ABB RELION REX640 Technical Manual page 616

Section 4
Protection functions
610
I
1 n
CT ratio correction =
I
nT
GUID-98F577FA-67F5-4EA6-8211-D9D9745896B7 V1 EN
I nominal primary current of the CT
1n
Next, the settings for the CT ratio correction can be calculated.
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
or percentage value of I
The rated input current (1A or 5A) of the relay does not have to be same for the HV
and the LV side. For example, the rated secondary current of 5 A can be used on the
HV side, while 1A is used on the LV side or vice versa.
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-3C6C42B3-B3A9-46F8-8D53-3043C12AAB35 V1 EN
Figure 340: 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"
(%I ).
r r
1MRS759142 C
(Equation 113)
(xI )
r r
REX640
Technical Manual