ABB RELION REX640 Technical Manual page 762

Protection functions
Figure 433: Connection scheme for the restricted earth-fault protection according
to the high-impedance principle
High-impedance principle
High-impedance principle is stable for all types of faults outside the zone of
protection. The stabilization is obtained by a stabilizing resistor in the differential
circuit. This method requires that all the CTs used have a similar magnetizing
characteristic, same ratio and relatively high knee point voltage. CTs on each
sides are connected in parallel along with a relay-measuring branch as shown in
Figure 434
protection relay.
762
s2 s1
VDR
Stabilizing
Resistor
. The measuring branch is a series connection of stabilizing resistor and
s1
s2
High impedance
protection
(HREFPDIF)
1MRS759142 F
REX640
Technical Manual