ABB RELION REX640 Technical Manual page 776

Protection functions
If there is a fault outside the zone, a high current, known as the through-fault
current, can go through the protected object. This can cause partial saturation
in the CTs. The relay operation is avoided with a stabilizing resistor (R
protection relay measuring branch. R
relay; hence the name high-impedance differential scheme.
Figure 440: Phase-segregated bus differential protection based on high-impedance
principle
CT secondary winding resistances (R
also shown in
Figure 441
outgoing feeder. To keep it simple, the voltage-dependent resistor (R
included. The wiring resistances are presented as total wiring resistances R
.
m2
The lower part of
system and no CT saturation.
776
Figure 441
.
demonstrates a simplified circuit consisting only of one incoming and
R is the maximum wiring resistance concerning all incoming feeder
m1
sets, whereas R is the maximum wiring resistance concerning all
m2
outgoing feeder sets.
Figure 441
shows the voltage balance when there is no fault in the
increases the impedance of the protection
s
) and connection wire resistances (R
in
1MRS759142 F
) in the
s
/2) are
m
) is not
u
and R
m1
REX640
Technical Manual