ABB REG650 Technical Manual page 113

1MRK 502 034-UEN -
Technical Manual
operate current
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5
4
3
2
1
TempIdMin
IdMin
0
0
EndSection1
IEC06000637 V2 EN
Figure 51: Operate-restrain characteristic
GENPDIF 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 shut reactor,
especially where a long DC time constant can be expected. The new limit is
superposed 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.
This DC desensitization is not active, if a disturbance has been
detected and characterized as internal fault.
Operate
unconditionally
UnrestrainedLimit
Operate
conditionally
Section 1 Section 2
SlopeSection2
1 2
EndSection2
Section 5
Differential protection
Section 3
SlopeSection3
Restrain
3 4 5
restrain current
[ times IBase ]
en06000637.vsd
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