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
Angle Decision in Internal and External Faults
For tripping to occur, the neutral-point current I
0
* must reach the value I
REF,off
. The characteristic curve in the
left part of the figure can be determined from the following relationship:
I
REF,off
= I
char
+ k · I
Angle,REF
where:
I
char. curve
Pickup value resulting from the pickup value increase
k Factor (permanently set to 4.05657. With this value, the limit angle at |I
0
*| = |I
0
**| is
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
Angle,REF
is used to establish
the tripping decision.
NOTE
With an external fault, the current I
Angle,REF
is always > 0.
The following figure shows the behavior under different fault conditions.
Also observe that the restraint current (I
Rest.,REF
) always results from the sum of all currents (phase currents of
the measuring points and the neutral-point current). For pickup, the differential current I
Diff,REF
is necessary.
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.
Protection and Automation Functions
6.43 Restricted Ground-Fault Protection
SIPROTEC 5, Overcurrent Protection, Manual 867
C53000-G5040-C017-8, Edition 07.2017
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