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Functions
Harmonic
Stabilization
84
Immediately after the fault (A), the short-circuit currents rise strongly, causing a
equally high stabilizing current (2xthrough-flowing current). Saturation occurring on
one side (B) now causes a differential current and reduces the stabilizing current, so
that the operating point I
In contrast, the operating point moves immediately along the fault characteristic (D)
when an internal fault occurs since the stabilization current will barely be higher than
the differential current. Therefore, an internal fault is assumed as soon as the ratio I
I
has exceeded an internal threshold for a fixed minimum time.
stab
Current transformer saturation in case of an external fault is thus characterized by a
high stabilizing current flowing at the beginning, i.e. by the operating point (diagram
see Figure 2-37) moving into an area that is typical for a high-current external fault
("add-on stabilization). The add-on stabilization area is limited by the parameter I-
ADD ON STAB. and the first straight line of the characteristic (with BASE POINT 1
and SLOPE 1) (see Figure 2-38). The saturation indicator makes its decision within
the first quarter of a period after fault inception. When an external fault is detected, the
differential protection is blocked for a selectable time. The blocking is cancelled as
soon as the operating point I
tripping area. This allows to detect evolving faults in the protected area reliably even
during an external fault with current transformer saturation.
I diff
---------------
10
I NObj
9
8
2031
I DIFF>>
7
6
5
4
3
2
b
1
a
2021
I–DIFF>
1
2
3
2044
BASE POINT 2
2042
BASE POINT 1
Figure 2-38
Add-on Stabilization During Current Transformer Saturation
In transformers in particular, high short-time magnetizing currents may be present
during power-up (inrush currents). These currents enter the protected zone but do not
leave it again, so that they act like fault currents entering from one side (Figure 2-39).
/I
may move into the tripping area (C).
diff
stab
/I
moves steadily (i.e. over 2 periods) within the
diff
stab
Fault characteristic
Trip area
4
5
6
7
8
9
2056
EXF–STAB
Block area
2043
SLOPE 2
c
Add-on stabilization
10 11 12 13 14 15 16 17 18
C53000-G1176-C149-3
/
diff
d
2041
SLOPE 1
I
stab
---------------
I
NObj
7UM62 Manual
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