Table of Contents
Advertisement
2 Functions
where I
parameter, 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 parameterization of the device. The line impedance is
calculated with the equation below similar to the calculation shown earlier.
Figure 2-38
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 (under-
reach 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 2-38) — it is possible that 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 2-38 may 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 2-38, the current I
is larger than I
ation; at II compensation is not applied.
Blocking of Zone
If the main protection functions - differential protection and distance protection -
Z1
operate in parallel, the distance protection of Zone Z1 may pick up before the differen-
tial protection (e.g. in the case of close-up faults). If this is desired, the distance pro-
tection works as a „booster" stage for fast tripping. If the fast tripping acts on one end
of the line only, 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 operates in differential protection
mode, zone Z1 can be blocked by setting a parameter (address 1533 Z1 blkd by
124
I
· Z
– I
· Z
– I
· Z
L3
L
E
E
EP
M
is the earth current of the parallel line and the ratio Z
EP
Earth fault on a double circuit line
: compensation is applied at I in that Z
EP
= U
L3-E
/Z
is a constant line
M
L
· I
is included in the evalu-
M
EP
7SD5 Manual
C53000-G1176-C169-1
E
Hide quick links:
Advertisement
Table of Contents
