ABB RELION RET670 Applications Manual page 277

1MRK 504 163-UUS A
Transformer protection RET670 2.2 ANSI
Application manual
Parallel line out of service and not grounded
A
OPEN
CLOSED
21
ANSI05000223 V1 EN-US
Figure 129: Parallel line is out of service and not grounded
When the parallel line is out of service and not grounded, the zero sequence on that
line can only flow through the line admittance to the ground. The line admittance is
high which limits the zero-sequence current on the parallel line to very low values. In
practice, the equivalent zero-sequence impedance circuit for faults at the remote bus
bar can be simplified to the circuit shown in figure
The line zero sequence mutual impedance does not influence the measurement of the
distance protection in a faulty circuit. This means that the reach of the underreaching
distance protection zone is reduced if, due to operating conditions, the equivalent zero
sequence impedance is set according to the conditions when the parallel system is out
of operation and grounded at both ends.
-
Z Z
0 0m
I
0
A
-
Z Z
0 0m
B
IEC09000255 V1 EN-US
Figure 130: Equivalent zero-sequence impedance circuit for a double-circuit line
with one circuit disconnected and not grounded
The reduction of the reach is equal to equation 127.
(
1
× × +
2 Z Z
1 0 E
3
=
K
U
( )
1
× × +
2 Z Z
1 0
3
EQUATION1284 V1 EN-US
B
OPEN
Z0m
CLOSED
21
en05000223_ansi.vsd
Z
0m
I
0
C
IEC09000255_1_en.vsd
)
+
R
Z
f
= -
1
(
× ×
Z 2 Z
+
R
1
0
f
Impedance protection
129
2
m 0
)
+ +
Z 3 R
0
f
Section 8
M17048-537 v5
(Equation 127)
271