ABB RELION REX640 Technical Manual page 652

Section 4
Protection functions
646
In three-winding transformer applications, currents from winding 3 are always
connected to the third three-phase current input set of the protection relay (Current
group 3 type to be set as "Winding 3").
In the normal two-winding transformer application with one set of three-phase
currents on both sides of the transformer, the Current group 3 type setting must be set
as "Not in use".
TR3PTDF
3dI>T
a)
GUID-CDF7C466-156F-40C2-BD52-FD3050ACE93F V1 EN
Figure 359: 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.
S
n
I
=
nT
U
⋅ 3
n
GUID-A7C2CC6B-30D9-4A41-A14E-B046B761A0B5 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
Two-winding transformer with one set of currents on both sides of the
transformer
The rated power of the transformer is 25 MVA, the ratio of the CTs on the 110 kV side
is 300/1 and that on the 21 kV side is 1000/1.
TR3PTDF
3dI>T
b)
1MRS759142 C
TR3PTDF
3dI>T
c)
(Equation 119)
REX640
Technical Manual