GUID-B2130C43-E82B-4617-9FC4-79AB2F735CD9 V1 EN (Equation 31)
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 with the formula:
CT ratio correction
I
I
n
nT
=
GUID-F950DE9C-6AFB-4DF9-B299-6631693C55C0 V1 EN (Equation 32)
I
n
nominal primary current of the CT
After the CT ratio correction, the measured currents and corresponding setting
values of LNPLDF 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.
GUID-74289FF5-E90D-4633-9FEE-04DB8E86A3AE V1 EN
Figure 184: Example of CT ratio correction setting calculation with in-zone
transformer
The rated load of the transformer is:
HV side (A): I
nT_A
= 5 MVA / (1.732 × 20 kV) = 144.3 A
1YHT530004D05 D Section 4
Protection functions
615 series 351
Technical Manual
To Next Page
Loading...
