ABB 615 series Technical Manual page 525

1MAC059074-MB A
615 series ANSI
Technical Manual
Next, the settings for the CT ratio correction can be calculated.
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
87T are expressed in multiples of the rated power transformer current I
value of I (%I ).
r r
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-ANSI V1 EN
Figure 260: 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"
Section 4
Protection functions
(Equation 90)
(xI ) or percentage
r r
519