ABB Relion Technical Manual page 147

1MRK 502 048-UUS A
Technical manual
operate current
[ times IBase ]
5
4
3
2
1
TempIdMin
IdMin
0
0
EndSection1
IEC06000637 V2 EN
Figure 54: 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 bounded 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.
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 6
Differential protection
Section 3
SlopeSection3
Restrain
3 4 5
restrain current
[ times IBase ]
en06000637.vsd
141