ABB 615 Series ANSI, Technical Manual

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1MAC050144-MB C
Section 4
Protection functions
615 series ANSI 355
Technical Manual
Example
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.
Figure 172: : 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 IED. If the neutral of a star-connected power transformer is grounded,
any ground fault in the network is perceived by the IED as a differential current. The
elimination of the zero-sequence component can be selected for that winding by setting the
Zro A elimination parameter