ABB RELION REX640 Technical Manual page 730

Section 4
Protection functions
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Figure 402: Secondary waveform of a saturated CT
At an internal fault, the secondary circuit voltage can easily exceed the isolation
voltage of the CTs, connection wires and the protection relay. To limit this voltage, a
voltage-dependent resistor (VDR) is used.
The whole scheme, that is, the stabilizing resistor, voltage-dependent resistor and
wiring, must be adequately maintained (operation and insulation tested regularly) to
be able to withstand the high-voltage pulses that appear during an internal fault
throughout the lifetime of the equipment. Otherwise, during a fault within the zone of
protection, any flashover in the CT secondary circuits or any other part of the scheme
can prevent the correct operation of MHZPDIF.
Flux-balancing principle
In a measuring configuration for the three-phase differential currents according to the
flux-balancing principle, no stabilizing resistors are needed. The configuration,
however, requires the use of core balance current transformers. The compared
currents, the one at the line end and the other at the neutral end, are both measured by
the same core balance current transformer.
In this scheme, the currents flowing through one core balance transformer cancel each
other out when there is no fault within the protected zone. When a fault occurs within
the protected zone, the currents flowing through the core balance transformer amplify
each other and the differential protection operates.
1MRS759142 C
REX640
Technical Manual