Functions
6-1457SA522 Manual
C53000-G1176-C119-2
Stub Protection When using the I STUB protection the pick-up thresholds ,SK!678% (address )
and ,!678% (address ) are usually not critical, as this protection function is
only activated when the line isolator is open which implies that every measured current
should represents a fault current. With a 1
1
/
2
–circuit breaker arrangement similar to
Figure 6-79 it is possible that large short circuit currents flow from busbar A to busbar
B or to feeder 2 via the current transformers. These currents could cause different
transformation errors in the two current transformer sets I
A
and I
B
, especially in the
saturation range. The protection should therefore not be set unnecessarily sensitive.
If the minimum short circuit currents on the busbars are known, the pick-up threshold
,SK!678%is set somewhat (approx. 10 %) below the minimum two phase short cir-
cuit current, ,!678% is set below the minimum single-phase current.
The time settings 7,SK678% (address ) and 7,678% (address ) are
set to ∞ for this application, to prevent the protection from operating while the line iso-
lator is closed. They may however also be used to implement an independent back-
up stage.
If this stage is applied differently, similar considerations as for the other overcurrent
stages apply.
The parameter ,678%7HOHS%, (address ) determines whether the delay
times 7,SK678% (address ) and 7,678%
(address ) can be
bypassed via a binary input „!2&,QVW75,3“. The binary input (if allocated) is
applied to all stages of the time-overcurrent protection. With ,678%7HOHS%, =
<(6 you can set the ,6WXE-stages to trip immediately after the pick-up, only if the
binary input is activated. Set time delays for ,678%7HOHS%, = 12 are always ac-
tivated.
If the,6WXE-stage is to trip when switching the line on to a fault with or without a
short delay, 627)7LPH'(/$< (address , see above and refer to Sub-section
“General”), the parameter ,678%627) (address ) is set to <(6. If using the
stub protection, then set to 12 as the effect of this protection function only depends on
the position of the isolator.
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