1MAC050144-MB C
Section 4
Protection functions
615 series ANSI
349
Technical Manual
Figure 167:
Setting Range for Restrained Differential
If the restraining current drops below 30 percent of the differential current or the phase
angle between the winding 1 and winding 2 phase currents is less than 50 degrees, a fault
has most likely occurred in the area protected by 87T. Then the internal blocking signals
of the restrained differential are inhibited
Unrestrained Differential (High Stage)
The unrestrained differential operation can be enabled and disabled using the Enable high
set setting. The corresponding parameter values are "TRUE" and "FALSE."
The unrestrained differential trips and the output OPR_HS is activated when the amplitude
of the fundamental frequency component of the differential current exceeds the set High
trip value or when the differential current exceeds 2.5 times the value of High trip value.
The factor 2.5 (=1.8 x _2) is due to the maximum asymmetric short-circuit current.
If the restraining current drops below 30 percent of the differential current or the phase
angle between the winding 1 and winding 2 phase currents is less than 50 degrees, a fault
has occurred in the area protected by 87T. Then the trip value set for the unrestrained
differential is automatically halved and the internal blocking signals of the restrained
differential are inhibited.
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