ABB REG650 ANSI, Technical Manual

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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
I
zs1
I
zs1
I
zs1
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
I
zs2
I
zs2
I
zs2
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 45: Zero sequence currents at an internal earth fault
1. For an external earth fault (figure 44 ), the residual current 3I
0
and the neutral
current I
N
have equal magnitude, but they are seen by the IED as 180 degrees out-
of-phase if the current transformers are connected as in figure 44 , 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
0
flows out into the connected power system. These two primary currents can
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
N
always has the same direction, towards the earth (Except in case of
autotransformers).
4. The two internally processed zero sequence currents are 3I
0
and I
N
. The vectorial
sum between them is the REFPDIF differential current, which is equal to Idiff =
I
N
+3I
0
.
1MRK 502 048-UEN A
Section 6
Differential protection
113
Technical manual