Siemens SIPROTEC 7UT613 series Manual page 157

Figure 2-70 Logic diagram of the definite time overcurrent protection for zero sequence current — example of IEC char-
acteristic (simplified)
Dropout
7UT613/63x Manual
C53000-G1176-C160-2
2.4 Time Overcurrent Protection for Phase and Residual Currents
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-energization 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 process-
es 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.
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