Siemens SIPROTEC 7UT613 series, Manual

Page: 208 / 638

2 Functions
208
7UT613/63x Manual
C53000-G1176-C160-2
Figure 2-93 Inverse time characteristic for unbalanced load protection
Dropout
It can be determined whether the dropout of the stage is to follow right after the thresh-
old 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-energisation. This process corresponds to the back turn of a Ferraris-
disk (explaining its denomination „Disk Emulation“). In case several faults occur suc-
cessively, 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 unbal-
anced load values. 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 incre-
menting 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
Figure 2-94 shows the logic diagram for the breaker failure protection with the thermal
stage (the IEC characteristic in the example) and the two definite time stages. The pro-
tection 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 protec-
tion (all phase currents below the minimum current
I
-REST
of the concerned mea-
suring location or side or at least one phase current is greater than 4 ·
I
N
), the pickups
of all unbalanced load stages drop off.