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
s
) in the
protection relay measuring branch. R
s
increases the impedance of the protection
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
in
) and connection wire resistances (R
m
/2) are
also shown in
Figure 441
.
Figure 441
demonstrates a simplified circuit consisting only of one incoming and
outgoing feeder. To keep it simple, the voltage-dependent resistor (R
u
) is not
included. The wiring resistances are presented as total wiring resistances R
m1
and R
m2
.
R
m1
is the maximum wiring resistance concerning all incoming feeder
sets, whereas R
m2
is the maximum wiring resistance concerning all
outgoing feeder sets.
The lower part of
Figure 441
shows the voltage balance when there is no fault in the
system and no CT saturation.
Protection functions
1MRS759142 F
776 REX640
Technical Manual
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