ABB RELION RET670 Applications Manual page 306

Section 8
Impedance protection
300
Z0
A
Z0
B
IEC99000038 V1 EN-US
Figure 142: Equivalent zero sequence impedance circuit of the double-circuit,
parallel, operating line with a single phase-to-ground fault at the
remote busbar
When mutual coupling is introduced, the voltage at the IED point A will be changed.
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-US
Figure 143: The parallel line is out of service and grounded.
-
Z0
m
Z0 m
-
Z0
m
B
Z<
en05000222.vsd
1MRK 504 163-UUS A
C
99000038.vsd
Transformer protection RET670 2.2 ANSI
Application manual
SEMOD154680-228 v1 SEMOD154680-226 v2