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60
3 zone impedance protection (Z(n)RW)
Z
=
RFPE
+
jX
1
1
1
and
(
⋅
) Z
⋅
Z
2 K
1
=
+
0
N
1
where:
RFPE, X and KN
are the reach setting parameters
In case of short lines the possibility of covering sufficiently large fault resistance is im-
portant. The different setting options for the range in the reactive and resistive reach
provide increased flexibility for the distance protection.
An optimized overreaching communication system can increase the resistance coverage
yet further. The optimal solution in some applications is to add the directional earth-
fault function to the distance protection.
In railway applications the margin to the load impedance is, in order to avoid load con-
flicts, of primary significance. Quadrilateral characteristics with different range of set-
tings in the reactive (to cover sufficient length of the line) and resistive (to avoid load
conflicts) directions will, to a large extent, reduce the conflicts that are extremely com-
mon for circular characteristics.
A large reactive setting range, that is independent for each zone, together with current
sensitivity down to 10% of the rated current is an important factor to increase the oper-
ability of the distance protection when used on long lines.
)XQFWLRQDOLW\
Measured current and voltage values are used for the fault loop equations. The apparent
impedances are calculated and checked against the set limits.
The calculation of apparent impedances at faults for single-phase catenary is:
U
L1
Z
---------
=
ap
I
L1
&KDSWHU
/LQH LPSHGDQFH
(Equation 15)
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