ABB RET650 Technical Manual page 120

Section 6
Differential protection
114
L1
Power
L2
system
L3
I + I +I = 3I
L1 L2 L3 0
-
3I = 3Izs2
0
(Summation in the IED)
Return path via
power system
External
External
IEC09000108-3 V1 EN
Figure 46: Zero sequence currents at an internal earth fault
1. For an external earth fault (figure 45), the residual current 3I
current I have equal magnitude, but they are seen by the IED as 180 degrees out-
N
of-phase if the current transformers are connected as in figure 45, which is the
ABB recommended connection. The differential current becomes zero as both
CTs ideally measure exactly the same component of the earth- fault current.
2. For an internal fault, the total earth-fault current is composed generally of two
zero sequence currents. One zero sequence current IN flows towards the power
transformer neutral point and into the earth, while the other zero sequence current
3I flows out into the connected power system. These two primary currents can
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be expected to have approximately opposite directions (about the same zero
sequence impedance angle is assumed on both sides of the earth fault). However,
on the secondary CT sides of the current transformers, they will be approximately
in phase if the current transformers are oriented as in figure 2, which is by ABB
recommended orientation. The magnitudes of the two currents may be different,
dependent on the magnitudes of zero sequence impedances of both sides. No
current can flow towards the power system, if the only point where the system is
earthed, is at the protected power transformer. Likewise, no current can flow into
the power system, if the winding is not connected to the power system (circuit
breaker open and power transformer energized from the other side).
3. For both internal and external earth faults, the current in the neutral connection I
always has the same direction, towards the earth (Except in case of
autotransformers).
4. The two internally processed zero sequence currents are 3I
sum between them is the REFPDIF differential current, which is equal to Idiff =
I +3I .
N 0
zone of protection
I
I zs1
zs2
I I
zs2
zs1
I
I
zs2
zs1
Uzs
Ifault
through transformer
operate
fault
block
region
3I
0
block
I
N
ROA
Internal
block
fault
fault
region region
1MRK 504 135-UEN A
3Izs1
Current in the neutral
(measured as I
as a directional reference
-
I = 3Izs1
N
because it has the same
direction for both internal
and external faults.
Return path
Zero-sequence differential
current for internal fault
Idiff = abs(3I
Idiff = 3Izs2 + 3Izs1 > 0
(reference) Idiff = Ifault
ROA = Relay Operate Angle
and the neutral
0
and I . The vectorial
0 N
Technical manual
L1
L2
L3
) serves
N
+ I )
0 N
IEC09000108-3-en.vsd
N