GUID-6014FAFC-12CB-4DB3-85A9-0EF254D1729D V2 EN (Equation 67)
Depending on the location of the star points of the current transformers, the polarity of the
local end remote currents may be different causing malfunction of the calculation
algorithms. The CT transformation ratio may be different and this needs to be
compensated to provide a correct differential current calculation result on both ends.
The operation characteristics related settings are given in units as percentage of the current
transformer secondary nominal current on each line end protection relay. For the actual
primary setting, the corresponding CT ratio on each line end has to be considered. An
example of how the CT ratio correction parameter values should be selected on both line
ends in the example case to compensate the difference in the nominal levels can be
presented.
Another example for differential application without in-zone transformer where line rated
current is 400 A. The ratio of CTs are 800/1 and 400/1.
800A/1
400A/1
P1
S1
P2
S2
P2
S2
P1
S1
Rated current (primary)
400A
(A) (B)
CT Ratio Correction
800A / 400A = 2.000
CT connection type
= Type 1
CT Ratio Correction
400A / 400A = 1.000
CT connection type
= Type 1
N IC IB IA IA IB IC N
GUID-4BCD4742-1DBC-4CAC-B409-7A603C4669B2 V1 EN
Figure 226: Example of CT ratio correction setting calculation in line differential
application
The settings for CT ratio Corrections for protection relay A and protection relay (B) are:
CT ratio Correction (A) = 800 A / 400 A = 2.000
CT ratio Correction (B) = 400 A / 400 A = 1.000
The CT secondary current often differs from the rated current at the rated load of the power
transformer. The CT transforming ratio can be corrected on both sides of the power
transformer with the CT ratio Correction setting.
Section 4 1MAC059074-MB A
Protection functions
466 615 series ANSI
Technical Manual
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