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This document is the exclusive property of Alstom Grid and shall not betransmitted by any means, copied, reproduced or modified without the prior
written consent of Alstom Grid Technical Institute. All rights reserved.
GRID
Technical Institute
Appl icat ion of Direct ional
Overcurrent
and Earthfaul t Protect ion
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> Directional Overcurrent and Earthfault Protection2 2
Direct ional Protect ion
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> Directional Overcurrent and Earthfault Protection3 3
Need for Direct ion al Con trol
Generally required if current can flow in both directions
through a relay location
e.g. Parallel feeder circuits
Ring Main Circuits
2.1 0.50.9 0.11.31.7
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> Directional Overcurrent and Earthfault Protection4 4
Need for Direct ion al Con trol
Generally required if current can flow in both directions
through a relay location
e.g. Parallel feeder circuits
Ring Main Circuits
Grading has now been lost !
2.1 0.50.9 0.11.31.7
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> Directional Overcurrent and Earthfault Protection5 5
Need for Direct ion al Con trol
Generally required if current can flow in both directions
through a relay location
e.g. Parallel feeder circuits
Ring Main Circuits
Relays operate for current flow in direction indicated
(Typical operating times shown)
0.9 0.10.5 0.90.50.1
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> Directional Overcurrent and Earthfault Protection6 6
Ring Main Circui t
With ring closed :
Both load and fault current may flow in eitherdirection along feeder circuits
Thus, directional relays are required
Note: Directional relays look into the feeder
Need to establish setting philosophy
51 67
51
Load
67 67
Load
6767 67
Load
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Ring Main Circui t
Procedure :
1. Open ring at A
Grade : A'
- E'
- D'
- C'
- B'
2. Open ring at A'
Grade : A - B - C - D - E
Typical operating times shown.
Note : Relays B, C, D, E may be non-directional.
1.30.1
0.1 0.90.5
0.9
0.5
B'
A'
B
E E'
A
1.7
D'
D
1.7
1.3
C' C
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Ring Sys tem w ith Two Sources
Discrimination between all relays is not possible due to differentrequirements under different ring operating conditions.
For F1 :- B must operate before A
For F2 :- B must operate after A
Not
Compatible
B' B C' C
D D'
F1
B
F2
A
A'
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Ring Sys tem w ith Two Sources
Option 1
Trip least important source instantaneously then treat as normal ringmain.
Option 2
Fit pilot wire protection to circuit A - B and consider as common sourcebusbar.
A
B
Option 1Option 1Option 1
Option 2 Option 2
50
PW PW
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Parallel Feeders
Non-Directional Relays :-
Conventional Grading :-
Grade A with C
and Grade B with D
Relays A and B have
the same setting.
51
51
A
D
Load
51 B
51 C
A & B
C & D
Fault level
at F
Operat
ingTime
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Parallel Feeders
Consider fault on one feeder :-
Relays C and D see the same fault current (I2). As C and
D have similar settings both feeders will be tripped.
51 A 51C
51 B 51D
LOAD
I1 + I2
I1
I2
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Parallel Feeders
Solution:- Directional Control at C and D
Relay D does not operate due to current flow in the reverse
direction.
51 A 67C
51 B 67D
LOAD
I1 + I2
I1
I2
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Parallel Feeders
Setting philosophy for directional relays
Load current always flows in non-operate direction.
Any current flow in operate direction is indicative of a fault
condition.
51 A 67
E
51 B 67
C
D
Load
51
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Parallel Feeders
Usually, relays are set :-
- 50% of full load current (note thermal rating)
- IDMT rather than DT
- Minimum T.M.S. (0.1)
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Paral lel Feeders - Applicat ion No te
P
B
B
D
D
Load
Load
If3
A C
If1If2/2 If2
BC
D
Ifmax
A
Grade A with B with C at If1
(single feeder in service)
GradeBwithDatIf3=If1
(upper feeder open at P)
Grade A with B at If2
(both feeders in service)
- check that sufficient margin existsfor bus fault at Q when relay A seestotal fault current If2, but relay Bsees only If2/2.
If1
If2
M
M = MarginM
M
Q
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Establ ish ing Direct ion
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Establ ishing Direct ion :- Polar ising Quant i ty
The DIRECTION of Alternating Current may only bedetermined with respect to a COMMONREFERENCE.
In relaying terms, the REFERENCE is called the POLARISING
QUANTITY.
The most convenient reference quantity is POLARISINGVOLTAGE taken from the Power System Voltages.
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Direct ional Decision by Phase Comparison (1)
S1 = Reference Direction = Polarising Signal = VPOL
S2 = Current Signal = I
OPERATION when S2 is within 90 of S1 :-
S 1
S 2
S 2
S 2S 2
S 2
S 2
S 2
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Direct ional Decision by Phase Comparison (2)
RESTRAINT when S2 lags S1 by between 90 and 270 :-
S2
S1
S2
S2
S2
S2S2
S2
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Polarising Voltage for A Phase Overcurrent Relay
OPERATE SIGNAL = IA
POLARISING SIGNAL :- Which voltage to use ?
Selectable from
VA
VB
VC
VA-B
VB-C
VC-A
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Directional Relay
Applied Voltage : VA
Applied Current : IA
Question :
- is this connection suitable for a typical power system ?
IA
VA
Operate
Restrain
VAF
IAF
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Polar ising Voltage
Applied Voltage : VBC
Applied Current : IA
Polarising voltage remainshealthy
Fault current is near centreof characteristic
IA
VBC
ZERO SENSITIVITY
LINE
VA
IAF
IVBC
VBC
MAXIMUM SENSITIVITY LINE
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> Directional Overcurrent and Earthfault Protection23 23
Relay Connect ion Angle
The angle between the current applied to the relay and thevoltage applied to the relay at system unity power factor
e.g. 90 (Quadrature) Connection : IA and VBC
The 90 connection is now used for all overcurrent relays.30 and 60 connections were also used in the past, but nolonger, as the 90 connection gives better performance.
IA
VA
90
VBC
VC VB
Relay Characteris tic Ang le (R C A )
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> Directional Overcurrent and Earthfault Protection24 24
Relay Characteris tic Ang le (R.C.A .)
for Electronic Relays
The angle by which the current applied to the relay must be
displaced from the voltage applied to the relay to produce maximumoperational sensitivity
e.g. 45
OPERATE
IA FOR MAXIMUM OPERATE
SENSITIVITYRESTRAIN
45
VA
RCA
VBC
90 C ti 45 R C A
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> Directional Overcurrent and Earthfault Protection25 25
90Connec tion - 45R.C.A .
RELAY CURRENT VOLTAGEA IA VBC
B IB VCA
C IC
VAB
IA
VA
90
VBVC
MAX SENSITIVITY
LINEOPERATE
IA FOR MAX
SENSITIVITYRESTRAIN 45
45
135
VA
VBC VBC
90 C ti 30 R C A
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> Directional Overcurrent and Earthfault Protection26 26
90Connec tion - 30R.C.A .
RELAY CURRENT VOLTAGEA IA VBC
B IB VCA
C IC
VAB
IA
VA
90
VBVC
VBC30
30
OPERATE
MAXSENSITIVITY
LINERESTRAIN
IA FOR MAX
SENSITIVITY
150
VA
VBC
S l ti f R C A (1)
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> Directional Overcurrent and Earthfault Protection27 27
Selec tion o f R.C.A . (1)
90 connection 30 RCA (lead)
Plain feeder, zero sequence source behind relay
Overcurrent Relays
S l ti f R C A (2)
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> Directional Overcurrent and Earthfault Protection28 28
Selec tion o f R.C.A . (2)
90 connection 45 RCA (lead)
Plain or Transformer Feeder :- Only Zero Sequence Source is inFront of Relay
Transformer Feeder :- Delta/Star Transformer in Front of Relay
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Direct ional Earth faul t Protect ion
Di t i l E th F lt
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> Directional Overcurrent and Earthfault Protection30 30
Directional Earth Fault
Requirements are similar to directional overcurrent
i.e. need operating signaland polarising signal
Operating Signal
obtained from residual connection of line CT's
i.e. Iop = 3Io
Polarising Signal
The use of either phase-neutral or phase-phase voltage as
the reference becomes inappropriate for the comparison withresidual current.
Most appropriate polarising signal is the residual voltage.
Residual Voltage (1)
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> Directional Overcurrent and Earthfault Protection31 31
Residual Voltage (1)
May be obtained from broken delta V.T. secondary.
Notes :
1. VT primary must be earthed.
2. VT must be of the '5 limb' construction (or 3 x single phase units)
VRES = VA-G + VB-G + VC-G = 3V0
A
BC
VRES
VC-GVB-GVA-G
Residual Voltage (2)
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> Directional Overcurrent and Earthfault Protection32 32
Residual Voltage (2)
Solidly Earthed System
Residual Voltage at R (relaying point) is dependant upon ZS / ZL ratio.
3ExZ2ZZ2Z
ZV
L0L1S0S1
S0
RES
E S R FZLZS
A-G
VCVC VC
VB VBVB
VRESVA
VA VRES
VBVCVCVC VBVB
VAVA
Residual Voltage (3)
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> Directional Overcurrent and Earthfault Protection33 33
Residual Voltage (3)
Resistance Earthed System
3Ex3ZZ2ZZ2Z
3ZZV
EL0L1S0S1
ES0
RES
VA-G
VA-GVA-G
VA-G
VB-GVC-G
G.FG.F
VB-G
VRES VRESVRES
VC-GVC-GVC-G
VC-G VB-GVC-G
VB-G VB-G VB-G
E
N
G
S R FZLZ
S
ZE A-G
G.F
Relay Characteris tic Ang le (R C A )
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> Directional Overcurrent and Earthfault Protection34 34
Relay Characteris tic Ang le (R.C.A .)
Voltage Polarising Signal
Rotate VRES by 180O
to obtain voltage polarisation signal0O, -45O or -60O R.C.A. applied for maximum sensitivity
e.g. -45VA
VC VB
VF
VRES
Rotate VRES by 180
MAX SENSITIVITY
LINE
IRES FOR MAX
SENSITIVITY-45
OPERATE
RESTRAIN
Residual Voltage Polarisat ion
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> Directional Overcurrent and Earthfault Protection35 35
Residual Voltage Polarisat ion
Relay Characteristic Angle
0 - Resistance/Petersen Coil earthed systems
-45 (I lags V) - Distribution systems (solidly earthed)
-60 (I lags V) - Transmission systems (solidly earthed)
+90 (I leads V) - Insulated systems
Zero Sequence Network :-
V0 = 0 - I0 (ZS0 + 3R)
(Relay Point)
ZL0ZS0 I0
V03R
Insulated Systems (1)
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> Directional Overcurrent and Earthfault Protection36 36
Insulated Systems (1)
a b c
Source
Icb
Ica
Ic
IcbIca
Ic
I
cb
Ica
2Ic3Ic
Location CT's
Insulated Systems (2)
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> Directional Overcurrent and Earthfault Protection37 37
Insulated Systems (2)
Faulty Feeder
VRES
a b
cIca
IcbIc -3
Ic
Healthy Feeders
VRES
Ic = Ica + IcbRCA
OperateRestrain
VPOL
-2IcRCA
OperateRestrain
VRES
VPOL
Peterson Coil Earthed Systems (1)
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> Directional Overcurrent and Earthfault Protection38 38
IL
Peterson Coil Earthed Systems (1)
a b c
Source
IcbIca
Ic
IcbIca
Ic
IcbIca
2Ic
Location of CT's
3Ic
Ic
IL
IL
Peterson Coil Earthed Systems (2)
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> Directional Overcurrent and Earthfault Protection39 39
Peterson Coil Earthed Systems (2)
Peterson Coil Earthed Systems (3)
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> Directional Overcurrent and Earthfault Protection40 40
Peterson Coil Earthed Systems (3)
Negative Phase Sequence Voltage Polarisat ion
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> Directional Overcurrent and Earthfault Protection42 42
Negative Phase Sequence Voltage Polarisat ion
Transmission Systems
Directional earth fault used as back-up protection
Can form part of a directional scheme
VRES might be unreliable due to mutual coupling
Unsuitable VT for VRES measurement (i.e. open delta, 3-limb)
Negative Sequence Network :-
V2 = 0I2 (ZS2)
ZL2ZS2 I2
V2
(Relay Point)
ZS1=ZS2
ZL1=ZL2
Current Polar ising
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> Directional Overcurrent and Earthfault Protection43 43
Current Polar ising
A solidly earthed, high fault level (low source impedance)system may result in a small value of residual voltage at the
relaying point. If residual voltage is too low to provide a reliablepolarising signal then a current polarising signal may be usedas an alternative.
The current polarising signal may be derived from a CT locatedin a suitable system neutral to earth connection.
e.g.
POL
OP
DEF Relay
Current Polarising (1)
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> Directional Overcurrent and Earthfault Protection44 44
Current Polarising (1)
POL
DEF RELAY
INCORRECTOP
Direction of current depends on faultposition
Current Polarising (2)
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> Directional Overcurrent and Earthfault Protection45 45
Current Polarising (2)
POLDEF RELAY
CORRECT
OP
Current Polarising (3)
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> Directional Overcurrent and Earthfault Protection46 46
Current Polarising (3)
POL
DEF RELAY
CORRECT IF
ZLO + ZSO IS
POSITIVE
S
OP
Current Polarising (4)
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> Directional Overcurrent and Earthfault Protection47 47
Current Polarising (4)
POL DEF RELAY
OP
CORRECT
Au to Trans formers (1)
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> Directional Overcurrent and Earthfault Protection48 48
Au to Trans formers (1)
ZT
ZLZHSOURCE
ZS SOURCE
DEF
RELAY
Neutral connection is suitable for currentpolarising if earthfault current flows up the
neutral for faults on H.V. & L.V. sides.
Au to Trans formers (2)
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> Directional Overcurrent and Earthfault Protection49 49
Au to Trans formers (2)
Check :
For correct application
(Note : there is also a possibility that neutral current may be zero.
Alternative : Use C.T. in one leg of winding)
1V
Vx
ZZZ
Z
L
H
T0L0S0
T0
Unloaded
Delta
Loaded
Delta
For LV Faults
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> Directional Overcurrent and Earthfault Protection51 51
o au s
T
H L
IN= 3 (ILO - IHO)
IH
IL
For HV Faults
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> Directional Overcurrent and Earthfault Protection52 52
T
H L
IN = 3 (IHO - ILO)
IH IL
Au to-Transfo rmer Example
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> Directional Overcurrent and Earthfault Protection53 53
p
T
H L
IN = 3 (IHO - ILO)
ZS
ZS0ZL0ZH0IH0 IL0
I0
ZT0
Au to-Transfo rmer Example
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> Directional Overcurrent and Earthfault Protection54 54
p
kAinkVx3
MVAx
p.u.in
H
base0
0H0
kAinkVx3
MVAx.
ZZZ
Z
p.u.in.ZZZ
Z
L
base0
L0S0T0
T0
0L0S0T0
T0L0
Au to-Transfo rmer Example
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> Directional Overcurrent and Earthfault Protection55 55
p
1ZZZ
Z
kV
kVor
ZZZ
Z
kV
1
kV
1ifveis
ZZZ
Z
kV
1-
kV
1
3
.MVA3
L0S0T0
T0
L
H
L0S0T0
T0
LHN
L0S0T0
T0
LH
base0N
Au to-Transfo rmer Example
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> Directional Overcurrent and Earthfault Protection56 56
p
T
H L
I
N= 3 (I
LO
- IHO
)
ZS
ZS0ZL0ZH0IH0 IL0
I0
ZT0
IH0 = 0
IN = 3IL0 which is +ve.
Direct ional Contro l
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> Directional Overcurrent and Earthfault Protection57 57
Static Relay (MCGG + METI)
Characteristic Selectable
51 I
Overcurrent Unit
(Static)
67
V
I
M.T.A. Selectable
Directional Unit
(Static)
Numerical Relay Direct io nal Characteris t ic
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y
Characteristic angle cc = -95 0 95
in 1 steps
Polarising thresholdsVp 2V to 320V
in 2V steps
VT supervisionselectively block operation
Zone offorward start
forward operation
Reverse start
(c - 90) (c + 90)c
+Is
-Is