CT ratio correction
I
I
n
nT
=
(Equation 145)
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.
Figure 367: 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
LV side (A): I
nT_B
= 5 MVA / (1.732 × 10.5 kV) = 274.9 A
So the settings for
CT ratio corrections
at HV (A) and LV (B) side are:
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
CT connection type
setting parameter is “Type 1”. The
connection examples of “Type 1” are as shown in the
Figure 368
and
Figure 369
.
• If the positive directions of the winding 1 and winding 2 protection relay currents
equate, the
CT connection type
setting parameter is “Type 2”. The connection
examples of “Type 2” are as shown in the
Figure 370
and
Figure 371
.
• The default value of the
CT connection type
setting is “Type 1”.
Protection functions
1MRS759142 F
650 REX640
Technical Manual
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