Section 1
Operate
conditionally
UnrestrainedLimit
Section 2
Section 3
Restrain
Operate
unconditionally
5
4
3
2
1
0
0 1 2 3 4 5
IdMin
EndSection1
EndSection2
restrain current
[ times IBase ]
operate current
[ times IBase ]
SlopeSection2
SlopeSection3
en06000637.vsd
TempIdMin
IEC06000637 V2 EN-US
Figure 55: Operate-restrain characteristic
GENPDIF (87G) can also be temporarily ‘desensitized’ if the Boolean setting
OperDCBiasing
is set
to 1 (TRUE). 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.
The DC component is always calculated from the instantaneous differential current in each phase
based on the sample values in the previous cycle.
This DC desensitization is not active, if a disturbance has been detected and
characterized as internal fault.
1MRK 502 066-UUS B Section 6
Differential protection
157
Technical manual
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