ABB REG650 Technical Manual page 136

Section 6
Differential protection
130
operate current
[ times IBase ]
5
4
3
2
1
TempIdMin
IdMin
0
0
EndSection1
IEC06000637 V2 EN
Figure 54: 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
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
1MRK 502 048-UEN A
Section 3
SlopeSection3
Restrain
3 4 5
restrain current
[ times IBase ]
en06000637.vsd
Technical manual