Siemens siprotec 7SA6, Manual

Page: 238 / 876

Functions
6-48
7SA6 Manual
C53000-G1176-C156-2
ground below the line. These line parameters are input to the device — along with all
the other line data — during the parameterisation of the device. The line impedance
is calculated with the equation below similar to the calculation shown earlier.
Figure 6-27 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 6-27) — it is possible that an overreach may oc-
cur.
The parallel line compensation only applies to faults on the protected line. For faults
on the parallel line, the compensation may not be carried out, as this would cause se-
vere overreach. The relay located in position
II
in Figure 6-27 may therefore not be
compensated.
An 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 6-27, the current I
E
is
larger than I
EP
: compensation is applied at I in that Z
M
· I
EP
is included in the evalua-
tion; at
II
compensation is not applied.
Switching onto a
Fault When the circuit breaker is switched onto a fault with a manual close command, fast
tripping by the distance protection is possible. By setting parameters it may be deter-
mined which zone(s) is/are released following a manual close (refer to Figure 6-28).
The line energization information (input “SOTF”) are derived from the state recogni-
tion, refer also to Sub-section 6.22.1.
Z
L
U
L3–E
I
L3
Z
E
Z
L
⁄
I
E
Z
M
Z
L
⁄
I
EP
⋅
–
⋅
–
--------------------------------------------------------------------------------
=
e.g. L3–E
Z
L
Z
E
Z
M
I
EP
I
E
I
L3
U
L3-E
A B
I
II