ABB RELION RET670 - Page 414

When the parallel line is out of service and grounded at both line ends on the bus bar
side of the line CTs so that zero sequence current can flow on the parallel line, the
equivalent zero sequence circuit of the parallel lines will be according to figure 195.
A
B
C
IEC09000252_1_en.vsd
I
0
I
0
Z
0m
Z
0
-
Z
0m
Z
0
-
Z
0m
IEC09000252 V1 EN-US
Figure 195: Equivalent zero sequence impedance circuit for the double-circuit line
that operates with one circuit disconnected and grounded at both ends
Here the equivalent zero-sequence impedance is equal to Z
0
-Z
0
m in parallel with (Z
0
-
Z
0m
)/Z
0
-Z
0m
+Z
0m
which is equal to equation
317.
2 2
0
0
om
E
Z Z
Z
Z
-
=
EQUATION2002 V4 EN-US (Equation 317)
The influence on the distance measurement will be a considerable overreach, which
must be considered when calculating the settings. It is recommended to use a separate
setting group for this operation condition since it will reduce the reach considerably
when the line is in operation.
All expressions below are proposed for practical use. They assume the value of zero
sequence, mutual resistance R
0m
equals to zero. They consider only the zero sequence,
mutual reactance X
0m
. Calculate the equivalent X
0E
and R
0E
zero sequence parameters
according to equation
318 and equation 319 for each particular line section and use
them for calculating the reach for the underreaching zone.
R R
X
R X
E
m
0 0
0
2
0
2
0
2
1= ⋅ +
+








DOCUMENT11520-IMG3502 V2 EN-US (Equation 318)
X X
X
R X
E
m
0 0
0
2
0
2
0
2
1= ⋅ −
+








DOCUMENT11520-IMG3503 V2 EN-US (Equation 319)
Section 8 1MRK 504 163-UUS A
Impedance protection
408 Transformer protection RET670 2.2 ANSI
Application manual