Functions
188
7SS52 V4 Manual
C53000-G1176-C182-3
5.16 Overcurrent Protection in the Bay Unit
5.16.1 Functional Description
An optional overcurrent protection is provided as a backup protection in the bay units
of the distributed busbar and breaker failure protection SIPROTEC 7SS52 V4. This
protection works independently of the busbar protection function and remains active
even if the link to the central unit is interrupted or if the bay unit is set with the param-
eter Bay status (XX12/CU) to bay out of service.
The overcurrent protection function can be activated and deactivated for phase cur-
rents and earth current separately by the parameters O/C PHASES (1201/BU) or O/
C EARTH (1501/BU). Likewise, the associated parameters can be set separately for
phase currents and earth current.
The overcurrent protection function can be blocked from the central unit as well as
from the bay unit
(Figure 5-43, page 203). Additionally, individual function elements
can be blocked selectively via the binary inputs of the bay unit (Figure 5-40, page 189
and Figure 5-41, page 189).
The overcurrent protection can be set to either a definite time or an inverse time char-
acteristic. For the inverse time characteristic of the overcurrent stage there are three
types according to IEC 255-3 (Figure 9-1, page 320):
• Normal inverse, acc. to IEC 255-3 (type A)
• Very inverse, acc. to IEC 255-3 (type B)
• Extremely inverse, acc. to IEC 255-3 (type C)
The characteristic is set with the parameters CHARAC. PH (7812/BU) for the overcur-
rent stage of the phase currents, and CHARAC. E (7815/BU) for the overcurrent stage
of the earth current. The characteristic of the inverse-time O/C protection is selected
with the parameter CHARACTER. (1211/BU) or (1511/BU).
The selected overcurrent characteristic curve can be superimposed by a high-set
stage I>> (1202/BU) or IE>> (1502/BU) that works always with a current-independ-
ent command time. The principle of the overcurrent protection for phase currents and
earth current is shown in Figures 5-40, page 189 and 5-41, page 189.
All stages can be set individually and independently of one another. For the selection
of threshold values and delay times, the interaction of the bay unit with the busbar pro-
tection should be considered.
If a circuit breaker is manually closed onto a fault current, it can be re-opened imme-
diately. It is possible to override the delay time for either the overcurrent stages with
the parameters MAN.CLOSE (1221/BU) or (1521/BU), or for the high-set current stag-
es with the binary input “>CB man.close” (FNo. 7618/BU). In that case an unde-
layed trip command is issued if a fault is detected on the respective stage.
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