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 316: 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
highimpedance principle or stabilized three-phase differential protection
must be used.
Protection functions
1MRS757644 H
600
620 series
Technical Manual
