ABB RET670 Applications Manual page 276

Section 3
IED application
270
Z0
A
Z0
B
IEC99000038 V1 EN
Figure 118: Equivalent zero sequence impedance circuit of the double-circuit,
parallel, operating line with a single phase-to-ground fault at the
remote busbar.
If the current on the parallel line have negative sign compared 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 the resistance is excluded, 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 terminal 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
21
ANSI11000088 V1 EN
Figure 119: The parallel line is out of service and grounded
-
Z0
m
Z0 m
-
Z0
m
B
21
ANSI11000088_1_en.vsd
1MRK504116-UUS C
C
99000038.vsd
Application manual