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Functions
2.5 Distance Protection
Parallel line measured value correction (optional)
During earth faults on parallel lines, the impedance values calculated by means of the loop equations are influ-
enced by the coupling of the earth impedance of the two conductor systems (Figure 2-48). This causes mea-
suring errors in the result of the impedance computation unless special measures are taken. A parallel line com-
pensation may therefore be activated. In this manner the earth current of the parallel line is taken into
consideration by the line equation and thereby allows for compensation of the coupling influence. The earth
current of the parallel line must be connected to the device for this purpose. The loop equation is then as shown
below, refer also to Figure 2-45.
I
· Z
– I
L3
L
E
where I
is the earth current of the parallel line and the ratios R
EP
eters, resulting from the geometry of the double circuit line and the nature of the ground below the line. These
line parameters are input to the device — along with all the other line data — during the parameterisation.
Figure 2-48
Without parallel line compensation, the earth current on the parallel line will in most cases cause the reach
threshold of the distance protection to be shortened (underreach of the distance measurement). In some cases
— for example when the two feeders are terminated to different busbars, and the location of the earth fault is
on one of the remote busbars (at B in Figure 2-48) — an overreach may occur.
The parallel line compensation only applies to faults on the protected feeder. For faults on the parallel line, the
compensation may not be carried out, as this would cause severe overreach. The relay located in position II in
Figure 2-48 must therefore not be compensated.
Earth current balance is therefore additionally provided in the device, which carries out a cross comparison of
the earth currents in the two lines. The compensation is only applied to the line end where the earth current of
the parallel line is not substantially larger than the earth current in the line itself. In example in Figure 2-48, the
current I
is larger than I
E
sation is not applied.
Blocking of zone Z1
If the main protection functions - differential protection and distance protection - operate in parallel, the distance
protection of zone Z1 may pick up before the differential protection (e.g. in the case of close-up faults). If this
is desired, the distance protection works as a „booster" stage for fast tripping. If the fast tripping acts only on
one end of the line, accelerated tripping of zone Z1 is not desired (see also Section 2.5.1.4).
There are two ways of blocking Z1. If the device is operated in differential protection mode, zone Z1 can be
blocked by setting a parameter (address 1533 Z1 blkd by diff). Another way of blocking the zone is to
set a binary input (No. 3610 „>BLOCK Z1-Trip").
130
· Z
– I
· (Z
/3) = U
E
EP
0M
L3-E
Earth fault on a double circuit line
: compensation is applied at I by including Z
EP
/3R
und X
/3X
are constant line param-
0M
L
0M
L
· I
in the evaluation; at II compen-
M
EP
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
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