ABB RELION 620 Series Technical Manual page 448

Section 4
Protection functions
442
GUID-983C9055-4686-4D5F-960E-0270BA365017 V1 EN
Figure 206: Example of two-winding power transformer differential protection
The rated load of the transformer can be calculated.
HV side: I = 25 MVA / (1.732 x 110 kV) = 131.2 A
nT_Wnd1
LV side: I = 25 MVA / (1.732 x 21 kV) = 687.3 A
nT_Wnd2
87T settings:
CT ratio Cor Wnd 1 = 300 A / 131.2 A = 2.29
CT ratio Cor Wnd 2 = 1000 A / 687.3 A = 1.45
CT ratio Cor Wnd 3 is not applicable in this case
Vector group matching and elimination of the zero-sequence component
The vector group of the power transformer is numerically matched on the windings on
different sides of the transformer by the Phase shift Wnd 1-2 and Phase shift Wnd 1-3
settings. Thus no interposing CTs are needed if there is only a power transformer inside
the protected zone. The matching is based on phase shifting and a numerical delta
connection in the function.
If the neutral of a star-connected power transformer is grounded, any ground fault in the
network is perceived by the protection as differential current. The elimination of the zero-
sequence-component can be selected to be on or off for that winding by the Zro A
elimination setting. By default, the elimination is not done for any winding.
1MAC504801-IB E
620 series ANSI
Technical Manual