ABB RELION REX640 Technical Manual page 698

Protection functions
Transforming ratio correction of CTs
The CT secondary currents often differ from the rated current at the rated load of
the power transformer. The CT transforming ratios can be corrected on both sides
of the power transformer with the
settings.
First, the rated load of the power transformer must be calculated on both sides
when the apparent power and phase-to-phase voltage are known.
I
nT
I
nT
S
n
U
n
Next, the settings for the CT ratio correction can be calculated.
CT ratio correction =
I
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
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
698
S
n
=
3 U
×
n
rated load of the power transformer
rated power of the power transformer
rated phase-to-phase voltage
I
1 n
I
nT
nominal primary current of the CT
(xI ) or percentage value of I
r r
CT ratio Cor Wnd 1 and CT ration Cor Wnd 2
(%I ).
r r
1MRS759142 F
(Equation 165)
(Equation 166)
REX640
Technical Manual