CT ratio correction =
I
I
n
nT
1
GUID-F5F45645-C809-4F99-B783-751C8CC822BF V1 EN (Equation 52)
I
1n
nominal primary current of the CT
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
r
(xI
r
) or percentage value of I
r
(%I
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
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 earthed, any earth fault in the network is perceived by the IED as a
Section 4 1YHT530004D05 D
Protection functions
402 615 series
Technical Manual
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