Figure 399: Example of two-winding power transformer differential
protection
The rated load of the transformer is calculated:
HV side: I
nT_Wnd1
= 25 MVA / (1.732 x 110 kV) = 131.2 A
LV side: I
nT_Wnd2
= 25 MVA / (1.732 x 21 kV) = 687.3 A
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
Winding 1 type
,
Winding 2 type
and
Clock number
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 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
Zro A
elimination
parameter.
Table 655: TR2PTDF settings corresponding to the power transformer vector
groups and zero-sequence elimination
Vector group of
the transformer
Winding 1 type Winding 2 type Clock number Zro A
Elimination
Yy0 Y y Clk Num 0 Not needed
YNy0 YN y Clk Num 0 HV side
YNyn0 YN yn Clk Num 0 HV & LV side
Yyn0 Y yn Clk Num 0 LV side
Yy2 Y y Clk Num 2 Not needed
YNy2 YN y Clk Num 2 Not needed
Table continues on the next page
1MRS759142 F Protection functions
REX640
Technical Manual
699
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