REFPDIF 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 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
0
and I
N
(as shown in figure
44 and figure 45) 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.
6.2.7.2 Operate and restrain characteristic
Restricted earth-fault protection, low impedance (REFPDIF) is a winding protection
of the differential type, whose settings are independent of any other protection.
Compared to the transformer differential protection it has some advantages. It is less
complicated as no current phase correction and magnitude correction are needed, not
even in the case of an On-Load Tap-Changer (OLTC). REFPDIF is not sensitive to
inrush and overexcitation currents. The only danger is current transformer saturation.
REFPDIF has a fixed operate-restrain characteristic, which is described in table
37,
and shown in figure 46.
Table 37: Data of the operate-restrain characteristic of REFPDIF
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
The bias (restrain) current is supposed to give stability to REFPDIF. The bias current
is a measure of how difficult the conditions are under which the CTs operate. The
higher the bias current, the more difficult conditions can be suspected, and the more
likely that the calculated differential current has a component of a false current,
primarily due to CT saturation. This "law" is formulated by the operate-bias
characteristic. The restrained part of the differential protection compares the
calculated fundamental differential currents, and the bias current, by applying them to
the operate-restrain characteristic. The operate-restrain characteristic is represented
by a double-slope, doublebreakpoint characteristic, as shown in
46. The restrained
characteristic is only determined by IdMin, all other parameters are fixed.
Section 6 1MRK 502 048-UEN A
Differential protection
114
Technical manual
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