ABB Relion Technical Manual page 130

Section 6
Differential protection
6.2.7.2
124
Operate and restrain characteristic
Restricted earth-fault protection, low impedance (REFPDIF, 87N) 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 (87N) is not sensitive to inrush and
overexcitation currents. The only danger is current transformer saturation.
REFPDIF (87N) 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(87N)
Default sensitivity Idmin Max. base
(zone 1) sensitivity Idmin
(zone 1)
% IBase % IBase
30 5
The bias (restrain) current is supposed to give stability to REFPDIF(87N). 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.
Min. base sensitivity End of zone
Idmin (zone 1) 1
% IBase % IBase
100 125
1MRK 502 048-UUS A
First slope Second
slope
% %
70 100
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