ABB RED615 Technical Manual page 357

1YHT530004D05 D
615 series
Technical Manual
S
n
I =
nT
3 × U
n
GUID-B2130C43-E82B-4617-9FC4-79AB2F735CD9 V1 EN
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:
CT ratio correction
=
GUID-F950DE9C-6AFB-4DF9-B299-6631693C55C0 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-74289FF5-E90D-4633-9FEE-04DB8E86A3AE V1 EN
Figure 184: 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
I
n
I
nT
Section 4
Protection functions
(Equation 31)
(Equation 32)
351