Differential Protection (ANSI 87G/87M/87T)
87
7UM62 Manual
C53000-G1176-C149-3
Figure 2-41
Pickup of the Differential Protection
If stabilization by higher-order harmonics is activated, the system first performs a
harmonics analysis (for about 1 period) to check the stabilization conditions, if
required. If it is not, the trip command is issued as soon as the tripping conditions are
fulfilled (hatched area in Figure 2-36).
For special cases, the trip command can be delayed.
Figure 2-42 shows a simplified diagram of the tripping logic.
Reset is initiated when, during 2 periods, pick-up is no longer recognized in the
differential values, i.e. the differential current has fallen below <70 % of the set value,
and the other pickup conditions are no longer fulfilled either.
If a trip command has not been initiated, the fault is considered to be over.
If a trip command had been initiated, it is maintained for the minimum command
duration set in the general device data for all protection functions (see also Section
2.3).
0.85
Steady-state
characteristic
I
diff
I
NObj
----------------
Pickup
I
stab
I
NObj
----------------
I–DIFF>
0.85 ·
I–DIFF>
EXF–STAB
Beginning of
add-on stabilization
