ABB RELION REX640, Техническое руководство, Страница: 809 / 1960

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.
Figure 462: Three-phase differential protection for motors based on fluxbalancing
principle
The advantage of this scheme is that the CT rated primary current can be selected
smaller than the rated current of the machine.
If six current transformers are used, the flux-balancing principle, that
is, summing two CTs in each phase, cannot be used. Instead, the high-
1MRS759142 F Protection functions
REX640
Technical Manual
809