ABB RELION 620 Series Technical Manual page 531

1MRS757644 F
620 series
Technical Manual
GUID-46FDF23A-7E78-4B17-A888-8501484AB57A V1 EN
Figure 268: Protection of the power transformer feeding the frequency converter
Transforming ratio correction of CTs
The CT secondary currents often differ from the rated current at the rated load of the
power transformer. The CT transforming ratios can be corrected on both sides of the
power transformer with the CT ratio Cor Wnd 1 and CT ration Cor Wnd 2 settings.
First, the rated load of the power transformer must be calculated on both sides when
the apparent power and phase-to-phase voltage are known.
S
n
I
=
nT
3 U
×
n
GUID-B5467DB8-17EB-4D09-A741-1F5BB23466AA 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.
Section 4
Protection functions
(Equation 76)
525