ABB RELION REX640 Technical Manual page 734

Protection functions
Figure 418: Differential protection of a three-winding transformer, transformer with
two output feeders and transformer with two input feeders
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 all sides of
the power transformer with the CT ratio correction settings.
I
=
nT
I
nT
S
n
U
n
Three-winding transformer with one set of currents on each side of the
transformer
100 MVA YNyn0d1 power transformer 154 kV/14.4 kV/4.37 kV. The
corresponding rated currents are 375 A, 3007 A and 3303 A. At the HV side,
the CT is 400 A, and on the MV sides both are 4000 A.
734
TR3PTDF
3dI>T
a)
S
n
⋅ 3 U
n
Rated load of the power transformer
Rated power of the power transformer
Rated phase-to-phase voltage
TR3PTDF
3dI>T
b)
1MRS759142 F
TR3PTDF
3dI>T
c)
(Equation 172)
REX640
Technical Manual