,1 ,u
Momemor;. ke~e~:-: Oulput Re1urns 10 Zerv
ta)
Cbl Mod,f,ed Type
No, mal fype
(+¡
R2
Figure 3-45: Flip-Flap
Ho
Rese,
Input
Figure
3-46
Flip-Flop
logic
Symbols.
Crrcutt,
l'he t hp t lup
1..2 ott until the voltage across CI builds u p again. Thus a series ol pulses continues as long as the input signa! cxists. Zener ZI provides surge-protection clipping al 20 V. The pulses are rectified and accurnulated on C2. C2-R4-R5 provide a steady de input to Q 1 until the input is removed. Q 1 conducts, turning Q2 on and provides a 20-V output.
hgun: 3 . .:¡ 1 I'he oui put rsulator circuir differs in that a [our-layer diode (DI), rather than a uni-junction transistor, provides a pulse chain through TI. An input voltage turns on Q l and Q2, charging C2. The voltage across C2 triggers D 1, as described above. The pulses-rectified and filtered are applied to the base of Q3, tuming it on and pr oducing an output , Zener Z I provides surge-protecticn clipping ar 20 V.
V.C.4.b. Output Isolator
V.C.4.c.
The input section of t he output isolator cu cuir U·igure 3-4lSJ is similar to the breaker t rip coil initialion circuir shown in
The input buffer circuir is shown in Figure 3-49. A normal 20- V signal will result in approximately 90 percent of the voltage appearing across R3 and capacitor Cl. When the
Input Buffer
3·25
voltage on CL builds up ro around 5 tu 7 V, cur reru t lows through Z2, Dl,and
RS is required when the output drives a PNP for a NPN stage. For interna! logic cír-
an output.
V.C.5.
Po wer Supply and Regularor
Cir cuits
R4. QJ then turnson, which produces
Since normal station battery voltages and solid-state devrces are not compatible, it is necessary to drop the common
stage but omitted
cuitry, Ql can be tumed on by the unbuffered input.
or 250-V supply to 20 Y for use in the logic circuits.
125-V
Two circuits for regulating this voltage reduction are shown
~ l + l 20Volts Oc LogoeS..ppty
in Figure 3-5 1 . R6
l.>nti.ke,ed Input Posmve
T~
Rl
Output
Zl
Input
R3
CI
Bottery Volloge
R4
J_~~~--~~--~_.__...~-
1
( - ) Nego11ve
Figure
3.49
Input
Buffer
C1rcu11
There are three t ypes uf buffering: high
1-i ) .:-. 1.6
= 20
IL
= 3 Amp
Then
carries one-half the burden, or
1.54 ohms on the 3-Amp tap.* N
in series.
Using the 100:5-tap,
turns
= 9.6 V Frorn the excitation
curve f'or V 5 uf 9 .6 V, Jt: wo ulu be 6
Arnp , and Nle equals 60. Theref'ore, the primary pickup current, IH,
= 4.62 Volts per transforruer "This valúe is slightly more than one-half of 3.0 ohms becausc of lhe secondary rcsistancc of the addcd currcnt transformer.
5·5
Then,
~eglccting t he e xciting current (Je). 1 his value woukl ht· come .?O times 5. 01 100 Arnp primary. when using t he 100:5 currcnr transtor mcr ratio.
from Figure 5-2. le= 0.33
3Cycles
2
3
VK
4
IRT Figure 5·11a: Current Transformer Time to Saturate.
6
2.0
2.5
Residual
flux
Any uon-core device will retarn a flux level even after the exciting curren! falls lo zero. Superimposed on this residual flux are variations in core flux, dictated by thc curren, transforrner secondary curren! and secondary burden. The residual flux may either aid or detract from transient flux performance, depending on the relative directions of the residual flux and the required flux variation.
Sotu,ot,on Curve
l-~
1.~
ÍRT
6.28
1 1 + 271' 6.28IRT
vt
)
While in t heory residual
flux can cause relayrng
there have been very few documented
proble ms
Vob
Ved' O
cases in which the
JIL(XH/n2 +XLI
residual flux has caused a relay misoperation.
lll. VOLl AGE TRANSFORMERS CAPACITANCE Vohage transformers
VOLTAGE (formerly
ers) and coupling capacitance
ANO COUPLlNG TRANSFORMERS
called potencial transtormvoltage transformers
lected according to two criteria: and the basic impulse insulation
111.B.
Couphng Capacitance
Volrage Transformen,
are se-
the system voltage level leve! required
Figure 5, 13: The Equivalent Circuit and Phasor Dlagram ot a Voltage Transformar.
by the sys-
Coupling capacuance bushing
capacitance
voltage transformers
tCCVT) and
voltage transformers are less expensive
5-12
than voltage transformers
but may be inferior in transient
With these potential devices, a subsidence
performance.
sients are introduced
a sud den reduction of voltage on the
transient
accompanies
primary.
This voltage may be oscillatory al 60 Hz or sorne
other Irequency , or it may be uni-directional. tive severe secondary
transient
voltage depends on the values of R, L, and C.
A representa-
is shown in Figure 5-14.
Other tran-
by the presence of ferroresonant
sup-
pression circuits and by the relays thernselves. A voltage transforrner
is not significantly
parable transients and will reproduce with excellent
affected
by com-
prirnary transients
Modero CCVTs, such as the PCA-7,
fidelity.
have capabililies approaching those of the voltage transformer. The subsidence transient of the CCVT rnay influence behavior of sorne relays. tan ce relays,
Solid-state
used in a zone 1 clirect trip function,
seriously affected by the ternporary during the
[3·!1
O ello Generolor
-
e
t'""º"" Reloy
ii-12
-
1¡12
Figura 6-3: Percentage Differential Relay Schematic for a Delta Connected Machina. (Only One Phase Connections are Shown)
.'\:, the grounding 1mpedance is incrcased, cu1renr·ly¡.>
Comparison or Relay and Generator Charactarrst ics (Time vs Negativa Sequence Current for an {12)2t Factor from 5 to 10).
K
2
i~ t he
lf 10 is small, íts elfect can be ignored. the rcla y w it h neutral made up inside.
rated 5000 kYA or larger.
transformer
for un·
bala nced faull protection are shown in Figure 6-1 O.
•
plicat ions .
is not normally
required
Othcrwíse , n wil] be
curren!
lhe COQ or SOQ relays are reco mme nded fer ali machines conncctions
2)
filter consiant
necessary to use either thc auxiliary Schcrnatic
is not uscd .
5 6 1 a 910
4
transf'orruer
or
The auxiliar y curretu in u nit-co nnectcd
ap-
6-7
tdd)'
vi lhi:. l)' pe vouiplemeuts llic: \...Ül..,1
O::,Ul..,1111 lC\..l.)g
Vl
ruzmg balanced !aults internal and externa!
to t he generatoi
lt also supplernents the COQ or SOQ relays by sensitively recogruzmg unbalanced [aults. lhe connection described above rnak es thc relay dircctional grves it reach
in
from the neutral, but
both drrcct ions trom thc voltagc truns-
tor mer locat ron A.,, a result, it will sense generator as wcll as cransformcr Iaults lhe k.D-11 is usually set to reuch thrcugh the unu transIorrner
Unlike single-phase
distance
relays, the reach of
che KO-type relays is not aff'ected by the phase shift through the bank. ..__4~6~\\
o
b
~~: }~;;·.{
e Mutual Reacio,
o)Connectlons When Neu1rol rs Mode Externolly (Omit 15:5 C1Jrrent T,onsfo,me, When Io "'d' o-. ts•v(l:,:,1:u lo Tr,p ,f Ocsirt!d
Cur.l!~ L ~od,ng PI lo,.,
Figure 6 12 Trlp Clrcults end R·X Diagram Show,ng Opera11on of The KLF (40) Loss of Fteld Relay
Maclune loss of [icld can result
f1u111 .,u> vi t11 .. lvlluw111ic:
K\.dU\,\.Ú
u::.ulb
LII .. diu¡,
ILL
llL\. ILLddllnc\
tc11111
IIJI voltugc ... ausing 11 tu dr.rw reactive power from t he system • An excessive voltage dccay mdicates l hat l he systcui IIIJ)' beco me unsiuble Conscqucnt ly, thc undcrvolt agc u1111 must be set to drop out at a voltage from wluch thc syst e m cannot re cove r dunng loss of excit auon. Alt hough lus 1s .i drf'hcult vulue to detcrrmne without ,1 transrent sluhiht y st ud y opcraung cx perrence will often mdrcate a cr iucal
• loss ot l1clu to the main excuer • J abcve use I mm. Advantagcs
units,
wh.:re there is danger uf explosion and fire from unburncd
1
,__
Ior (al
relay is commonly used with diesel engine generaling
Zone I volrage contact shorted Zone 2 dropout voltage sel ,t 80'.t
fue l. ~lotoung generator.
, c:.ults trom a low pri me·mover
When this input cannot meet ali the losses, lhe
deficiency is supplied by absorbing
1 umer = 1 u1io
tem.
input to the ac
real power from lhc sy:,-
Since ficld cxcitation should rema in the same, the
:.ame reactive power would flow as before motoring.
Less sensiuve to stable sysrem sw1ng~
1 ) More sensiuve 10 LOI' con-
1) Same JS l ). 2). and 31 Jl lefl
du son
lJ t'Jn cperare on part ut LOF
Thus,
on motoring, the real power will be into the machine,
whilt:
the reactive power may be cither ílowing out or into the
21 Pro,.,..,d~ ba"'k -up pro·
recuon
machine.
Usually the reactive powcr will be supplied lo the
system as machines are not generally operatcd underexcitcd.
3) Provide alarm
Icatures Ior manual opera·
Dunng generator motoring,
(ion
the current phasor will fall in in Figure 6· I 1.
the second or third quadrant, as shown
ally, the phasor will fall in lhe third quadrant, chine supplymg
S•t1ina
Zone 1 ( slone)
Zone 2 (alone)
Sce l-igure 6·16
See F ,gurc 6-16
Vo lt age
(a) unde rvolrage conract sbort ed or
gn
Sertrng
\O)
>Cf O(
801h 2011• 1 and Zonc 2
-
Zone 1 vohJg( t:onlJd shortcd w,1 h Zone 2 set ar 87 ¡,
87',1, for
1/4 lo 1 -ec
1/4 to 1 sec 11 /4 sec adequate l
fD·2 (See Fig. 6-17)
No1 required above
Ior
(J)
For ( b) above use I O sec íor J ircct ly cooled, 25 scc tor mdirect ly coolcd
Zone 1 tuner = 114 >1ot..:..:L1011
and cross-cornpound,
connectcd turbine gencrators. Figures 6-24 and 6-25 show thc recommcnded protect ion for machines t hat are not urut connected. Gencrally , such gcnerat ors are uscd in industrial
í
Wuh une
PRU'l l:.CI ION
Rl:.CUMMl:.NUl:.U
87G
A.:1..111ve--1-----
87T
Protecnve
Retoys
Oevice Number Char1 2 21
40 86G
41
46
51/27
SIN Negollv~------
59N
59NT 63
TO 5 1(0 11 KLF
o-
KLF I
F,eld Breaker 500
cov
ca
SV 7 16V
Figure 6-22 Overall Protecricn for a Tandem-Compound Unit Connected Genera1or
64
B6G
87G 87T ( J
CV8
SPR
DGF l "Brush" Machones Onty} WL Generator Lockout CA o, SA 1 Generator o,rrerent,at HU-1 Overau D1fferen1,al Number of Retays Aequired
6-17
e
4
83
41
~
4
1..P~LP ~9 NT
~9N
2
re
8
NCQ01,ve-
Alle,nole Ac Curren! Ctrcu,t W,lh E11he, Ring
+32N
(Refereocel
----~ 32N
-
lCS
-Vo
1
32N. ~
~
.
Resislot
~
. Phose ..' Re1Qys ¡,_ • __ .. ~-
~ t.+
...
l ,_
res
,r,•
~
1
g
·i_r
. .
J
32N
G1oundlng
'
r----1e
1
1 ,_
-
+ 3.?N
8
!
--
.
•
.Q
Pos,1,,Je--------
o b e
or Convent,onol Type Curren! Tronstormers
+
Atorm (Allernole Connecuon] CuneOI Should Not Exceed
32N
.
Figure 7-4· Typical Connections of The Product Tvpe CWP·l (32NJ tor Hígh Resistance Grounded Systems.
7-5
pickup, and n and K are constants that depend on the relay type and on the tune dial setting. lf a linear-linear plot oí' (I - 1 )º and t is used, a varying current and time can be compared with the relay characterístíc on an area basis. In Figure 7-7, for exarnple, the CO re lay contact will not close if the current drops below the CO pickup befare area A equals area B.
Typ,cal Motor Ccocbihty Curve
,/ Time
Locked
Rotor Time
1
Llneor-Llneo1 Ploí
Srcrtlnq Curve
Curren!
Figure 7·5: Motor Staning TIme Exceeding Permissible Locked Rotor Time.
is a trace of current against time (Figure 7-6). Ir ILR is ap-
/
Time
Current ~
CO Choroctensnc (51)
Are.o "A'' ~
plied to the CO relay for time ta• tbe contacts are very nearly cJosed. Current does noí drop below the CO pickup value untíl time lb. Contact closure occurs al sorne point te, e ven though the CO re lay characteristic is always a bove U1e current trace.
CO Pick-up
(I·llnScolF
lLR
Currenl
Figure 7-7, Area Comparison.
An alternatlve solution to lockcd-rotor problems for large motors is to use a distan ce relay and timer, The Impedance of the motor will remain fixecl (low at low pewer factor) If
CO Cnoroc1eris11c 151)
CO Corüocrs Clase CO Contncts Very
/
Nearl y crosed Here
Current TroeE
ca Pick-up Figure 7·6, CO Characteristic Compared to Current Trace.
Overa narrow range, such as that between two and three times pickup, a CO relay can be assurned to operare if the integral of (J - l )" dt exceeds K, where I is the m ultlple of
the motor does not accelerate, Tfthe motor aecelerates, both the irnpedance and the power factor will increase (Figure 7-8). The impedance of a motor wilh a locked rotor is essentially independent of terminal voltage, and, as the motor accelerates, íts impedance changes as indicated. This change of Impedance with motor acceleratíon makes the
distance relay particularly well suited to this application. lt also affords tirne-delayed backup protection for three-phase faults and lnstantaneous backup prctection for two-phase and the ground faults. If neither of the above schemes is applicable to a motor which has a starting time grearer than the allowable lockedrotor time, a mechanical zero-speed switch may be used. This devíce supervises an overcurrent unit and prevents its operatíng a timer once rotation is detected. Thís scherne will not detect a Iaílure to accelerate to full speed nor pulíout wíth continued rolation. as the other t wo schernes wUI.
VoltsAc
1
21~30
__ 52o
, 2
2
CV-4
21
1
00
GQod Pro1ect1on
TIme
lllllEf
/
lhaóeQiJOte ProteenonSrorttng
CV.LJ
Time
Note: Pnase Pnnse Onll cannot Opera te on loatT or on Starhng Cu«eo1
X
Cunenl
I a)
00Un,t IJOIOOdeó Motor
Loooao
: >-L-o:~~t----:;:::
Molar
W1th Locked Rotor Motor lmpedonce
Time
30Unlt
Good Rrotcct1on C0·5Rell'.ly (51~
Chonge of lmpedonce W1t11 Time os Motor Acéelérotes
-..
--....e;_"-"'.: :¡
Ovei Prorects
..,.,L.ocked
Slor11nQ T ime
Figure 7-8¡ K0-1 O Oistence Relay (211 U~ed for locked Rotor and Backup Protection for Larga Motor.
Rotor
Trme Is
Currem
(bl
V. OVERLOAD PROTECTION Heating curves are difficult to obtain and vary considerably wíth motor síze and ücsign. Further, these curves are an approximate average of an lmprecíse tbennaJ zone, where varying degrees of darnage or shortened insulation life rnay occur. lt is difficul L, then, for any .relay design to approximate these variable curves adequately over the tange from light sustained overloads to severe Iocked-rotor overload.
BL-1 (49)
Mo11;ir Cotrob1111 y
Time
-
? CO·S fletci~ (511 Currenr
Thermal overload relays offer good protection Ior light and
"
~ked ROtQI
llmb
IlR
Figure 7-9. Typícal Motor and Relay Time Current Characteristics.
medíum (long-duration) overloads, but may nor Ior heavy
overloads (Figure 7-9a). The long-time induotion overcurrent rele.y offers good protectíon for the heavy overloads, bu t overprotects for lígh; and medium overloads ( Figure 7-9b). A combination of the two devíces provides complete thermal protectíon (Figure 7·9c). Toe National Eleotric Code requires that an ovecload device be used in each phase of a motor "unless protected by other
approved means." This requirement is necessary because single phasing (opening one supply lead) in the primary of a delta-wye transforrner that supplles a motor will produce three-phase motor currents in a 2 1.1 relationshlp. If the [\VO uníts of current appeared in a phase with no overload device, the motor would be unprorected. Thus, the NEC tequtres three overload devices, or two overíoad devlces and an approved substituto, such as a CM or CVQ relay.
7-7
VI. THERMAL RELA YS
The DT-3 relay is a d' Arsonval-type de con tact-m aking milliammeter which is connecred across the bridge. Thc bridge
There are two types of thermal relays, Those such as the
is energized by either 125 or 250 Vdc or supplied with 120
CT and DT-3 opérate
Vac through a transforrner and full-wave bridge recrifier in
from exploring coils embedded in the
is callbrated Erom either so? lo
machíne windlngs. They are applled only to large motors,
the relay , The relay scale
usuaííy 1500 H P and up, where exploríng coils are available
l 90°C ( or 1 00° to l 60ºC). Toe right- or left-hand con-
when specified.
tacts e lose when the ternperatu re rises or falls to the preset
Replíca-type therrnal overcurre nt relays,
so? and
value between
Vl.A.
No eurrent-responsive relay can protcct a motor subjected
CT and DT-3 Relays (49)
l 90°c (or
looº and
such as the BL-1, operate directly from the cutrent dra wn by the machíne.
160ºC). The normal setting for class B machines is 120°c.
to block ed ventilation, The D1'-3 relay overcornes this
The CT and DT-3 are bridge-rype relays, The exploring coils form part of a Wheatstone Bridge circult, which is balanced at a given temperature.
As che motor ternperature in-
creases above the balance temperature, operating torque is produced (Figures 7-1 O and 7-11). With the DT-3 relay, only one resistance temperature detector (RTD) (10 ohm,
100 ohm, or 120 ohm) or exploring coil is requíred: lhe CT relay requires two (10 ohm only),
shortcorning by rcsponding to temperature alone.
Vl.B. Therrnal Replica Relays The replica-type relays (BL-l) are designed to replicare, within the relay operating unlt, the heating characteristics of the machine. Thus, when current frorn the current transformer secondary passes through the relay , its timeovercurrent characteristlc approximately parallels thal of
U1e machine capability curve a! modérate overload. Extreme variarious in load, such as jogging, produce a difñcult relayíng problern,
In general, thermal replica relays
cool ata different rate fron, the motor they protect. Variations in load 1nay produce a ratcheting effect on the relay a.nd cause premature tripping. Neither the DT-3 nor the
CT relay is susceptible to lhis proble1n. Since motor insuJation can support
Re5,stance Temceroture Oetecto, ,n Moch,ne W,nchnqs
temperature ín e.xcess
of its rating for a short tin1e, ternpera(ure
sensitivit-y is not
the ultin1ate criterion of relay capability.
Where w!tle load
varja tions are expected, the CT relay is preferred to Ute replica type, beca use there 1nust be overteruperature and over-
Figure 7-1 O: Typical Schematic of The Type DT-3 Relay (49) for Motor Overload Protection (lt s Aclln1n1ages are G(>Od Protecuon lor Overroacl. Blocked Ventllatton and 11,gh Arnbiaru lempermure Operanon J
curten t sln1ultane_ously for tripping to take place. As the titne curves of Fíg,urt: 7-11 sllow, i:he CT relay wi11 tolerate the allowabJe loading condition, 1vhereas a relay solely responsive to tcmperature woulJ trip off the motor.
The CT operating unit is an induction disc, with the two torque-producing windings connected across the bridge. Cur-
The thermal repHca relay is recornmencled
rent Ilowing through a current transrormer from one phase
lays are reco1nn1enlllilU~
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•pflhuullon. 'l li,U dJ111dlln • ,li,;c 110111 ) ilf I L1 MVA, 11l ..;yl¡qc "'ffi!I fmm ~Ñ J ~ tu "?6! kV n11;r 911 M .-1111l* fi!h,.., • ,,,.._ U!m-. •il fflf"i~ fy~ ..,¡¡~~1*-' t.r •::f'p,t'cl-lJtm•..»ll'4\, r1w coilll1H'liolll!llíQ)' IIC ( 1 1 ibm11Jy 10 t\.r ,...,.-,,m,11,un, Íl'Nllf4lfl tn'f!011lt1nh1y:o¡,mflfflf•!'l1,. Ulll.Ul1"tlU1t t11111.11L~1 !1ik•.h.•w• •n4111fll l'•~•t,$UA11t.11t1 w'i1jl tbt J,.nt ~ur.m.._••nft- 1 l'ltM ,~.¡ 11111,ll.,.flflnf •111m ~•!r,,fflllfl, 1
--
~--.
Pffutlf ~ ;Jto..t~
~'ltlT)
'.
W't•l(II 1u, C11:11111tq¡tlllll 1111¡:t:.flV \ll
!k1•J1•11• ~
~tu.i.t ....: 111 •b ,nr • pii11 ul! LW,...u..1..u.uw:alUll unuiu 1..-(o cuj! b,-¡;..,,..,. 11111iflui lhct1 C:\P).{Ílllll1'i1 rmuucn \ "~ ) "'º•I ti
S.ip,1111.1• 111~tli:u·p11111u:uut1 ll!Li•o 4u: ·~11 ... wl11,,I Wi.c::i., rn.1.u.....6 U\r .h:uu:Uir1111:r j1,,,..111m1 m111,w,e1l,11••.hu1111• !\•1111;! 111 Ji.a:111, 1 j1t• 611rM 1111t1lh1l TM111tL110 ••.iu., m, pr1lfH'hO!l fn ti, '"fr1IOl1d
llldM1i1 1.11 ,ia 11urntm trn t.. ,tn lf!.Hlil~ ni, muJ'\11 1•1•t01J< Uult, ~ 1.1u.. li&llat.wU11\&J.l.tl..,~11t1 lll.w.~t!llt:f"
1111 1 'º'l'f'l 1111 n11,,.1, ... l11111ht fo '411111 •••"" llti! p,,,-11 Hh) ,11 10111111 '"l+l1111• Umll'11 ! la.Oett!II
Ílllcitl¡IUI .._.nti !ll!"'Jlt1HW'U:tl1 llt111114'Jfllfl fllu IUIU11Ulll1 .. ll..il.11g,
J~wu,, ,..µ
JI! '1111Wrl !1t
••f, h• l\ltffill
wtl
o 1 s A na,,,
lllh.1 ._ ,.J"lí'' A 111, tol"', lV{tM11l ,•do11• l•ulit whltildO m,l lt1... w, ;t«>11ml c11mtt).l oc.:w u ffllil!t w11 ,~,n (h.,i 1;.iink. Tn,,vt\Jrll\ 1h11 JiOd t ,1,1l.y1111t• ~~ ... rt'l,Ü;,Uc.iü litt~li .i..u.ld.JI. ..uaU.wJ... U111 ~
,-."'ª'
Sin,i.~ Lb: #iluo (:t$Uldl.Uu:t to uouu1¡ lrnptt¡:1111111 nJ I lm .i.Jl•~ .. ta:,1 r~)ll.:n',.... ~,~y
.tll!J.IJ
(WI 4.Ü;•wll
III t.b,r.-11,n·1111Jt1tnu
dÚ§UUl:t..
Wl1J" ti~ in iflHr) l,lll'f'Qf fa1 1' ¡mnml (1~,I ii¡ !.IUI( I 1!11.ifJ. il\~ Mco1'11h11 wnnl •,u lill l;ff~ U W.ll
wfih.
IDCV.f IMl.1Ull imr,,tllll'l'-4
or 7.a:
, 111n..t•i> ai111i.11w- m~wulh íftl' O.O'JmNlll 1b
im·SAL
Fo1
,ni. c.01110·
º" i1tm f .82 (0.93 + 1 .07) 60,000 400
= 246
~ .....
/
1
100
+ 1 .07) 60,000 = O B
400
,
....
..
200
(0.93
= 345
550
s
For the three-phase fault condition:
400
In Figure 9-12 using 345, the lT unit setting is determined to be 43 amperes for 4 disc KAB and 12 amperes for onedisc KAB.
+
V.A. Settings for the V Voltage Unit
F
+ J.07) 60,000 = 300
vi. PROTECTING A BUS THAT INCLUDES A TRANSFORMER BANK
For the phase-to-ground fault condition: (0.93
+2
.07) 45,000 400 X 375 X J
= o.sz
(9-5)
And from Figure 9-10, K>0.77, therefore: VR2: .77 (.93
= 266
45,000
+ 2 x 1.07) ~
Volts
(9-4)
Ideally , where the bus includes a power transformer bank, separate protection should be provided for the bus and for the transformer, even though both protection schemes must trip ali breakers around the two units. Such a system offers máximum continuity of service, since faults are easier to loca te and isolate. Also, using a bus-diff'erential relay for bus protection and a transforrner-differential relay for transformer protection provides maximum sensitivity and security with mínimum application engineering.
9-12
However, econornics and location of current transformers often dictate that both units be protected in one differential zone. For these applications, either the rnulti-restraint HU-4 or the CA-26 relays should be used. The HU-4 relay is similar to the HU and HU-1 relays, except that it has four restraint windings. Also, the rectified outputs of the restraint transformers are connected in series, providing a higher restraint force when a through fault occurs on the bus. Since the de saturation of current transformers will allow current to pass in the HRU transformers and possibly pick up the IIT, the llT unit of the HU-4 relay is set at 15 times the rms tap value to prevent false tripping for externa! faults. Similar to the CA-16, the CA-26 relay has a stronger contact spring and a higher pickup of 1.25 ±5% amperes to help override inrush. Its variable restraint curve is more inverse than the CA-16, and its operating time is approximately 3 cycles.
Of the two types, the HU-4 relay is preferred, as it is immune frorn operation 011 transforrner magnetizing inrush. The HU-4 should always be applied for large transformer banks or for banks associated with 1-IV and EHV busses. A typical application, shown in Figure 9-13, protects a threewinding transformer bus with four circuits. Figure 9-14 shows another typical application used in EHV systems. The CA-26 relay is applicable for relatively small transformers remote from generating stations, HV, and EHV busscs. Here, inrush will usually be light and will not cause the CA-26 to operate. If', however, complete security against inrush is required, the HU-4 must be applied, With the CA-26 relays, the Iour-circuit bus connections of Figure 9-7 are not recommended Ior bus protection, since the relay may have too much restraint when energlzing the bus on a f'ault. The bus CA-16 relay should not be used for transformer differential, since it is too sensitive to override magnetizing inrush.
VII. OTHER BUS PROTECTIVE SCHEMES H
Harmonic Restroint Unit (HRUJ Differentiol Unit Operating Winding ond lnstontoineous Trip lT Unit Figure 9-13:
Typical Applicatíon of HU-4 Relay for Protecting a Large Transformar Bank Associated With HV and EHV Busses. (Auxiliary Current Transformers for Ratio Matching are Not Shown)
Restro,nt
Auxiliory CT forRotio
Restrolnt Figure 9· 14: Protection of a Typical Transformer Section Where The Transformer Tertiary is Brought Out for Loador Connected toan Externa! Source.
Other methods for protecting busses are in limited use: ( J) current-differential schemes with overcurrent relays, (2) partial-differential schernes, (3) directional-cornparison relaying, and (4) the fault-bus method. Except for the latter, these schemes are most often applied as economic compromises for the protection of busses that have been in service for many years. VII.A. Overcurrent-Differential
Relaying
The differcntial circuit is obtained by paralleling ali the current transformers with an induction-disc overcurrent relay across their output. Relays must be set above thc maximum false-difference current for an externa! fault. That is, very little saturation can be allowed if any degree of internalfault sensitivity is to be obtained. A certain amount of de or ac saturation can be tolerated, beca use ( 1) the operation of induction-disc relays on the de component is less efficient, and (2) the relay operation is not instantaneous. To increase the response of these schemes, the de time decrement must be short. This requirement virtually limits applications to substation busses remete from large generaring stations. The most common application is in a grounddifferential systern where the critica! externa! fault is a
9-13
double line-to-ground
fault.
The phase-fault
can cause the current transformers these applications,
to saturate unequally.
In
is required
Ali contacts
While the relay cost is low, the
engineering cost is usually high, since considerable
into the bus section. lays open contacts
the C0-2 relay is used with operating
times of I O to 18 cycles. experience
The directional relays close contacts when fault power flows
components
study or
the dírection
re·
on the feeder.
in series, and, when the fault is is energized
time delay of severa! cycles will perrnit
to assure correct operation.
Partial-Differential
are connected
on the bus, the trip circuit cide correctly
Vll.B.
Back contacts on the overcurrent when the fault is externa!
through
a timer.
ali the relays
A
to de·
of the fault.
Relaying of this scherne is the large number of con· tacts and complex connections required. There is also the remote possibility of the directional elements not operating on a solid three-phase bus fault as a result of zero voltage.
The disadvantage In this scheme, connected,
using an overcurrent
relays protecting ferential.
only the source circuits are differentially relay with time delay.
The
thc feeders or circuits are not in the dif-
Essentially , this arrangement
delay bus protection
combines
time-
with feeder backup protection.
VII.O. Fault Bus (Ground Fault Protection Only)
Where some or ali of the feeder circuits have current-lirniting reactors, a partial-differential type relays.
circuit
These distance-type
through , the reactor impedance. used to select between on the feeders. VII.e.
is used with distance-
relays are set into, but not The reactor impedance
The scheme is both fast and sensitive.
Dírectional-Comparison
Directional-comparison
is
faults on the bus and externa! faults
Relaying
relaying uses individual
relays on ali sources and overcurrent
directional
relays on all feeders.
This method requires that all the bus-supporting structure and associated equiprnent be interconnected and have only one connection to ground. An overcurrent relay is connected in this ground path. Any ground fault will cause fault current to flow through the relay circuit, tripping the bus through the multi-contact auxiliary tripping relay. A Iault detector, energized from the neutral of the grounded transformer or generator, prevents accidental tripping. This scheme requires special construction measures and is expensive,
10-1
Chapter 10
Line and Circuit Protection Author: J. V. Kresser & J. L. Blackburn l. Introduction A. Techniques for Line Protection B. Relay Protection Systems for Line Faulls C. Selecting a Protection Systern II. Distribution Circuit Protection A. B. C. D. E. F. G. H. J. J. K. L.
Relay Application to Radial Feeders Coordination Coordination Time Interval Selecting an Overcurrent Relay Tap Effect of Extended Load Outage/Cold-Load Inrush Selecting an Overcurrent Relay Time Curve Jnstantaneous Trip Applications Phase and Ground Relays Fuse and Relay Coordination Coordinating with Reclosers and Sectionalizers Distribution Feeder Protection Systerns Coordinating with Low Voltage Breakers
III. Subtransmission Circuit Protection A. Criteria for Directional Relays B. Using Overcurrent Relays to Protect Loop Circuits with a Single Source C. Coordination on Single-Source Loop Systerns D. Using Inverse Time-Distance Relays to Protect Single-Source Loop Circuits E. Protecting Loop Circuits with Taps F. Short Loop Circuits G. Using Overcurrent Relays to Protect Loop Circuits with Multiple Sources H. Coordination on Múltiple-Loop Systems 1. Setting for Relays M and N 2. Setting for Relay L to Coordinate with Relays M and N 3. Setting for Relay B to Coordinate with Relays H and J
4. Setting for Relay D to Coordinate with Relays B and L 5. Setting for Relay C to Coordinate with Relays E and J 6. Setting for Relay A to Coordinate with Relays C and L 7. Final check: Relay B Coordination with Relay H l. Protecting Loop Circuits with Multiple Sources Using Inverse-Tirne Distance Relays
IV. Transmission Circuit Protection A. B. C. D. E. F.
Fundamentals of Distance Relaying The K-DAR Phase-Distance Relays The lnfeed Effect on Distance Relays Effect of Tapped Transformer Banks Zone Application of Distance Relays Distance Relays with Transformer Banks at the Terminals G. Distan ce Re lay Characteristics H. Are Resistance and Phase-Dístance Relays J. An Example of Distance Relay Application and Setting V. Ground-Fault Protection A. Distribution Circuit Protection B. Subtransmission Circuit Protection C. Directional Ground Relay Polarization 1. Voltage (Potential) Polarization 2. Current Polarization D. Mutual lnduction and Ground Relay Directional Sensing E. Negative Sequence Directional Units for Ground Relaying F. Evaluation of Ground Relay Polarizing Methods G. Fundamentals of Ground-Distance Relaying 1. Using Zero Sequence Quantities 2. Using Modified Phase-to-Ground Voltage and Zero Sequence Current 3. Using Phase-to-Ground Voltage and Modified Phase Current H. The KDXG Reactance Ground-Distance Relay l. The SDG Solid-State Ground-Distance Relays J. Fault Resistan ce and Ground-Distance Relays K. Mutual lmpedance and Ground-Distance Relays L. Transmission Circuit Protection VI. Series-Compensated Transmission Line Protection VII. Protecting Direct-Current Systems A. Principie Types of Direct-Current Relays B. DC Equipment Protection C. Circuit Protection D. High Voltage Direct-Current Transmission
10-2
Chaptcr
l. INTRODUCTION
10
Line and Circuit Protection
CLASSIFICATION
OF ELECTRIC POWER LINES
Alternating-currcnt lines are commonly classified by function, which is rclatcd to voltage level. While there are no utility-wide standards, typical classifications are as follows: a. Distribution (2.4-34.S kV): Circuits transmitting power to t he final retail outlets. b. Suhtransrnission ( 13.8-138 kV): Circuits transmitting power to distribu Lion substations and to bulk retail outlets. c. Transmission (69-765 k V): Circuits transmiu ing power between major substations or interconnecting systems, and to wholesale outlets. Transmission lines are further divided into: l. 1-ligh voltage ()IV):
345-765 kV
3. Ultra high voltage (UHV):
greater than 765 kV
Low voltagc (24-250 V): Auxiliary power in power plants and substations; control circuits; and, occasionally, utilization power in sorne industrial plants, industry.
c. High voltagc (grcater than 600 V): Long-distance bulk transrnission, submarino, and major system interconnections.
There are seven protective techniques commonly used for isolating faults on power lines: a. lnstantaneous overcurrent b. Tirne-overcurrent c. DirectionaJ instantaneous and/or tirne-overcurrent d. Step tirne-overcurrent e. Inverse tirnc-distance
g. Pilot relaying (see Chapters 14, IS, and 16).
2. Extra high voltage (EHV):
b. Medium vollage (300-600 V): Transportation
I.A. Techniques for Line Protection
f. Zone distance
11 S-230 kV
Direct-current systcms can be classified as follows: a.
Most faults experienced in a power system occur on thc lines connecting generating sources with usage points. Justas these circuits vary widely in their characteristics, configurations, lengths, and relativo importance, so do lheir protection schemes. * Sorne of the more common and widely used protection schcrnes wiJI be detailed below and in Chapters 14, 1 S, and 16.
I.B.
Relay Protection Systems for Line Faults
Relay systerns for phase-fault protection of power lines are outlined in Table 10-I, those for ground faults in Table 10-11. The last column in each table gives the approxirnate relative per-unit relay equipment system cost of thc basic timeovercurrent (CO) systern. I.C. Selecting a Protection System Severa! fundamental factors influence the final choice of the protcction applied to a power line: d.
Type of circuir: cable, overhead, single line, parallel línes, multi-terrninal, etc.
b. Line function and irnportance: effect on service continuity, realistic and practica! time requirements to isolate the fault from the rest of the system.
10-3
Coordination and matching requirernents: cornpatibility with equipment on the associated lines and systerns. To these three considerations must be added economic factors and the relay engineer's preferences based on his technical knowledge and experience. Because of these many considerations, it is not possible to establish firm rules for line protection. Toe remainder of this chapter, however, will focus on basic application rules and coordination proeedures to aid the engineer in the selection of proper protective systerns for distribution, subtransmission, and transmission circuits for both phase and ground faults. Also covered are series-cornpensated transmission lines; and de
Table 10-II Relay Protection Systems for Ground Faults
Basic Relay Type
Number Required
co
1
1.00
Time-Overcurrent
COwith IIT
1
1.30
Product Overcurrent
CWCorCWP
1
2.20
CWC or CWP with IIT
1
2.SO
1
2.65
CRD
1
2.85
CRC or CRP with IIT
1
2.95
CRD with IIT
1
3.IS
KRCorKRP
1
3.60
KRD
1
3.80
IRCor IRP
1
4.10
IRD
1
4.40
CRQ
1
4.65
1
4.75
KRQ
1
6.10
IRQ
1
7.00
plus SDG-4T, plus Tl).5
3
27.50
three KDXG,
4
30.00
5
43.70
Type oí Protection Tirne-Overcurrem lnstantaneous
Instantaneous
and
and
Product Overcurrent Directional Time-
Overcurrent
systems.
Relative Cost (pu)
CRCorCRP
Directional Time· Overcurrent
Table 10-1 Relay Protection Systerns for Phase Faults
lnstantaneous and Directional Time· Overcurrent
Type oí Protection
Basic Rtlay Type
Nu,n~r Required
Relative Cost (pu)
lnstantaneous
and
Directional Time·
co
3
1.00
CO with IIT
3
1.30
Time-Overcurrent
Directional
lmtantaneous and Time· 0.ercurrent
CR
2.70
and Oirectional
Directional TimeCR with IIT
3
2.90
O.ercurrent
KRV
3
3.40
Slep Time-Overcurrent
C0-4
3
3.80
lnstantaneous
and Directional Time· Overcurrent Directional
Time·
Overcurrent
Directional lnstantaneous and Directional Time·
lnstantancous
0.ercurrent
IRV lWO
and Direc-
4.10
tional Time-Overcurrent
KD-10,
plus 2-Element
Directional 3
6.75
Overcurrent
or K D·S plus SI). 2
2
8.30
and Dlrectional
KD-10.
Directional
plus TD-4
Instantaneous Time-
4
10.20
Zone Distance
SDG-2T
two SKD-T plus SK D· l T, plus TD-4
Zone Distance
IIT
two KD·IO, plus KD-11.
Zone Distance
CRQw11h
lnstantancous
CO
Overcurren t Zone Distance
Time·
Overcurrent Direcrional
Directional lnstantaneous
lnverse Time-Distance
Overcurren t Directional lnstantaneous
lnsuntaneous and
lnverse Time-Distance
lnstantaneous
Overcurrent
Dlrectíonal lnstantaneous
Direclional Time· 0.ercurrent
Oeercurrent
Overcurrcnt
4
13.20
Zone Distance
two SKD-T plus SKD·IT,
plus TD-52
Zone Distance
14.20
plus KRT
two SDG-2T plus SDG-4T, plus two TD-5
L0-4
U. DISTRIBUTION CIRCUIT PROTECTION Lines from distribution substations are usually radial to the load area. Open loops, operating normally as radial lines, provide alternate supplies to the load from different substations. These loops may be temporarily closed for po wer to flow between the substations; that is, in either direction in the line. For the purpose of relay application, a feeder is considered to be radial if, ata particular relay location, the máximum back-feed (fault current in the non-trip direction) is less than 25 percent of the mínimum fault current for which the protective relay must operate. The following discussion covers protection systems for radial distribution circuits. Il.A.
Relay Application to Radial Feeders
Radial feeders can be protected by non-directional overcurrent relays. Figure 10-1 shows severa! sections of a typical radial feeder. Because the feeder is radial, each section requires only one circuit breaker at the source or distribution substation end. To clear faults at (1) and other faults to the right, then, only the brcaker at R need be tripped. To clear faults at (2) and (3) and in the area between thern, the breaker at H must be tripped. Likewise, to clear faults at (4) and (5) and between them, the breaker at G must be tripped.
R ZHR
Lood Figure 10.1:
Lood
(2)
( 1)
Lood
A Typical Radial Feeder
None of the relays at the breaker locations can distinguish whether the remote fault is on the protected line, on the remote bus, or on an adjacent line. The relays at H, for exarnple, cannot distinguish between faults at (1) and at (2), since the current magnitude measured at H will be the same in either case. Opening breaker H for fault (1) is not desirable, since it would interrupt the load at R unnecessarily. Two techniques are available to solve this problem: time delay or pilot relaying. The later requires a communication channel between the two statíons and will be covered in later chapters.
Time relaying delays the operation of the relay for a remote fault, allowing relays and breakers closer to the fault to clear it, if possible. In the exarnple shown in Figure 10-1, relays at H will delay for faults at (1) or (2). lf the fault is at ( 1 ), this dela y will allow the R relays and breaker to opera te before H. Thus, although H would not open for a fault at (1) (unless the R relays & associated breaker failed), it would operate for a fault at (2). This technique, called coordination or selectivity , is designed to combine mínimum operating time for the close-in faults with a long enough delay for remo te faults. In Figure 10-1, for exarnple, the relays and breaker at R must coordinate or select with those to the right (not shown); H must coordinate with R, and G with H. Relays are coordinated in pairs. lf, in Figure 10-1, breaker H relay trip characteristics have already been coordinated with whatever protective devices exist at R and beyond, the breaker at G must then be coordinated with those at H. For the three critica! fault points-(5), (3), and (2)-the following data are required: a. Fault at (5):
Maximum and mínimum fault currents.
b. Fault at (3 ): Maximum fault current, which determines the required coordination between breakers G and H. c. Fault at (2): Mínimum fault current, which determines when the G relays must opérate to provide backup protection for faults on line HR not cleared by the breaker at H. U.B. Coordination Relays within a systern can be coordinated using graphs or tables, although graphs are generally more useful for radial systems. Serni-log (log absissae for current and linear ordinate for time) or log-log paper can be used. Log-log is preferred when a number of different types of devices, including Cuses, are being coordinated on one graph. The current scale can be in primary amperes or per unit. Any dífference in current transformer ratíos must be taken into consideration when determining actual relay currents at different locations. The coordination procedure is conducted as follows (Figure 10-2). First, assume that the desired relay type (tap tange and time characteristic) and current transformer ratio have been determined. (The selection of these variables will be díscussed later.) Then:
10-5
Moximum Foulls
(1)-(2)
Mínimum
!
1
11
( 1 )-(2)
Mox,mum
1 1
I
1 1 1
(3)-(4)
1 1
(3)-(4)
Mox,mum
Mnu;num
1
1
1
'1
1 1
(5)
1
1
(5)
Moximum
i
l
Set Points
1
e. Select a tap for relay G to operate for fault (2) minimum and, for a phase relay, not to opérate on rnaximum load. The fault (2) minirnum should operate the relay on at least twice pickup, although compromises may be necessary (see Section 11.D. below). For phase relays, the setting must always be above the maximum load.
1 1
t
1 1 1
~
1
Curren! BoclóOº) of the pickup
linc protcction.
For time-overcurrent
ovcrcurrent
units (ussurning sufficicnl
tault ami thc remole bus). a d ircct ional rclay should
opera te only
when fault current flows in the spccif'ied lripping direct ion, they avoid both this cornplex
lines, such as ar
10-15.
an
11 max
--> ITpu -- 0.90 111.B.
(or 0.80 for source angles
t:sing Dircctional Loop Circuits
Ovcrcurrent
Relays
>
60 o )
to Protccl
with a Single Source
A loop circuit with a single sourcc is shown in Figure 10-16. a. The máximum
fault current (11)
through
the relay for a
For the purposes of thc following tliscussion,
fault on bus C excccds 0.25 times thc mínimum fault
in this circuit will be considered
current 12 through the relay for a fault on the remote
at least for a significan!
bus H for the protcction
of the line only or 13 thru uie
relay for a tault on thc re mote bus R for the protecrion
ali brcakers
closed during operation,
amount of rime.
open, the system becomes radial.)
(Should a brcakcr
The re lay application
rules are similar to those for radial circuits, cxt:cpt that di· rectional relays will be rcquirctl I O. Non-directional
ovcrcurrcnt
al all localions
but I ancl
relays can be used at 1 and
I O. since no current ftows through
these locations
for faults
in thc sourcc syslém an
