Recommendations For Current Transformers - ABB RELION REX640 Technical Manual

1MRS759142 C
4.3.8.7
REX640
Technical Manual
GUID-FB11FEE8-6FFD-4539-BA9F-56695DFB82E5 V1 EN
Figure 403: 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.

Recommendations for current transformers

High-impedance principle
The sensitivity and reliability of the protection depend on the characteristics of the
current transformers. The CTs must have an identical transformation ratio. It is
recommended that all current transformers have an equal burden and characteristics
and that they are of the same type. This means that they should be preferably from the
same manufacturing batch, that is, an identical construction is used. If the CT
characteristics and the burden values are not equal, the calculation for each branch in
the scheme should be done separately and the worst-case result is used. If the CT
winding resistance and the burden of the branches are not equal, the maximum burden
equal to 3.2 Ω should be used for calculating the stabilized voltage.
Section 4
Protection functions
725