ABB RELION REX640 - Page 698

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 CT ratio Cor Wnd 1 and CT ration Cor Wnd 2
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
S
U
nT
n
n
=
×3
(Equation 165)
I
nT
rated load of the power transformer
S
n
rated power of the power transformer
U
n
rated phase-to-phase voltage
Next, the settings for the CT ratio correction can be calculated.
CT ratio correction =
I
I
n
nT
1
(Equation 166)
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
).
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
Protection functions
1MRS759142 F
698 REX640
Technical Manual