ABB RELION REX640 Technical Manual page 650

Protection functions
CT ratio correction
I
n
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.
Figure 367: Example of CT ratio correction setting calculation with in-zone
transformer
The rated load of the transformer is:
HV side (A): I
LV side (A): I
So the settings for
CT ratio correction (A) = 200 A / 144.3 A = 1.386
CT ratio correction (B) = 300 A / 274.9 A = 1.091
CT connections
The connections of the primary current transformers are designated as "Type 1" and
"Type 2".
• If the positive directions of the winding 1 and winding 2 protection relay
currents are opposite, the
connection examples of "Type 1" are as shown in the
• If the positive directions of the winding 1 and winding 2 protection relay currents
equate, the
examples of "Type 2" are as shown in the
• The default value of the
650
I
n
=
I
nT
Nominal primary current of the CT
= 5 MVA / (1.732 × 20 kV) = 144.3 A
nT_A
= 5 MVA / (1.732 × 10.5 kV) = 274.9 A
nT_B
CT ratio corrections at HV (A) and LV (B) side are:
CT connection type setting parameter is "Type 1". The
CT connection type setting parameter is "Type 2". The connection
CT connection type setting is "Type 1".
Figure 368
Figure 370 Figure 371
and
1MRS759142 F
(Equation 145)
Figure 369
and .
.
REX640
Technical Manual