ABB RELION RET670 Applications Manual page 276

Section 8
Impedance protection
270
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 128.
-
Z Z
0 0m
I
A
0
-
Z
Z
0 0m
B
IEC09000252 V1 EN-US
Figure 128: 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
of (Z -Z ) and Z which is equal to equation 124.
0 0m 0m
2 2
-
Z Z
0 om
=
Z
E
Z
0
EQUATION2002 V4 EN-US
The influence on the distance measurement will be a considerable overreach, which
must be considered when calculating the settings.
All expressions below are proposed for practical use. They assume the value of zero
sequence, mutual resistance R
mutual reactance X . Calculate the equivalent X
0m
according to equation 125
them for calculating the reach for the underreaching zone.

X
0 m
R R 1
= ⋅  +
0 E 0
2

R
+
 0
DOCUMENT11520-IMG3502 V2 EN-US

X
0
X X 1
= ⋅  −
0 E 0
2

R
+
 0
DOCUMENT11520-IMG3503 V2 EN-US
Z
0m
I
0
C
IEC09000252_1_en.vsd
equals to zero. They consider only the zero sequence,
0m
and equation 126 for each particular line section and use
2


2

X
0 
2

m

2

X
0 
1MRK 504 163-UUS A
-Z m in series with parallel
0 0
and R zero sequence parameters
0E 0E
Transformer protection RET670 2.2 ANSI
Application manual
(Equation 124)
(Equation 125)
(Equation 126)