Digital Signal Processing Reference
In-Depth Information
Fig. 10.15 Conceivable
versions of adaptive adjusting
of the differential curve:
a vertical shifting b slope
change c additional restrain
area
(a)
t
=
t
s
1
t
=
t
s
2
(b)
t
=
t
s
1
t
=
t
s
2
(c)
t
=
t
s
1
t
=
t
s
2
The design of the adaptive differential protection included also research on
appropriate versions and level of adaptation. Adaptive adjusting of the differential
characteristic may be realized according to one of the following scenarios (see
Fig.
10.15
):
(a)
shifting up the stabilizing curve before the saturation began, then shifting it
down to the standard position after CT saturation disappeared,
(b)
changing the slope m
1
of the differential curve (stabilizing section) to a
higher value, then return to standard settings,
(c)
forming an additional transitory restraining area above the stabilizing curve
e.g. polygonal shape (covering the area of foreseeable location of current
trajectory), then removing the area.
Versions (a) and (b) were examined in more detail, for both of them appropriate
formulae for the required level of adaptation have also been determined [
9
].
In Fig.
10.16
an example of the adaptive protection operation for a real external
fault close to generator terminals is presented. The fault was accompanied with DC-
induced CT saturation. In Fig.
10.16
a the current signals from both sides of the
protected generator (phase L1) are shown. The effects of adaptive adjusting of the
differential curve can be seen in Fig.
10.16
b, c. Appropriate changes (version a—
shifting up, version b—setting new value of the stabilizing slope) were introduced at
time instant t
s1
= 62.3 ms, i.e. before the non-adaptive relay picks up (curve Out (no
ad.) in Fig.
10.16
b). The adaptation by shifting up (version a) was not effective
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