ABB RET670 Applications Manual page 299

1MRK504116-UUS C
Application manual
If the current on the parallel line have negative sign compare to the current on the
protected line that is, the current on the parallel line has an opposite direction compare
to the current on the protected line, the distance function will overreach. If the currents
have the same direction, the distance protection will underreach.
Calculation for a 400 kV line, where we for simplicity have excluded the resistance,
gives with X1L=0.303 Ω/km, X0L=0.88 Ω/km, zone 1 reach is set to 90% of the line
reactance p=71% that is, the protection is underreaching with approximately 20%.
The zero-sequence mutual coupling can reduce the reach of distance protection on the
protected circuit when the parallel line is in normal operation. The reduction of the
reach is most pronounced with no infeed in the line IED closest to the fault. This reach
reduction is normally less than 15%. But when the reach is reduced at one line end, it is
proportionally increased at the opposite line end. So this 15% reach reduction does not
significantly affect the operation of a permissive under-reach scheme.
Parallel line out of service and grounded
A
Z0m
Z<
DOCUMENT11520-IMG867 V1 EN
Figure 132: The parallel line is out of service and grounded.
When the parallel line is out of service and grounded at both ends on the bus bar side
of the line CT 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 132.
Z
I
A
0
Z
B
DOCUMENT11520-IMG7100 V1 EN
Figure 133: Equivalent zero-sequence impedance circuit for the double-circuit line
that operates with one circuit disconnected and grounded at both ends.
B
Z<
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Z
0 m0
Z m0
Z
0 m0
Section 3
IED application
I
0
C
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