Siemens SIPROTEC 7ST6, Руководство пользователя, Страница: 76 / 356

2 Functions
76
7ST6 Manual
E50417-G1176-C251-A3
2.3 Overcurrent Protection
The 7ST6 device has an integrated time overcurrent protection function. This function
can be used as a backup overcurrent protection which works independently of the
other protection and monitoring functions, including distance protection. The back-up
time overcurrent protection could for instance be used as the only short-circuit protec-
tion if the voltage transformers are not yet available when the feeder is initially com-
missioned.
2.3.1 Functional Description
General The time overcurrent protection has three stages for each phase current:
• Two overcurrent stages with a definite time characteristic (O/C with DT),
• One overcurrent stage with definite-time or inverse-time characteristic (selectable)
These three stages are independent of each other and are freely combinable. Instan-
taneous tripping can be set in all stages when the protected line is switched onto a
fault. If some stages are not needed, those not needed can be deactivated by setting
the pickup value to
∞
.
The time overcurrent protection can be switched on or off by parameter settings, or via
binary inputs. The figure below shows the control logic:
Figure 2-13 Control logic of the overcurrent protection
Definite Time Very
High Set Current
Stage I>>> The current of the overhead contact line (or in auto-transformer systems optionally the
summated overhead contact line current and the negative feeder current; can be set
at address
213
) are numerically filtered and compared with the setting
I>>>
. Currents
above the associated pickup value are detected and signalled. After expiry of the as-
sociated time delay
T I>>>
a trip command is issued. The dropout value is approxi-
mately 5 % less than the pickup value, but at least 1.8 % of the rated current, below
the pickup value.
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