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 (3I
ZS1
) flows towards the power transformer
neutral point and into the ground, while the other zero sequence current (3I
ZS2
) flows 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
62,
which is the orientation recommended by ABB. 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, which is towards the ground (except in case of autotransformers
where the direction can vary).
4. The two internally processed zero sequence currents are 3I
o
and I
N
. The vectorial sum is the
REFPDIF (87N) differential current, which is equal to Idiff = I
N
+3I
o
.
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
o
and I
N
(as shown in
Figure
64 and Figure 65) 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.
6.4.7.2 Restricted ground fault protection, low impedance differential protection
M5447-20 v12
Restricted earth-fault protection, low impedance (REFPDIF, 87N) is a protection of differential
type, a unit protection, whose settings are independent of any other protection. It has some
advantages compared to the transformer differential protection. It is less complicated, as no
current phase correction or magnitude correction are needed, not even in the case of an eventual
on-load tap changer (OLTC). REFPDIF (87N) is not sensitive to inrush and overexcitation currents.
The only danger is an eventual current transformer saturation.
REFPDIF (87N) has only one operate-bias characteristic, which is described in the table
77 and
shown in Figure 66.
Table 77: Data of the operate-bias characteristic of REFPDIF(87N)
Default sensitivity
Idmin (zone 1)
Max. base
sensitivity Idmin
(zone 1)
Min. base
sensitivity Idmin
(zone 1)
End of
zone 1
First slope Second
slope
% IBase % IBase % IBase % IBase % %
30 5 100 125 70 100
1MRK 502 066-UUS B Section 6
Differential protection
171
Technical manual
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