Digital Signal Processing Reference
In-Depth Information
Impedance plane
-
fault
-
normal conditions
(a)
(b)
jX
A
R
F
1
2
B
3
-
Mho characteristic
F
1
Z
′
AB
Z
AB
Z
A
min
F
2
1
A
B
Load
2
A
R
A
Z
A
F
2
F
1
Fig. 10.3 Illustration of the decision making in distance protection: a simple system with faults,
b impedance plane
X
Zone
2
Zone
1
R
R
PH
Zone
3
Fig. 10.4
Polygonal shape of distance characteristic
The relay manufacturers offer protection devices with distance function of
various pick-up characteristics. Except Mho type the polygonal shapes are also in
use, one of them is shown in Fig.
10.4
. Independent of the impedance charac-
teristic the condition (
10.3
) is checked in numerical way, by analysis of the
equations describing the curve.
It is also a frequent practice to apply protection schemes where one criterion
quantity is confronted with another one on a common decision plane. A repre-
sentative example of such a scheme is the differential protection of transmission
line, transformer, etc. Independent of the type of object to be protected the trip
decision is met when the trajectory of measurement loci on the Diff-Bias plane
(differential versus stabilization currents) exceeds the pick-up characteristic (see
Fig.
10.5
). After the consecutive points of the curve enter the tripping region
located above the section-wise characteristic the relay picks up, and eventually the
procedure of checking additional criteria (second harmonic level—for excluding
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