ABB REG650 Technical Manual page 129

1MRK 502 043-UUS B
Technical Manual
operate current
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5
4
3
2
1
TempIdMin
IdMin
0
0
EndSection1
IEC06000637 V2 EN
Figure 50: Operate-restrain characteristic
GENPDIF (87G) can also be temporarily 'desensitized' if the Boolean setting
OperDCBiasing is set to 1 (TRUE). In this case, the DC component is extracted on-
line from the instantaneous differential currents. The highest DC component is taken as
a kind of bias in the sense that the highest sensitivity of the differential protection is
inversely proportional to the ratio of this DC component to the maximum fundamental
frequency differential current. Similar to the 'desensitization' described above, a
separate (temporary) additional limit is activated. The value of this limit is limited to
either the generator rated current, or 3 times IdMin, whichever is smaller. This
temporary extra limit decays exponentially from its maximum value with a time
constant equal to T = 1 second. This feature must be used when unmatched CTs are
used on the generator or shunt reactor, especially where a long DC time constant can
be expected. The new limit is superposed on the otherwise unchanged operate-bias
characteristic, and temporarily determines the highest sensitivity of the differential
protection. This temporary sensitivity must be lower than the sensitivity in section 1 of
the operate-bias characteristic.
Operate
unconditionally
UnrestrainedLimit
Operate
conditionally
Section 1 Section 2
SlopeSection2
1 2
EndSection2
Section 6
Differential protection
Section 3
SlopeSection3
Restrain
3 4 5
restrain current
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