Siemens SIPROTEC 4 7UT6 Series Manual page 142

Functions
2.4 Time Overcurrent Protection for Phase and Residual Currents
[logik-umz-abh-amz-nullstrom-iec-kennlinie-121102-st, 1, en_GB]
Figure 2-72
Dropout
You can determine whether the dropout of a stage is to follow right after the threshold is undershot or
whether it is to be evoked by disk emulation. "Right after" means that the pickup drops out when approx. 95 %
of the set pickup value is undershot and in a new pickup the time counter starts at zero.
The disk emulation evokes a dropout process (time counter is decrementing) which begins after de-energiza-
tion and this process corresponds to the reset rotation of a Ferraris disk (explaining its denomination "disk
emulation"). In case several faults occur successively, it is ensured that due to the inertia of the Ferraris disk
the "history" is taken into consideration and the time behaviour is adapted. Reset begins as soon as 90 % of the
setting value is undershot, in accordance with the dropout curve of the selected characteristic. In the range
between the dropout value (95 % of the pickup value) and 90 % of the setting value, the incrementing and the
decrementing processes are in idle state.
Disk emulation offers advantages when the grading coordination chart of the time overcurrent protection
must be coordinated with other devices in the system on an electro-mechanical or an induction base.
142
Logic diagram of the definite time overcurrent protection for zero sequence current —
example of IEC characteristic (simplified)
SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016