ABB RELION REX640 Technical Manual page 699

1MRS759142 F
Figure 399: Example of two-winding power transformer differential
protection
The rated load of the transformer is calculated:
HV side: I
LV side: I
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"
Vector group matching and elimination of the zero-sequence component
The vector group of the power transformer is numerically matched on the high
voltage and low voltage sides by means of the
and
a power transformer inside the protected zone. The matching is based on phase
shifting and a numerical delta connection in the protection relay. If the neutral of
a star-connected power transformer is earthed, any earth fault in the network is
perceived by the protection relay as a differential current. The elimination of the
zero-sequence component can be selected for that winding by setting the
elimination parameter.
Table 655: TR2PTDF settings corresponding to the power transformer vector
groups and zero-sequence elimination
Vector group of
the transformer
Yy0
YNy0
YNyn0
Yyn0
Yy2
YNy2
Table continues on the next page
REX640
Technical Manual
= 25 MVA / (1.732 x 110 kV) = 131.2 A
nT_Wnd1
= 25 MVA / (1.732 x 21 kV) = 687.3 A
nT_Wnd2
Clock number settings. Thus no interposing CTs are needed if there is only
Winding 1 type
Y
YN
YN
Y
Y
YN
Winding 1 type , Winding 2 type
Winding 2 type Clock number
y Clk Num 0
y Clk Num 0
yn Clk Num 0
yn Clk Num 0
y Clk Num 2
y Clk Num 2
Protection functions
Zro A
Zro A
Elimination
Not needed
HV side
HV & LV side
LV side
Not needed
Not needed
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