ABB Relion REG670, Техническое руководство

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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 (3I
ZS1
) flows towards the
power transformer neutral point and into the earth, 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 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
59 ,
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 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, which is towards the earth (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 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 should also be stable against heavy phase-to-phase internal faults, not
including earth. 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 61 and Figure 62 ) are both within the operating region. By
taking a smaller ROA, REFPDIF 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 earth-fault protection, low impedance differential protection
Restricted earth-fault protection, low impedance (REFPDIF) 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).
Section 6
1MRK502052-UEN B
Differential protection
170
Technical manual