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In an external short circuit (180°), the current becomes positive. At a smaller angle (<180°) due to transformer
saturation, the angle remains positive. The amount also decreases.
[dwwinken-011112-01.tif, 1, en_US]
Figure 6-269
For tripping to occur, the neutral-point current I
left part of the figure can be determined from the following relationship:
I
= I
+ k · I
REF,off
char
where:
I
Pickup value resulting from the pickup value increase
char. curve
k
Factor (permanently set to 4.05657. With this value, the limit angle at |I
precisely 100°. No tripping is possible from this angle on.)
Processing of Several Measuring Points on One Side
If several measuring points are present on one side of a star winding, as with a 1 1/2 circuit-breaker layout, the
angle decision occurs separately for each measuring point. The maximum current I
the tripping decision.
NOTE
i
i
With an external fault, the current I
The following figure shows the behavior under different fault conditions.
Also observe that the restraint current (I
the measuring points and the neutral-point current). For pickup, the differential current I
This differential current results from the geometric sum of all adapted residual currents. The residual currents
of the measuring points on the outgoing side and of the neutral-point current are meant here.
SIPROTEC 5, Overcurrent Protection, Manual
C53000-G5040-C017-8, Edition 07.2017
Angle Decision in Internal and External Faults
Angle,REF
Angle,REF
Rest.,REF
* must reach the value I
0
is always > 0.
) always results from the sum of all currents (phase currents of
Protection and Automation Functions
6.43 Restricted Ground-Fault Protection
. The characteristic curve in the
REF,off
*| = |I
**| is
0
0
is used to establish
Angle,REF
is necessary.
Diff,REF
867
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