ABB 615 series Technical Manual page 473

1MAC059074-MB A
615 series ANSI
Technical Manual
First, the rated load of the power transformer is calculated on both sides when the apparent
power and phase-to-phase voltage are known.
S
n
I
=
nT
⋅ 3 V
n
GUID-FD7DAEE6-80AB-4C5E-98C7-863063099802 V1 EN
I rated load of the power transformer
nT
S rated power of the power transformer
n
V rated phase-to-phase voltage
n
Next, the settings for the CT ratio correction can be calculated with the formula:
CT ratio correction
=
GUID-F950DE9C-6AFB-4DF9-B299-6631693C55C0 V1 EN
I nominal primary current of the CT
n
After the CT ratio correction, the measured currents and corresponding setting values of
87L are expressed in multiples of the rated power transformer current Ir (×Ir) or
percentage value of Ir (%Ir).
An example shows how the CT ratio correction settings are calculated; when the rated
power of the transformer is 5 MVA, the ratio of CTs on the 20 kV side is 200/1 and that
on the 10.5 kV side is 300/1.
I
n
I
nT
Section 4
Protection functions
(Equation 68)
(Equation 69)
467