ABB 650 series, Technical Manual

Page: 129 / 778

1. For an external ground 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 ground fault current.
2. For an internal fault, the total ground 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 ground, 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 ground 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 grounded, 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 ground faults, the current in the neutral connection I
N
always has the same direction, towards the ground (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 (87N) differential current, which is equal to Idiff = I
N
+3I
0
.
REFPDIF (87N) is a differential protection where the line zero sequence (residual) current
is calculated from 3 line (terminal) currents, a bias quantity must give stability against
false operations due to high through fault currents. To stabilize REFPDIF at external
faults, a fixed bias characteristic is implemented.
REFPDIF (87N) should also be stable against heavy phase-to-phase internal faults, not
including ground. These faults may also give false zero sequence currents due to saturated
line CTs. Such faults, however are without neutral current, and can thus be eliminated as
a source of danger.
As an additional measure against unwanted operation, a directional check is made in
agreement with the above points 1 and 2. Operation is only allowed if the currents 3I
0
and
I
N
(as shown in figure 44 and figure 45 ) are both within the operating region. By taking a
smaller ROA, REFPDIF (87N) can be made more stable under heavy external fault
conditions, as well as under the complex conditions, when external faults are cleared by
other protections.
1MRK 502 048-UUS A
Section 6
Differential protection
123
Technical manual