Dropout
It can be determined whether the dropout of the stage is to follow right after the threshold undershot or
whether it is evoked by disk emulation. "Right after" means that the pickup drops out when approx. 95 % of
the set pickup value is undershot. For a new pickup the time counter starts at zero.
The disk emulation evokes a dropout process (time counter is decrementing) which begins after de-energisa-
tion. This process corresponds to the back turn of a Ferraris-disk (explaining its denomination “Disk-Emula-
tion”). 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. This ensures a proper simulation of the
temperature rise of the protected object even for extremely fluctuating unbalanced load values. Reset begins
as soon as 90 % of the setting value is undershot, in accordance with the dropout curve of the selected charac-
teristic. In the range between the dropout value (95 % of the pickup value) and 90 % of the setting value, the
incrementing and the decrementing process is in idle state. If 5 % of the setting value is undershot, the
dropout process is completed, i.e. when a new pickup occurs, the timer starts again at zero.
Logic
Das Figure 2-96 shows the logic diagram of the unbalanced load protection with the inverse time stage (the
IEC characteristic in the example) and the two definite time stages. The protection may be blocked via a binary
input. That way, pickups and time stages are reset.
When the tripping criterion leaves the operating range of the unbalanced load protection (all phase currents
below the minimum current PoleOpenCurr. of the concerned measuring location or side or at least one
phase current is greater than 4 · Ι
N
), the pickups of all unbalanced load stages drop off.
Functions
2.8 Unbalanced Load Protection
SIPROTEC 4, 7UT6x, Manual 189
C53000-G1176-C230-5, Edition 09.2016
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