Application of Non-Directional Overcurrent and Earthfault Protection
J.W. WrightEngineering Manager
Non-Directional Overcurrent and Earth Fault Protection
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
Overcurrent ProtectionCo-ordination
Co-ordinate protection so that relay nearest to fault operates first
Minimise system disruption due to the fault
F1 F2 F3F3F2F1
Fuses
Overcurrent ProtectionFuses
Simple Can provide very fast fault
clearance <10ms for large current
Limit fault energy
Pre Arc TimeArcing Time
Prospective Fault Current
TotalOperatingTime
t
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
Tripping Methods
Overcurrent ProtectionDirect Acting AC Trip
AC series trip common for electromechanical O/C relays
51
IF
Trip Coil
Overcurrent ProtectionDirect Acting AC Trip
Capacitor discharge trip used with static relays where no secure
DC supply is available
IF'
SensitiveTripCoil
IF
51
+
-
Overcurrent ProtectionDC Shunt Trip
Requires secure DC auxiliary No trip if DC fails
IF'IF
DCBATTERY
SHUNTTRIP COIL
51
Overcurrent Protection
Overcurrent ProtectionPrinciples
Operating Speed Instantaneous Time delayed
Discrimination Current setting Time setting Current and time
Cost Generally cheapest form of protection
relay
IF1IF1IF2
Overcurrent ProtectionInstantaneous Relays
Current settings chosen so that relay closest to fault operates
Problem Relies on there being a difference in fault
level between the two relay locations Cannot discriminate if IF1 = IF2
50
B
50
A
IF1IF2
Overcurrent ProtectionDefinite (Independent) Time Relays
TOP
TIME
IS Applied Current(Relay Current Setting)
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 where fault level is highest
510.9 sec 0.5 sec
51
Overcurrent ProtectionIDMT
Inverse Definite Minimum Time characteristic
TIME
Applied Current(Relay Current Setting)
IS
Overcurrent ProtectionDisc Type O/C Relays
Current setting via plug bridge
Time multiplier setting via disc movement
Single characteristic Consider 2 ph & EF or 3 ph
plus additional EF relay
Overcurrent ProtectionStatic Relay
Electronic, multi characteristic Fine settings, wide range Integral instantaneous elements
RESET
A B CINST
t
I > Is
INST
t
I > Is
NoPh+
I nVx 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
DLT1
S1V1E1 I
t
sI =
x Is
sI =
x Is
x t =
x t =
I =INST x sI
I =INST x sI
MCGG
Overcurrent ProtectionNumerical Relay
Multiple characteristics and stages Current settings in primary or secondary values Additional protection elements
Current
Time
I>1
I>2
I>3
I>4
Co-ordination
Overcurrent ProtectionCo-ordination Principle
Relay closest to fault must operate first
Other relays must have adequate additional operating time to prevent them operating
Current setting chosen to allow FLC
Consider worst case conditions, operating modes and current flows
T
IS1IS2MaximumFaultLevel
I
R2R1
IF1
Overcurrent ProtectionCo-ordination Example
C AB
0.01
0.1
1
10Op
erat
ing
time
(s)
Current (A) FLB FLC FLD
ED
CB
DE
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 10010O
pera
ting
Tim
e (s
)
VI
EI
SI
LTI
Overcurrent ProtectionOperating Time Setting - Terms Used
Relay operating times can be calculated using relay characteristic charts
Published characteristcs are drawn against a multiple of current setting or Plug Setting Multiplier
Therefore characteristics can be used for any application regardless of actual relay current setting
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
Ope
rati
ng T
ime
(s)
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 current
flowing PU/DO for static/numerical = 95% PU/DO for EM relay = 90%
e.g for numerical relay, Is = 1.1 x IFL/0.95
Overcurrent ProtectionCurrent Setting
Current gradingensure that if upstream relay has started
downstream relay has also started
Set upstream device current setting greater than downstream relay
e.g. IsR1 = 1.1 x IsR2
R1 R2IF1
Overcurrent ProtectionGrading Margin
Operating time difference between two devices to ensure that downstream device will clear fault before upstream device trips
Must include breaker opening time allowance for errors relay overshoot time safety margin
GRADING MARGIN
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
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
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
to ensure co-ordination
Overcurrent ProtectionGrading Margin - relay with upstream fuse
1.175Tr + 0.1 + 0.1 = 0.6Tf
or Tf = 2Tr + 0.33s
Allowance for CT and relay error
CB Safety margin Allowance for fuse error (fast)
Tf
Tr
IFMAX
Overcurrent ProtectionTime Multiplier Setting
Used to adjust the operating time of an inverse characteristic
Not a time setting but a multiplier
Calculate TMS to give desired operating time in accordance with the grading margin
Current (Multiples of Is)
0.1
1
10
100
1 10010
Ope
rati
ng T
ime
(s)
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
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 whole
curve
Grading curves should be drawn to a common voltage base to aid comparison
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
Overcurrent ProtectionCo-ordination Example
Relay B is set to 200A primary, 5A secondary Relay A set to 100A If (1400A) = PSM of 14
relay 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
IFMAX = 1400 Amp
B A
200/5 100/5
Is = 5 Amp
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 7
relay 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 = 0.15 Op time TMS=1 3.52 Set relay B to 200A, TMS = 0.15, SI
Is = 5 Amp; TMS = 0.05, SI
IFMAX = 1400 AmpB A
200/5 100/5
Is = 5 Amp
Overcurrent ProtectionLV Protection Co-ordination
ZA2118B
Relay 1Relay 2Relay 3Relay 4Fuse
1
23
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 ProtectionLV Protection Co-ordination
ZA2119
1000S
100S
10S
1.0S
0.1S
0.01S0. 1kA 10kA 1000kA
TX damage
Veryinverse
MC C B ( co ld )
Relay 2
Relay 3Relay 4
Fuse
Overcurrent ProtectionLV Protection Co-ordination
ZA2120C
Relay 1Relay 2Relay 3Relay 4Fuse
1
23
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 ProtectionLV Protection Co-ordination
ZA2121
1000S
100S
10S
1.0S
0.1S
0.01S0. 1kA 10kA 1000kA
TX damage
Long timeinverse
MC C B ( co ld )
Relay 2
Relay 3
Relay 4
Fuse
ZA2135
R3
R2
R1
Block t >
I > StartIF2
IF1M(Transient backfeed ?)
Gradedprotection
Blockedprotection
Overcurrent ProtectionBlocked OC Schemes
Use of High Sets
Fast clearance of faults ensure good operation factor, If >> Is (5 x ?)
Current setting must be co-ordinated to prevent overtripping
Used to provide fast tripping on HV side of transformers
Used on feeders with Auto Reclose, prevents transient faults becoming permanent AR ensures healthy feeders are re-energised
Consider operation due to DC offset - transient overreach
Overcurrent ProtectionInstantaneous Protection
Set HV inst 130% IfLV
Stable for inrush No operation for LV fault Fast operation for HV fault Reduces op times required
of upstream relays
HV2 LVHV1
HV2
LVTIME
CURRENT
HV1
IF(LV) IF(HV)1.3IF(LV)
Overcurrent ProtectionInstantaneous OC on Transformer Feeders
Earthfault Protection
Earth fault current may be limited Sensitivity and speed requirements may not be met
by overcurrent 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
Overcurrent ProtectionEarth Fault Protection
Combined with OC relays
E/F OC OC OC E/F OC OC
Economise using 2x OC relays
Overcurrent ProtectionEarth Fault Relay Connection - 3 Wire System
EF relay setting must be greater than normal neutral current
Independent of neutral current but must use 3 OC relays for phase to neutral faults
E/F OC OC OC E/F OC OC OC
Overcurrent ProtectionEarth Fault Relay Connection - 4 Wire System
Solid earth 30% Ifull load
adequate
Resistance earth setting w.r.t earth fault
level special considerations for
impedance earthing - directional?
Overcurrent ProtectionEarth Fault Relays Current Setting
Settings down to0.2% possible
Isolated/highimpedance 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
BC
E/F
A
Overcurrent ProtectionSensitive Earth Fault Relays
Need to take care with core balance CT and armoured cables
Sheath acts as earth return path
Must account for earth current path in connections - insulate cable gland
NO OPERATION OPERATION
CABLE BOX
CABLE GLAND
CABLE GLAND/SHEATHEARTH CONNECTION
E/F
Overcurrent ProtectionCore Balance CT Connections