ABB REG670 - Page 175

Return path via
power system
L2
L3
L1
L2
L3
I
N
= -3Izs1
Uzs
Current in the neutral
(measured as I
N
) serves
as a directional reference
because it has the same
direction for both internal
and external faults.
zone of protection
Izs1
Izs1
Izs1
Ifault
ROA
block
I
N
External
fault
region
L1
Zero-sequence differential
current for external fault
Idiff = abs(3I
0
+ I
N
)
Idiff = 3Izs1 - 3Izs1= 0
3I
0
block
block
operate
Return path through
transformer
I
L1
+ I
L2
+I
L3
= 3I
0
3I
0
= 3Izs1
(Summation in the IED)
Power
system
External
fault
region
Internal
fault
region
ROA = Relay Operate Angle
3Izs1
IEC09000107-3-en.vsd
These three
zero-sequence
currents are not measured
IEC09000107-3 V1 EN
Figure 61: Zero sequence currents at an external earth fault
IEC09000108-3-en.vsd
Return path via
power system
L1
L2
L3
I
N
= -3Izs1
Uzs
Current in the neutral
(measured as I
N
) serves
as a directional reference
because it has the same
direction for both internal
and external faults.
zone of protection
Izs1
Izs1
Izs1
Return path
through transformer
I
L1
+ I
L2
+I
L3
= 3I
0
3I
0
= -3Izs2
(Summation in the IED)
3Izs1
L2
L3
Ifault
L1
Power
system
Izs2
Izs2
Izs2
ROA
block
I
N
(reference)
3I
0
block
block
operate
Zero-sequence differential
current for internal fault
Idiff = abs(3I
0
+ I
N
)
Idiff = 3Izs2 + 3Izs1 > 0
Idiff = Ifault
ROA = Relay Operate Angle
External
fault
region
External
fault
region
Internal
fault
region
IEC09000108-3 V1 EN
Figure 62: Zero sequence currents at an internal earth fault
1. For an external earth fault (Figure 61), the residual current 3I
o
and the neutral
current I
N
have equal magnitude, but they are seen within the IED as 180 degrees
out-of-phase if the current transformers are connected as in Figure
61, which is
1MRK502052-UEN B Section 6
Differential protection
169
Technical manual