ABB RED615 Technical Manual page 408

Section 4
Protection functions
402
I
1 n
CT ratio correction =
I
nT
GUID-F5F45645-C809-4F99-B783-751C8CC822BF V1 EN
I nominal primary current of the CT
1n
After the CT ratio correction, the measured currents and corresponding setting
values of TR2PTDF are expressed in multiples of the rated power transformer
current I (xI ) or percentage value of I
r r
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
GUID-DC9083B2-CB07-4F6B-8C06-52979E5F484A V1 EN
Figure 217: Example of two-winding power transformer differential protection
The rated load of the transformer is 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
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 earthed, any earth fault in the network is perceived by the IED as a
(%I ).
r r
1YHT530004D05 D
(Equation 52)
615 series
Technical Manual