The required differential current for tripping at a certain stabilizing current level can be
calculated using the formulae:
For a stabilizing current lower than End section 1
I I Low trip values
doperate n
[% ] = Set
GUID-4FAC8F94-FBB0-47DB-A952-F995889969C6-ANSI V1 EN (Equation 110)
For a stabilizing current higher than End section 1 but lower than End section 2
I I Low trip value I I End section 1 Slop
doperate n b n
[% ] ( [% ] )= + − ⋅ ee section 2
GUID-EB0A327A-4079-45A5-87E6-AA30A8D2220C-ANSI V1 EN (Equation 111)
For higher stabilizing current values exceeding End section 2
I I Low trip value End section End section
doperate n
[% ] (= + −2 1)) ( [% ] )⋅ + −Slope section I I End section
b n
2 2
GUID-E9DA330C-84A8-40C2-BCFB-1F7E517395D8-ANSI V1 EN (Equation 112)
When the differential current exceeds the operating value determined by the operating
characteristics, the OPR_LS output is activated. The TRIP output is always activated
when the OPR_LS output activates.
The trip signal due to the biased stage can be blocked by the activation of the
BLK_OPR_LS or BLOCK input. Also, when the operation of the biased low stage is
blocked by the waveform blocking functionality, the INT_BLKD output is activated
according to the phase information.
The phase angle difference between the two currents I_A1 and I_A2 is theoretically 180
electrical degrees for the external fault and 0 electrical degrees for the internal fault
conditions. If the phase angle difference is less than 50 electrical degrees or if the biasing
current drops below 30 percent of the differential current, a fault has most likely occurred
in the area protected by 87G, 87M. Then the internal blocking signals (CT saturation and
DC blocking) of the biased stage are inhibited.
1MAC059074-MB A Section 4
Protection functions
615 series ANSI 573
Technical Manual
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