ABB RELION 620 Series Technical Manual page 532

Section 4
Protection functions
526
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
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-DC9083B2-CB07-4F6B-8C06-52979E5F484A V1 EN
Figure 269: 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
(%I ).
r r
1MRS757644 F
(Equation 77)
(xI )
r r
620 series
Technical Manual