ABB RED615 Technical Manual page 356

Section 4
Protection functions
350
I − I
LOC REM
=
I
b
2
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Depending on the location of the star points of the current transformers, the
polarity of the local end remote currents may be different causing malfunction of
the 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 IED. 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-E9087A8C-1F10-45E0-A47C-754C30CB801C V1 EN
Figure 183: Example of CT ratio correction setting calculation in line differential
application
The settings for CT ratio Corrections for IED A and IED (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.
1YHT530004D05 D
(Equation 30)
615 series
Technical Manual