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Application of Non-Directional Overcurrentand Earthfault Protection
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Non-Directional Overcurrent and Earth
Fault Protection
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Overcurrent ProtectionPurpose of Protection
Detect abnormal conditions
Isolate faulty part of the system
Speed
Fast operation to minimise damage and danger
Discrimination
Isolate only the faulty section
Dependability / reliability
Security / stability
Cost of protection / against cost of potential hazards
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Fuses
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Overcurrent ProtectionFuses
Simple
Can provide very fast fault clearance
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Overcurrent ProtectionFuses - disadvantages
Problematic co-ordination
IFA approx 2 x IFB
Limited sensitivity to earth faults
Single phasing
Fixed characteristic
Need replacing following fault clearance
Fuse A Fuse B
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Tripping Methods
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Overcurrent ProtectionDirect Acting AC Trip
AC series trip
common for electromechanical O/C relays
51
IF
Trip Coil
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Overcurrent ProtectionDirect Acting AC Trip
Capacitor discharge trip
used with static relays where no secure DCsupply is available
IF'
SensitiveTripCoil
IF
51
+
-
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Overcurrent ProtectionDC Shunt Trip
Requires secure DC auxiliary
No trip if DC fails
IF'IF
DCBATTERY
SHUNTTRIP COIL
51
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Overcurrent Protection
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Overcurrent ProtectionPrinciples
Operating Speed
Instantaneous Time delayed
Discrimination
Current setting
Time setting
Current and time
Cost
Generally cheapest form of protection relay
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IF1IF1IF2
Overcurrent ProtectionInstantaneous Relays
Current settings chosen so that relay closest tofault operates
Problem
Relies on there being a difference in fault levelbetween the two relay locations
Cannot discriminate if IF1 = IF2
50
B
50
A
IF1IF2
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Overcurrent ProtectionDefinite (Independent) Time Relays
TOP
TIME
IS Applied Current
(Relay Current Setting)
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Overcurrent ProtectionDefinite (Independent) Time Relays
Operating time is independent of current
Relay closest to fault has shortest operating time
Problem
Longest operating time is at the source wherefault level is highest
51
0.9 sec 0.5 sec
51
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Overcurrent ProtectionIDMT
Inverse Definite Minimum Time characteristic
TIME
Applied Current(Relay Current Setting)
IS
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Overcurrent ProtectionDisc Type O/C Relays
Current setting via plug bridge
Time multiplier setting via discmovement
Single characteristic
Consider 2 ph & EF or 3 phplus additional EF relay
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Overcurrent ProtectionStatic Relay
Electronic, multi characteristic
Fine settings, wide range
Integral instantaneous elements
RESET
A B C
INST
t
I > Is
INST
t
I > Is
No
Ph +
In
Vx V
Hz
0.05000000
111
0.050.050.10.20.30.4
124810
0.05000000
111
0.050.050.10.20.30.4
124810
0.10.10.20.40.40.40.8
000
0.02500000
000000
D
LT1
S1V1
E1 I
t
sI =
x IssI =
x Is
x t =
x t =
I =INST
x sII =INST
x sI
MCGG
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Overcurrent ProtectionNumerical Relay
Multiple characteristics and stages
Current settings in primary or secondaryvalues
Additional protection elements
Current
Time
I>1
I>2
I>3
I>4
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Overcurrent ProtectionCo-ordination Principle
Relay closest to faultmust operate first
Other relays must haveadequate additionaloperating time to
prevent them operating Current setting chosen
to allow FLC
Consider worst caseconditions, operatingmodes and currentflows
T
IS1IS2MaximumFault
Level
I
R2R1IF1
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Overcurrent ProtectionCo-ordination Example
C AB
0.01
0.1
1
10
Operatingtime(s)
Current (A) FLB FLC FLD
ED
C
B
DE
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Overcurrent ProtectionIEC Characteristics
SI t = 0.14(I0.02 -1)
VI t = 13.5(I2 -1)
EI t = 80(I2 -1)
LTI t = 120(I - 1)
Current (Multiples of Is)
0.1
1
10
100
1000
1 10010
Operating
Time(s)
VI
EI
SI
LTI
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Overcurrent ProtectionOperating Time Setting - Terms Used
Relay operating times can becalculated using relay
characteristic charts
Published characteristcs aredrawn against a multiple of
current setting or Plug SettingMultiplier
Therefore characteristics can beused for any applicationregardless of actual relay currentsetting
e.g at 10x setting (or PSM of 10)
SI curve op time is 3s
Current (Multiples of Is)
0.1
1
10
100
1000
1 10010
Operat
ingTime(s)
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Overcurrent ProtectionCurrent Setting
Set just above full load current
allow 10% tolerance Allow relay to reset if fault is cleared by
downstream device
consider pickup/drop off ratio (reset ratio)
relay must fully reset with full load currentflowing
PU/DO for static/numerical = 95%
PU/DO for EM relay = 90%
e.g for numerical relay, Is = 1.1 x IFL/0.95
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Overcurrent ProtectionCurrent Setting
Current grading
ensure that if upstream relay has started
downstream relay has also started
Set upstream device current setting greater thandownstream relay
e.g. IsR1 = 1.1 x IsR2
R1 R2 IF1
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Overcurrent ProtectionGrading Margin
Operating time difference between two devices toensure that downstream device will clear fault beforeupstream device trips
Must include
breaker opening time
allowance for errors
relay overshoot time
safety margin
GRADINGMARGIN
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Overcurrent ProtectionGrading Margin - between relays
Traditional
breaker op time - 0.1 relay overshoot - 0.05
allow. For errors - 0.15
safety margin - 0.1
Total 0.4s
Calculate using formula
R2R1
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Overcurrent ProtectionGrading Margin - between relays
Formula
t = (2Er + Ect) t/100 + tcb + to + ts
Er = relay timing error
Ect = CT measurement error
t = op time of downstream relay
tcb = CB interupting time
to = relay overshoot time
ts = safety margin
Op time of Downstream Relay t = 0.5s
0.375s margin for EM relay, oil CB
0.24s margin for static relay, vacuum CB
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Overcurrent ProtectionGrading Margin - relay with fuse
Grading Margin = 0.4Tf + 0.15s over whole characteristic
Assume fuse minimum operating time = 0.01s
Use EI or VI curve to grade with fuse
Current setting of relay should be 3-4 x rating of fuse toensure co-ordination
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Overcurrent ProtectionGrading Margin - relay with upstream fuse
1.175Tr + 0.1 + 0.1 = 0.6Tf
or
Tf
= 2Tr
+ 0.33s
Allowance for CTand relay error
CB Safety margin Allowance for fuseerror (fast)
Tf
Tr
IFMAX
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Overcurrent ProtectionTime Multiplier Setting
Used to adjust the operatingtime of an inversecharacteristic
Not a time setting but a
multiplier
Calculate TMS to givedesired operating time inaccordance with the gradingmargin
Current (Multiples of Is)
0.1
1
10
100
1 10010
OperatingTime(s)
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Overcurrent ProtectionTime Multiplier Setting - Calculation
Calculate relay operating time required, Treq
consider grading margin
fault level
Calculate op time of inverse characteristic
with TMS = 1, T1
TMS = Treq /T1
O P i
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Overcurrent ProtectionCo-ordination - Procedure
Calculate required operating current
Calculate required grading margin
Calculate required operating time
Select characteristic
Calculate required TMS
Draw characteristic, check grading over wholecurve
Grading curves should be drawn to a commonvoltage base to aid comparison
O t P t ti
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Overcurrent ProtectionCo-ordination Example
Grade relay B with relay A
Co-ordinate at max fault level seen by both relays =1400A
Assume grading margin of 0.4s
Is = 5 Amp; TMS = 0.05, SI
IFMAX= 1400 Amp
B A
200/5 100/5
Is = 5 Amp
O t P t ti
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Overcurrent ProtectionCo-ordination Example
Relay B is set to 200A primary, 5A secondary
Relay A set to 100A If (1400A) = PSM of 14relay A OP time = t = 0.14 x TMS = 0.14 x 0.05 = 0.13(I0.02 -1) (140.02 -1)
Relay B Op time = 0.13 + grading margin = 0.13 + 0.4 = 0.53s
Relay A uses SI curve so relay B should also use SI curve
Is = 5 Amp; TMS = 0.05, SI
I
FMAX= 1400 Amp
B A
200/5 100/5
Is = 5 Amp
Overcurrent Protection
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Overcurrent ProtectionCo-ordination Example
Relay B Op time = 0.13 + grading margin = 0.13 + 0.4 = 0.53s
Relay A uses SI curve so relay B should also use SI curve
Relay B set to 200A If (1400A) = PSM of 7relay B OP time TMS = 1 = 0.14 x TMS = 0.14 = 3.52s
(I0.02 -1) (70.02 -1)
Required TMS = Required Op time = 0.53 = 0.15Op time TMS=1 3.52
Set relay B to 200A, TMS = 0.15, SI
Is = 5 Amp; TMS = 0.05, SI
IFMAX= 1400 Amp
B A
200/5 100/5
Is = 5 Amp
Overcurrent Protection
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Overcurrent ProtectionLV Protection Co-ordination
ZA2118B
Relay 1Relay 2
Relay 3Relay 4Fuse
1
2
3
4
F
350MVA4 4
3 3
2
F
11kV
MCGG CB
ACB CTZ61 (Open)CTZ61
ACBMCCB
27MVA
20MVALoad
Fuse
2 x 1.5MVA11kV/433V
5.1%
K
1
Overcurrent Protection
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Overcurrent ProtectionLV Protection Co-ordination
ZA2119
1000S
100S
10S
1.0S
0.1S
0.01S
0. 1kA 10kA 1000kA
TX damage
Veryinverse
MC
CB
(cold
)
Relay 2
Relay 3
R
elay4
Fuse
Overcurrent Protection
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Overcurrent ProtectionLV Protection Co-ordination
ZA2120C
Relay 1Relay 2
Relay 3Relay 4Fuse
1
2
3
4
F
350MVA4 4
3 3
2
1
F
11kV
KCGG 142 CB
ACB (Open)KCEG 142
ACBMCCB
27MVA
20MVALoad
Fuse
2 x 1.5MVA11kV/433V
5.1%
K
Overcurrent Protection
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Overcurrent ProtectionLV Protection Co-ordination
ZA2121
1000S
100S
10S
1.0S
0.1S
0.01S
0. 1kA 10kA 1000kA
TX damage
Long timeinverse
MC
CB
(co
ld)
Relay 2
Relay 3
Relay 4
Fuse
Overcurrent Protection
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ZA2135
R3
R2
R1
Block t >
I > StartIF2
IF1
M (Transient backfeed ?)
Graded
protection
Blockedprotection
Overcurrent ProtectionBlocked OC Schemes
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Delta / Star Transformers
Overcurrent Protection
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A phase-phase fault on one
side of transformerproduces 2-1-1 distributionon other side
Use an overcurrent elementin each phase (cover the 2xphase)
2 & EF relays can be usedprovided fault current > 4xsetting
Iline
0.866 If3
Turns Ratio
= 3 :1
Idelta
Overcurrent ProtectionTransformer Protection - 2-1-1 Fault Current
Overcurrent Protection
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Istar = E-/2Xt = 3 E-n/2Xt
Istar = 0.866 E-n/Xt Istar = 0.866 If3 Idelta = Istar/3 = If3 /2 Iline = If3
Iline
0.866 If3
Turns Ratio
= 3 :1
Idelta
Overcurrent ProtectionTransformer Protection - 2-1-1 Fault Current
Overcurrent Protection
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Grade HV relay withrespect to 2-1-1 for
- fault
Not only at maxfault level
51HV
/
51LV
If386.6%If3
Overcurrent ProtectionTransformer Protection - 2-1-1 Fault Current
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Use of High Sets
Overcurrent Protection
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Fast clearance of faults
ensure good operation factor, If >> Is (5 x ?)
Current setting must be co-ordinated to preventovertripping
Used to provide fast tripping on HV side of transformers
Used on feeders with Auto Reclose, prevents transientfaults becoming permanent
AR ensures healthy feeders are re-energised
Consider operation due to DC offset - transientoverreach
Instantaneous Protection
Overcurrent Protection
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Set HV inst 130% IfLV
Stable for inrush
No operation for LV fault
Fast operation for HVfault
Reduces op times
required of upstreamrelays
HV2 LVHV1
HV2
LVTIME
CURRENT
HV1
IF(LV) IF(HV)
1.3IF(LV)
Instantaneous OC on Transformer Feeders
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Earthfault Protection
Overcurrent Protection
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Earth fault current may be limited
Sensitivity and speed requirements may not be met byovercurrent relays
Use dedicated EF protection relays
Connect to measure residual (zero sequence) current
Can be set to values less than full load current
Co-ordinate as for OC elements May not be possible to provide co-ordination with
fuses
Earth Fault Protection
Overcurrent Protection
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Combined with OC relays
E/F OC OC OC E/F OC OC
Economise using 2x OCrelays
Earth Fault Relay Connection - 3 Wire System
Overcurrent ProtectionE th F lt R l C ti 4 Wi S t
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EF relay setting must begreater than normalneutral current
Independent of neutralcurrent but must use 3 OCrelays for phase to neutralfaults
E/F OC OC OC E/F OC OC OC
Earth Fault Relay Connection - 4 Wire System
Overcurrent ProtectionE th F lt R l C t S tti
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Solid earth
30% Ifull loadadequate
Resistance earth
setting w.r.t earth fault
level
special considerationsfor impedance earthing
- directional?
Earth Fault Relays Current Setting
Overcurrent ProtectionSensitive Earth Fault Relays
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Settings down to0.2% possible
Isolated/high
impedance earth networks For low settings cannot use residual connection, use
dedicated CT
Advisable to use core balance CT CT ratio related to earth fault current not line current
Relays tuned to system frequency to reject 3rd
harmonic
B
C
E/F
A
Sensitive Earth Fault Relays
Overcurrent ProtectionCore Balance CT Connections
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Need to take care with corebalance CT and armouredcables
Sheath acts as earth return
path
Must account for earth currentpath in connections - insulatecable gland
NO OPERATION OPERATION
CABLEBOX
CABLE GLAND
CABLE GLAND/SHEATHEARTH CONNECTION
E/F
Core Balance CT Connections
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