ABB RELION 670 Series Technical Manual page 134

Section 6
Differential protection
operate current
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IdMin
IEC05000187 V2 EN-US
Figure 36: Description of the restrained, and the unrestrained operate characteristics
where:
slope
EQUATION1246 V1 EN-US
The operate - restrain characteristic is tailor-made and can be designed freely by the user after his
needs. The default characteristic is recommended to be used. It gives good results in a majority of
applications. The operate - restrain characteristic has in principle three sections with a section-
wise proportionality of the operate value to the bias (restrain) current. The reset ratio is in all parts
of the characteristic equal to 0.95.
Section 1: This is the most sensitive part on the characteristic. In section 1, normal currents flow
through the protected circuit and its current transformers, and risk for higher false differential
currents is relatively low. An un-compensated on-load tap-changer is a typical reason for existence
of the false differential currents in this section. The slope in section 1 is always zero percent.
Section 2: 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.
128
5
4
3
2
Section 1
1
0
0
EndSection1
D
Ioperate
= ×
100%
D
Irestrain
Operate
unconditionally
UnrestrainedLimit
Operate
conditionally
Section 2
SlopeSection2
1 2 3
EndSection2
Section 3
SlopeSection3
Restrain
4 5
restrain current
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1MRK 502 066-UUS B
Technical manual