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A Appendix
A.7
Current Transformer Requirements
504
The differential protection is of decisive importance for the requirements that the cur-
rent transformers must meet. The highspeed trip stage (IDiff >>) uses instantaneous
values and can therefore reliably trip high-current internal short-circuits.
The other decisive factor for the requirements that the current transformers must meet
are external short-circuits with their possible DC component. In case of a short-circuit
flowing through the power transformers, a minimum of 5 ms should elapse before cur-
rent transformer saturation occurs. Recommendations for dimensioning are given in
Tables A-3 and A-4 on the basis of the IEC 60044-1 and 60044-6 standards. Table A-
5 shows the necessary equations for converting the requirements into the knee-point
voltages.
The primary nominal CT current is defined according to the rules of good engineering
practice. It must be chosen higher than or equal to the nominal current of the protected
object.
Table A-3
Overcurrent Factors
Required Actual Accuracy
Limiting Factor
I
pSSC
⋅
K
K
---------------
'
=
ALF
td
I
pN
with
K
Rated transient dimensioning factor
td
I
Primary short-circuit current
pSSC
Rated primary ct current
I
pN
R
Connected burden resistance
BC
R
Rated burden resistance
BN
R
Secondary winding resistance
Ct
Table A-4
Transformer Requirements
Transient
dimensioning factor K
td
Resulting Rated Accuracy
Limiting Factor
R
R
+
BC
K
-------------------------- - K
=
ALF
R
+
R
BN
Transformer
Generator
> 4
> (4 to 5)
τ
≤ 100 ms
τ
N
N
Ct
⋅
'
ALF
Ct
> 100 ms
7UM62 Manual
C53000-G1176-C149-3
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