ABB RELION REX640 Technical Manual page 681

1MRS759142 C
REX640
Technical Manual
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GUID-5E9C7D3D-656B-4736-BCFE-F4AD96F2B805 V1 EN
Figure 374: 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
The measuring branch is a series connection of stabilizing resistor and protection
relay.
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VDR
Stabilizing
Resistor
High impedance
protection
(HREFPDIF)
Section 4
Protection functions
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Figure 375.
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