ABB RELION 620 Series Technical Manual page 447

1MAC504801-IB E
620 series ANSI
Technical Manual
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".
87T3
3dI>T
a)
GUID-4325CA89-B94D-4637-AB0F-98D515BB16BA V1 EN
Figure 205: Differential protection of 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
3 × U
n
GUID-01BA92BE-6C25-46FD-934D-5AD76DC7ACC1 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.
Protection functions
87T3
3dI>T
b)
Section 4
87T3
3dI>T
c)
(Equation 48)
441