Tit le Titl e of Bo Book:: ok
ower
ys em ro ec ve e ay ng
By:: By
. BSEE 79 / BSECE 80 – MIT 1ST EDITION
INTRODUCTION Since the mid twentieth century, electricity has been an essential part of our lives. Electricity powers, our appliances, office equipment and countless other devices and apparatus that we use to make life safer, easier and more interesting. Use of electric power is something we take ,
,
man’s useful resources making our life convenient and relaxing through its various usefulness.
2 – TIME DELAY STARTING OR OR CLOSING CLOSING RELAY. RELAY. 3 – CHECKING OR INTERLOCKING INTERLOCKING RELAY. RELAY. 21 – DISTANCE RELAY. 25 – SYNCHRONIZING OR SYNCHRONISM CHECK RELAY. – . 30 – ANNUCIATOR RELAY. 32 – DIRECTIONAL POWER RELAY. RELAY. 37 – UNDERCURRENT OR UNDERPOWER UNDERPOWER RELAY. RELAY.
– . 46 – REVERSE PHASE OR PHASE BALANCE CURRENT RELAY. 49 – MACHINE OR TRANSFORMER THERMAL RELAY. 50 – INSTANTANEOUS OVERCURRENT OR RATE OF RISE RELAY. 51 – AC TIME OVERCURRENT RELAY. 52 – AC CIRCUIT BREAKER. 52A – CIRCUIT BREAKER UXILLIARY SWITCH – NORMALLY OPEN. 52B – CIRCUIT BREAKER AUXILLIARY SWITCH – NORMALLY CLOSED. 55 – POWER FACTOR RELAY. 56 – FIELD APPLICATION RELAY. 59 – OVERVOLTAGE RELAY. –
64 – EARTH FAULT PROTECTIVE PROTECTIVE RELAY. RELAY. 67 – AC DIRECTIONAL OVERCURRENT RELAY. 68 – BLOCKING RELAY. RELAY. 74 – ALARM RELAY. – . 78 – PHASE ANGLE MEASURING OR OR OUT OUT OF OF STEP STEP PROTECTIVE RELAY. 79 – AC RECLOSING RELAY. 81 – FREQUENCY RELAY. 83 – AUTOMATIC SELECTIVE CONTROL OR TRANSFER RELAY. – . 86 – LOCKING OUT RELAY. 87 – DIFFERENTIAL PROTECTIVE RELAY.
51N – RESIDUAL GROUND OVERCURRENT RELAY 0.5 – 2.5 A. A. 63 – SUDDEN PRESSURE RELAY. RELAY. 63X – AUXILLIARY RELAY FOR SUDDEN PRESSURE. 79 – RECLOSING RELAY, 33 SHOT SHOT DC DC OPERATED OPERATED TIMER. TIMER. – . 87B – BUS DIFFERENTIAL RELAY, HIGH SPEED, HIGH INPEDANCE VOLTAGE UNIT WITH LOW INPEDANCE INSTANTANEOUS OVERCURRENT UNIT. 87T – TRANSFORMER BANK DIFFERENTIAL RELAY, PERCENTAGE, 2 RESTRAINTS.
87G – RESTRICTED EARTH FAULT RELAY. – . A – AMMETER KWH – BILLING KILOWATT HOUR METER W/ DEMAND INDICATOR. VAR – VARMETER – W – WATTMETER AS – AMMETER SWITCH VS – VOLTMETER SWITCH – , GT OVERCURRENT RELAY. 12 – ZONE PACKAGE, MHO CHARACTERISTICS, COMPENSATOR DISTANCE RELAY – . 21 Z-2
– COMPENSATOR ZONE PACKAGED, MHO CHARACTERISTICS WITH OFFSET OPTION, DISTANCE RELAY (ZONE – 2).
21 -
- ZONE PACKAGED, MHO CHARACTERISTICS WITH ,
– 3).NONDIRECTIONAL INSTANTANEOUS 50H – (ZONE HIGH SET OVERCURRENT RELAY. 2 – TOW – ZONE TIMING AUXILLIARY RELAY (ATARTING OR CLOSING). 27 – UNDERVOLTAGE RELAY, 115 VOLTS. – SCHEME. 50 – INSTANTANEOUS OVERCURRENT RELAYS. 50--51– PHASE OVERCURRENT RELAY WITH INSTATANEOUS 50 1.0 – 12 A (6NEUTRAL -14 A ITT).GROUND OVERCURRENT 51G – UNIT TRANSFORMER RELAY 0.5 – 2.5 A.
Part 1:
P o w e r S s t e m P r o t e ct i o n
Electri c Pow er System
Electricity is generated at a power plant (1), voltage is “stepped-up” for transmission (2). energy travels along a transmission line to the area where the power is needed (3). voltage is decreased or “stepped-down,” at another substation (4), and a distribution power line (5) carries that electricity until it reaches a home .
Power System
Delivery Substation
Delivery Substation B
A
ac one Looped Lin es C
Distribution Radial Lin es
Power System Protection
INCOMING 115kV LINE 2
INCOMING 115kV LINE 1
LEGEND: Initial
Bus No. 1
115KV Switch yard
us
o.
Bank #3
Bank #2
Bank #1
83 MVA PXF with OLTC
34.5kV switchgear
34.5KV Switchy ard
F1
F2
F3
No. 1
F4
Bus tie 7.2 MVAR Capacitor Bank
F1
F2
F3
F4
7.2 MVAR Capacitor Bank
Bus tie
F1
F2
TYPICAL DISTRIBUTION SUBSTATION Single Line Diagram
F3
F4
7.2 MVAR Capacitor Bank
What i s Pow er System Protection? also known as Protective Relaying ranc o ec r c ower Engineering Science,, Art and Skill in applying Science devices.
Protective relays are devices which monitor power system conditions and o p e r a e o q u c y a n a c c u ra e y isolate faults or dangerous conditions. A well desi ned protective system can limit damage to equipment, as well as minimize the interruption.
Pur po se of Sys tem Pro tect io n To prevent injury to personnel components To limit the extent and duration of service interruption
Com po nent s of Pow er Sys tem Protection Relays Circuit Breakers Tripping and Auxiliary Supplies
Components of Power System Protection Decid es wh ether sys tem quantities are normal or abnormal (Brain of the
Power System
Transducers (PT&CT)
Circuit Breaker
Relay
* electrical quantities to level relays can use, i.e., 5 ampe res, 115 vol ts
,
no si gnal is s ent to brea ker * If qu antiti es are abno rm al, signal is sent to br eaker to
Sensor (Inst rum ent Transformers)
Feedback Signals
Relay Power Circ uit Brea ker
Components of Power System Protection
CB
CT
Transmission Line Trip Coil
Station
Relay Contacts
Abnormalities in Power ystems Overcurrent (overloa d, shor t c irc uit , open Ground Potential (ung rou nded e qui pment, , Surge Voltages (ligh tning s trok es, swit chin g sur es harmonics
Frequ enc y of Typ es of Faults Type Faultof
% Occurrence
SLG LL DLG 3L
85 8 5 2 or less
Equipment
% of Total
Overhead lines Cables
50 10
Transformers CTs and PTs
12 2
Control Equipment Miscellaneous
3 8
Fact or s Whi ch Inf lu enc e Desi gn of a Pro tect iv e Sys tem Reliability Dependability ecur y Sensitivity Selectivit Speed Economics Experience Industry Standards
Fact or s Whi ch Inf lu enc e Desi gn o a ro ec ve ys em Reliability The level of assurance that the rela will function as intended. Reliability denotes:
Dependability - certainty of correct operation
Security - assurance against incorrect incorrect operation operation
Fact or s Whi ch Inf lu enc e Desi gn o a ro ec ve ys em Sensitivity that it will operate when required Must discriminate normal from abnormal conditions.
Fact or s Whi ch Inf lu enc e Desi gn o a ro ec ve ys em Selectivity Performance of rotective devices to select between those conditions for which prompt operation and those for which no operation, or time delay operation . Isolate faulted circuit resulting in minimum interruptions. Implemented through “Zone of Protection”
Fact or s Whi ch Inf lu enc e Desi gn o a ro ec ve ys em Speed Remove a fault from the power system as quickly as possible Classification:
ns an aneous - no n en ona e ay High Speed - less than 3 cycles -
-
Fact or s Whi ch Inf lu enc e Desi gn o a ro ec ve ys em Economics Maximum protection at minimum cost the cost of installation, operation, and maintenance of the protection system which must be weighted service interruption.
Fact or s Whi ch Inf lu enc e Desi gn o a ro ec ve ys em Experience be encountered: Actual Relay Performance Nature of Faults Operation and Maintenance
Factors Which Influence Design of a Protective System Industry Standards The Institute of Electrical and Electronic Engineers (IEEE) and other . include specific standards for many applications. ANSI-C37.90-1989 Relays and Relay System
IEEE STD 242-1975
Apparatus Recommended Practice for Protection and Coordination of Industrial and Commercial Power System
Principles of
D e v e lo m e n t o f P ro t e ct iv e R e la s
• Electro-mechanical relay • Solid-state relay • Digital relay
Electro-mechanical Electromechanical Relay: ( 1st Generation ) The most commonly used (watthour meter) Provides individual phase protection
Electro-mechanical Electromechanical Relay: ( 1st Generation )
Time Adjustment: ension contro e Operator Rod
Contacts Sensing Coil
Disc
Core
Electro-mechanical Electromechanical Relay: ( 1st Generation ) Critical Components: • Composition of the rotating disc & Coil •
.
• Rotating & Tripping mechanism • Lubrication & alignments. • Spring & tension adjusting mechanism • Fatigue & Temperature dependence.
Design Approach: • Periodic re-calibration & maintenance •‘
-
’
.
• High burden CT, low sensitivity at higher currents.
Static Relay: ( 2nd Generation ) Characteristic curve is obtained through use of RC timing circuits Used to retrofit electro electro--mechanical relays Fast reset Less maintenance
Static Relay: ( 2nd Generation ) Set Value
AC - DC Conversion
From CTs
Comparator
Timing
Output
RC Based Timin
Static Relay: ( 2nd Generation ) r ca
omponen s:
• RC Timing circuit. • Tem erature de endence • Low repeatability
V
Time
Static Relay: ( 2nd Generation ) r t ca
omponents: cont nue …
•AC/DC Conversion. • Offset roblem • Effects of harmonics & Noise
Static Relay: ( 2nd Generation ) • Periodic re-calibration & maintenance • ‘Draw-out’ connections. • RC & LC Based filters - Slow tripping actions
Without Filter
With RC Filter
Digital Relay: ( 3rd Generation ) Computer-based Computerbased--with CPU Selectable characteristic curves and protection functions Metering and control functions ven an or s ur ance recording Remote communication Self-monitoring Self“All in”
Digital Relay: ( MicroProcessor Based ) Crystal Micro-processor
CPU
Data & Address Bus Memory
EPROM u pu
on ro
ontro e ay ontacts
AC - DC Conversion Current Sampling
Digital Relay: ( MicroProcessor Based ) Design Principle: •AC/ DC • • Analog to Digital Conversion
A to D
Digital Relay: ( MicroProcessor Based ) r t ca
omponents:
• High Frequency Bus • Susce tible to EMI/ RFI. •AC/ DC & Digital Conversion • Effect of Offset/ Harmonics, etc. CPU
RAM EPROM A/D
Hi-Freq Bus
Digital Relay: ( MicroProcessor Based ) r t ca
omponents:
• High Frequency Bus • Susce tible to EMI/ RFI. •AC/ DC & Digital Conversion • Effect of Offset/ Harmonics, etc.
Noise A to D
Digital Relay: ( MicroProcessor Based ) • RC & LC Based filters • Slow tri
in actions
• Shielding & Watch-dog timers • Problem reduced not eliminated • May reset randomly CPU
RAM EPROM Watch DOG
A/D
Digital Relay: ( MicroController Based ) Crystal -controller
Out ut Control
Steady state data
AC - DC Conversion A
B
C
E
Current Sampling
Digital Relay: ( MicroController Based ) es gn r nc p e: • Lesser no. of components, hence less chance of failure. •
-
• More functions can be built in a compact space. μP
μC
• Controller • RAM
• Controller • A2D Converter
• EPROM • Decoding logic • A2D Converter • or s • Output driver
• Output driver
Digital Relay: ( MicroController Based ) es gn r nc p e: • Absence of exposed high frequency bus •
-
.
Cost effectiv e technolo gy appro pri ate for MV appl ications
CPU RAM
EPROM
A/D
Digital Relay: ( Numerical Relay )
Serial Port
Crystal
To Outside world
-controller Output Control
Relay Contacts
Steady state data
irect Sampling
A
B
C
E
Current Sampling
Digital Relay: ( Numerical Relay ) Design Principle: • AC attenuation • • Numeric filtering & measurement
A to D
Digital Relay: ( Numerical Relay ) es gn r nc p e: con nue … • Digital Signal Processing concepts
• The normal representation i.e. with time in the X axis • The signal is sampled periodically, a different value obtained every , . Amplitude Disadvantages • No Phase Angle information. • No Frequency information.
Time
Digital Relay: ( Numerical Relay ) es gn r nc p e: cont nue … • Digital Signal Processing concepts
• X Axis now represents frequency, instead of time. • Amplitude
50Hz
Frequency
.
Digital Relay: ( Numerical Relay ) es gn r nc p e: con nue … • Digital Signal Processing concepts
Any signal can be represented by infinite Nos. of Sine waves ny r
rary g na
=
+ Fundamental (50Hz).
2nd Harmonic (100 )
Digital Relay: ( Numerical Relay ) es gn r nc p e: con nue … • Digital Signal Processing concepts Fourier Analysis • X Axis now represents frequency, instead of time. • Amplitude
50Hz
100Hz
Frequency
.
Digital Relay: ( Numerical Relay ) • Very quick operation. • In-built immunit to DC & harmonics tuned characteristics) • Possibility of providing additional filtering or inhibiting actions without sacrificing speed of response. •
oss
yo
s ur ance recor ng
us,
‘post-mortem’ analysis of fault & relay behavior. • • Very less no. of components.
.
Digital Relay: ( Numerical Relay ) Additional Features: • Communication to external laptop/ computer.
programmed in a single enclosure, instead of a combination of multi le discreet rela s. • Possibility of using non-conventional transducers for input sensing. Eg. Hall effect Current Transducers. • A ‘Bay-level controller’ instead of just a pro ec on re ay .
C o m a ris o n s o f D if f e re n t t arac er s c
ec ro-
a c
mechanical
Speed ofresponse TimingAccuracy
Size Draw-outrequired CT Burden
Slow Temp.
Fast Temp.
e o f R e la s
cro-
cro-
processor
Controller
based
based
Fast
Fast
VeryFast
Stable
Stable
Stable
Very Compact
Dependant
Dependant
Bulky
Small
Small
Small
Required
Required
Notrequired
Notrequired
High
umer ca
Notrequired
Low
Low
Low
Low
Less
Less
Less
Less
Reset Time
Very High
Functions
Single
Single
Multi
Multi
Multi
function
function
function
function
function
Frequent Yes
Frequent No
Low No
Low No
VeryLow No
Possible
Possible
Yes
Maintenance Deterioration due to frequent operations
SCADACompatibility
No
No
Zones of Protection
Regions (zones) of power system that can be protected adequately with fault recognition and remova resu ng n so a on o a m n mum amoun o equipment. Re uirements: All ower s stem elements must be encompassed by at least one zone Zones of protection must overlap to prevent any sys em e emen rom e ng unpro ec e no n spots”).
Zones of Protection
3
6 52 87B 50/51
1
52 87B 50/51
G
OVERLAPPING ZONES
Zones of Protection
Overla ppi ng zone s of p rot ectio n
Zones of Protection
3 5
1 - Bus Protection 2 - Generator Protection 3 - Subtrans Line Protection
1
4 - Fee er Protection 5 - Transformer Protection G
4
Fault
Fault
Fault
Fault
Fault
Pri mary and Back - up
Primary Protection - Main protection system for Back--up Protection - Alternate protection Back
Back--up Back up P ro tect io n
Local BackBack-up - Alternate protection at the . Remote Back Back--up - Alternate protection at the fails.
Lo cal B Local Backack -up P Protection ro tect io n B1
B4
B7
B2
B5
B8
B3
B6
B9 F1
Faul t F1 Main: B 8,B9, L2 remot e prot ectio n Local ¾ If Backup: B9 fails, trips B6 and B3 ¾ If B8 fa ils, trips B7 and s ends sig nal to tr ip L1 remote prote ct on
Remote Back Back--up P Protection ro tect io n B2
B1
B7
B8
F1 F3
F2 B5
B3
B6 B9
B4
Faul t F1 Main : B 5,B6 , Faul t F3 Main : B 3,B4 Backup: B1,B6
,
,
B10
Faul t F2 Main : B2,B4 ,B5 Backup : B1,B3,B6
Methods of Discrimination
Current Magnitude
100 A
60 A
20 A
0.35 sec
Inst.
Time
1.0 sec
Methods of Discrimination
Distance Measurement Z = V/I
ohms
Methods of Discrimination
Time
Current
Methods of Discrimination
Zon e 3
Time Zone 2 Zone 1
Distance
Methods of Discrimination
87T
Methods of Discrimination
Types Of Protective Relays
Overcurrent Relays - are operated when the current passing to the relay exceeds a preset value. They are not directional in nature. Bus
51
I
a a
ne ro ec on CT
A B
52
C
51N 51 Phase Relays
Ground e ay
Types Of Protective Relays
r e c on a
vercurren
e a y s - a r e ope r a e
when the current passing to it exceeds a reset value but with the addition of another condition and that is provided that the direction of the fault is correct A
I@CB2
B
I@CB3
Directional Overcurrent Relays I @ CB4
1
A
2
3 I
CB3
B
4
So f or a fault jus t after C B4 a directional ov er-current relay a t CB3 wil l see the fa ult in reverse whi le at CB4 the fa ult is f orward looking thus i t will trip CB4 only to iso late the fault
The sc heme is now sele ct iv e and a lit tl e secur e
Types Of Protective Relays I @ CB4
1
A
2
3 I
CB3
B
4
, dir ecti onal overcurr ent r elay of CB2 migh t see the fault . Therefor e, th e sch eme is n ow n ot s ecure.
The us e of d is tance re lay is pr eferred.
Types Of Protective Relays
s a n c e e ay s
Measures the im edance of the line it bein proportional to the distance of the line from the substation hence the name implies.
Widely used protection scheme for DoubleEnded Transmission Line from 69 kV up to as g as y r a n s co .
Types Of Protective Relays
Differential Relay A rela that b its desi n or a lication is intended to respond to the difference between incoming and outgoing electrical quantities associated with the .
Input
=
Output
CT
PCB
50/51 86T
87T
POWER TRANSFORMER
PCB
CT
151G
TRIPS AND LOCKS-OUT HS AND LS PCB
Typical Bus Arrangements: Single bus Double bus, double breaker Breaker-and-a-half Ring bus
Bus differential connection sin le-bus 87B 86B TRIPS AND LOCKSOUT ALL BREAKERS CONNECTED TO BUS
BUS
NOTE: All CTs connected to the bus differential must have same ratios.
Differential Protection
Primary Element 600/5
600/5 600A
CT
5A
5A
Relay
5A
0A
5A
CT
600A
Differential Protection
Fault
Primary Element 600/5
600/5 600A
CT
5A
5A
Relay
5A
CT
10A
5A
600A
Part :
R e l a i n P h i l o so h
P ow e r S ste m
Delivery Substation
Delivery Substation B A
Looped Lines (System B ackbone)
Power Transformer 34.5kV Bu s Distribution
Standard Protection
Radial Lines
Power Transformer
Capacitor Bank
ELECTRO-MECHANICAL OVERCURRENT RELAY BUS TARGET TARGET TIME
3
CT
50 51
50N 51N
LEGEND: Î50
– INSTANTANEOUS, PHASE OVERCURRENT RELAY – , Î50N – INSTANTANEOUS, GROUND OVERCURRENT RELA Y Î51N – TIME, GROUND OVERCURRENT RELAY Î43R – RECLOSER SWITCH Î79 – AUTO RECLOSER RELAY Î52 – POWER CIRCUIT BREAKER ÎCT – CURRENT TRANSFORMER
INST.
a a A
B
ne ro ec on single-phase
C CT
overcurrent relays are neede d t o protect a feeder
52
a Ib
Ic
Phase Relays
In=Ia+Ib+Ic
roun Relay
BUS
21
21G
67
67N
79
85
CT
CT
52
43R
METERING BUS PROTECTION
LEGEND: 21 – PHASE DISTANCE RELAY 21G – GROUND DISTANCE RELAY 67 – PHASE DIRECTIONAL OC RELAY 67N – GROUND DIRECTIONAL OC RELAY 43R – RECLOSER SWITCH 79 – AUTO RECLOSER RELAY 85 – TELEPROTECTION 52 – POWER CIRCUIT BREAKER CT – CURRENT TRANSFORMER
oope
ne ro ec on
Dista nce Rela ¾A relay that measures the impedance of the line Principle of Operation
ZLINE
FAULT
IFAULT
VFAULT
FAULT
FAULT
LINE
Therefore: ZLINE = VFAULT / IFAULT ZRELAY = ZLINE * CTR/PTR
X LINE
R
Impe dance R elay is non -direction al
oope
ne ro ec on
Types of Distance Relay Impedance - Used mainly as a fault detector in most of Reactance - Needs the supervision of mho to obtain its directionality. o - os w e y use s ance c arac er s c curve due to its inherent directionality. Quadrilateral - Made possible by combining reactance type principle rotated along different axes. Composite - Combined mho and quadrilateral
yp es o
st anc e e ay
X
X
Zone 2
Zone 1
R
REACTANCE
R
MHO
yp es o
st anc e e ay X
X
R R
QUAD
COMPOSITE
oope
ne ro ec on
phase-phase or 33-phase Mho Distance Relay, 21 – for phasefaults , – or nene- o-groun au s o cover arc resistance Phase Directional OC Relay, 67 – back back--up for phasephasephase or 33-phase faults back--up for linelineGround Directional OC Rel ay, 67N – back to-- round aults to
Teleprotection, 85 - POTT (Permissive Overreaching Transfer Trip) – , , shot
oope
ne ro ec on
Im lementin Distance Rela Characteristic with Time Graded Scheme also called Step Distance Scheme
Z2 Z1
me e aye Z2 Time Delayed
Time = 1.0 sec
Time = 0.35 sec A
Time = 0.0 sec
21
Relay Location
B
C
D
oope
ne ro ec on -
21
Must under-reach end of the line for selectivity Typically set for 80-90% of line impedance
oope
ne ro ec on -
21
.
Must over-reach end of the line Must not over-reach the Zone 1 of adjacent line adjacent line impedance
oope
ne ro ec on y–
ZL3 ZL2
. Typically set for 100% ZL1 + 100% ZL2 (longest) + 0% - 120% ZL3 (shortest) impedance.
oope
ne ro ec on 90% of ZL = instant A 21
B 90% of ZL = instant 120% of ZL = 0.35 s
What if th e as shown?
On this condition CB B will trip instantaneously via Zone 1 operation while CB A will trip after 0.35 second via Zone 2. This is not good since the fault has to be cleared .
So th ere is a ne ed to sol ve this p rob lem to co ver e pro ec on o e rema n ng o e ne
oope
ne ro ec on
Communication-Aided Protection
Z2 = 120% of ZL = 0.35 s
A =
21
o
B
= ns an
Z2 = 120% of ZL = 0.35 s
TRIP Z2
XMTR A
AND RCVR A
TRIP
XMTR B
AND
RCVR B
Z2
oope
ne ro ec on
Comm uni cati on-Aided Prot ecti on =
o
= nstant
Z2 = 120% of ZL = 0.35 s
A =
21
o
B
= nstant
Z2 = 120% of ZL = 0.35 s
No TRIP Z2
XMTR A
AND RCVR A
No TRIP
XMTR B
AND
RCVR B
Z2
oope
ne ro ec on
67 67N
67 67N
Relay is set at fault at the adjacent bus -
Time is 0.70 second Instantaneous unit is blocked (distance relay must
No auto-reclosure when tripping initiated by back-up protection
oope u
ne ro ec on
With Auto -Reclos ing Zone 1 Tripping Zone 2 Communication-Aided Tripping With out Au to-R eclo sin g Zone 2, Zone 3 or Zone 4 (reverse zone) Tripping Back-up Protection Tripping Note: Auto-reclosing is single-shot and 300 msec time delay to give time for the PCB to extinguish the fault
Normal maximum working temp. = 95 °C rs r se e ww transformer.
av a ve e e e ee oo e e
vercurren Fuses for distribution transformer Overcurrent relaying for 5MVA and above Characteristics:
Must be below the damage curve
Must be above magnetizing inrush
Differential - 87T Overload - 51
Back--up Ground - 151G Back
Overheating - Thermal Relay Relay
Gas Detection - Buchholz Relay
Sudden Pressure - Pressure Relief Valve
CT
52 50/51
86T 87T
LEGEND: 87T – TRANSFORMER DIFFERENTIAL RELAY
52 151G CT
86T – AUXILIARY LOCK-OUT RELAY 50 – INSTANTANEOUS OC RELAY 151G – BACK -UP GROUND RELAY 52 – POWER CIRCUIT BREAKER –
Differential Relay A relay that by its design or application is intended to respond to the difference between incoming and outgoing electrical quantities associated with the protected apparatus.
Constant
Percent Slope Characteristic Differential Relay K = 40 %
K = 25 % Operate I1-I2 K = 10 %
Min. Pick-up Restraint = (I1+I2)/2
Percentage Differential Protection Variable
Percent Slope Characteristic Differential Relay
Operate I1-I2
Pick-up
TRIP ZONE
Slope 1
Slope 2
NO TRIP ZONE
Restraint = (I1+I2)/2
Typi cal glB e us bu sArr angeme nts : Sin ou e us, ou e rea er Breaker Breaker--and and--a a-half breaker Ring b us
-
87B 86B OUT ALL BREAKERS CONNECTED TO BUS
BUS
NOTE: All CTs connected to the bus differential must have same ratios.
TRIPS AND LOCKSOUT ALL BREAKERS CONNECTED TO BUS 1
,
-
breaker) 87B
87B
86B
86B
TRIPS AND LOCKSOUT ALL BREAKERS BUS 2
BUS 1
BUS 2
-
- -
86B TRIPS AND BREAKERS CONNECTED TO BUS 1
87B
87B 86B TRIPS AND LOCKS-OUT ALL BREAKERS CONNECTED TO BUS 2
BUS 1
BUS 2
BUS 1 BUS 2
TRIPS AND LOCKS-OUT ALL BREAKERS CONNECTED TO BUS 1
86B1 87B1 87B2 BUS IMAGING RELAY
86B2 TRIPS AND LOCKS-OUT ALL BREAKERS CONNECTED TO BUS 2
NOTE: No bus differential protection is needed. The busses are covered by line or transformer protection.
Low
Impedance - using time overcurrent rela s 9 9
inexpensive but affected by CT saturation. low voltage application; 34.5kV and below
High Impedance - using overvoltage relays this scheme loads the CTs with a hi h im edance to force the differential current through the CTs instead of the relay operating coil.) . 9 115kV and above voltage application or some 34.5kV bus voltages which require high protection
