ABB Relion Technical Manual page 146

Section 6
Differential protection
140
currents in this section can be tolerances of the current transformers used on both sides of
the protected generator. Slope in section 1 is always zero percent. Normally, with the
protected machine at rated load, the restrain, bias current will be around 1 p.u., that is,
equal to the machine rated current.
In section 2, a certain minor slope is introduced which is supposed to cope with false
differential currents proportional to higher than normal currents through the current
transformers.
The more pronounced slope in section 3 is designed to result in a higher tolerance to
substantial current transformer saturation at high through-fault currents, which can be
expected in this section.
Temporarily decreased sensitivity of differential protection is activated if the binary input
DESENSIT is (temporarily) set to 1 (TRUE). In this case, a new, separate limit is
superposed to the otherwise unchanged operate-bias characteristic. This limit is called
TempIdMin and it is available as a setting. The value of the setting TempIdMin must be
given as a multiple of the setting IdMin. In this case no trip command can be issued if all
fundamental frequency differential currents are below the value of the setting
TempIdMin.
AddTripDelay: If the input DESENSIT is activated also the operation time of the
protection function can be increased by using the setting AddTripDelay.
1MRK 502 048-UUS A
Technical manual