2.2 Distance protection
71
7SA522 Manual
C53000-G1176-C155-3
the other line data — during the parameterisation of the device. The line impedance is
calculated similar to the calculation shown earlier.
Figure 2-16 Earth fault on a double circuit line
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 2-16) — 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 2-16 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 Figure 2-16, the current I
E
is
larger than I
EP
: Compensation is applied at I by including Z
M
· I
EP
in the evaluation; at
II compensation is not applied.
Switching onto a
Fault
When the circuit breaker is switched onto a dead fault with a manual close command,
fast tripping by the distance protection is possible. By setting parameters it may be de-
termined which zone(s) is/are released following a manual close (refer to Figure 2-17).
The line energization information (input “Line closure”) are derived from the line ener-
gization recognition.
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