ABB RELION REX640 Technical Manual page 567

1MRS759142 C
REX640
Technical Manual
I Rated load of the power transformer
nT
S Rated power of the power transformer
n
U Rated phase-to-phase voltage
n
Next, the settings for the CT ratio correction can be calculated with the formula:
I
n
CT ratio correction
=
I
nT
GUID-10C49B76-7D13-486B-A65F-8A03655A5048 V1 EN
I Nominal primary current of the CT
n
After the CT ratio correction, the measured currents and corresponding setting values
of LNPLDF are expressed in multiples of the rated power transformer current Ir (×Ir)
or percentage value of Ir (%Ir).
An example shows how the CT ratio correction settings are calculated; when the rated
power of the transformer is 5 MVA, the ratio of CTs on the 20 kV side is 200/1 and that
on the 10.5 kV side is 300/1.
GUID-E6A9C133-B99C-4CEA-AC81-FCFDB626439F V1 EN
Figure 308: Example of CT ratio correction setting calculation with in-zone
transformer
The rated load of the transformer is:
HV side (A): I = 5 MVA / (1.732 × 20 kV) = 144.3 A
nT_A
LV side (A): I = 5 MVA / (1.732 × 10.5 kV) = 274.9 A
nT_B
So the settings for CT ratio correction at HV (A) and LV (B) side are:
CT ratio correction (A) = 200 A / 144.3 A = 1.386
Section 4
Protection functions
(Equation 91)
561