ABB RELION 620 Series Technical Manual page 598

Section 4
Protection functions
592
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.
GUID-0D639B1E-54D9-45A9-8B44-BCF3FA5E01D1 V1 EN
Figure 306: Three-phase differential protection for motors based on flux-
balancing 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-
impedance principle or stabilized three-phase differential protection
must be used.
1MRS757644 F
620 series
Technical Manual