ABB RELION REX640 Technical Manual page 566

Section 4
Protection functions
560
calculation algorithms. The CT transformation ratio may be different and this needs to
be compensated to provide a correct differential current calculation result on both
ends.
The operation characteristics related settings are given in units as percentage of the
current transformer secondary nominal current on each line end protection relay. For
the actual primary setting, the corresponding CT ratio on each line end has to be
considered. An example of how the CT ratio correction parameter values should be
selected on both line ends in the example case to compensate the difference in the
nominal levels can be presented.
Another example for differential application without in-zone transformer where line
rated current is 400 A. The ratio of CTs are 800/1 and 400/1.
GUID-F3D20F08-5760-4BF7-B72B-48A61A5B9F83 V1 EN
Figure 307: Example of CT ratio correction setting calculation in line differential
application
The settings for CT ratio correction for protection relay A and protection relay (B) are:
CT ratio correction (A) = 800 A / 400 A = 2.000
CT ratio correction (B) = 400 A / 400 A = 1.000
The CT secondary current often differs from the rated current at the rated load of the
power transformer. The CT transforming ratio can be corrected on both sides of the
power transformer with the CT ratio correction setting.
First, the rated load of the power transformer is calculated on both sides when the
apparent power and phase-to-phase voltage are known.
S
n
I
=
nT
⋅ 3 U
n
GUID-BF85B8C5-A12B-4BDF-856E-E18C851605A6 V1 EN
1MRS759142 C
(Equation 90)
REX640
Technical Manual