ABB Relion 620 Series - Page 536

Figure 278: 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.
I
S
U
nT
n
n
=
×3
(Equation 77)
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.
CT ratio correction =
I
I
n
nT
1
(Equation 78)
I
1n
nominal primary current of the CT
Protection functions 1MRS757644 H
536 620 series
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