ABB RELION REX640 - Page 649

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.
Figure 366: Example of CT ratio correction setting calculation in line differential
application
The settings for CT ratio Corrections 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.
I
S
U
nT
n
n
=
×3
(Equation 144)
I
nT
Rated load of the power transformer
S
n
Rated power of the power transformer
U
n
Rated phase-to-phase voltage
Next, the settings for the CT ratio correction can be calculated with the formula:
1MRS759142 F
Protection functions
REX640
Technical Manual
649