A Appendix
504
7UM62 Manual
C53000-G1176-C149-3
A.7
Current Transformer Requirements
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.
with
K
td
Rated transient dimensioning factor
I
pSSC
Primary short-circuit current
I
pN
Rated primary ct current
R
BC
Connected burden resistance
R
BN
Rated burden resistance
R
Ct
Secondary winding resistance
Table A-3
Overcurrent Factors
Required Actual Accuracy
Limiting Factor
Resulting Rated Accuracy
Limiting Factor
Table A-4
Transformer Requirements
Transformer
Generator
Transient
dimensioning factor K
td
> 4
τ
N
≤
100 ms
> (4 to 5)
τ
N
>
100 ms
K
AL F
'
K
td
I
pSSC
I
pN
---------------
⋅
=
K
ALF
R
BC
R
C t
+
R
BN
R
C t
+
---------------------------
K
AL F
'
⋅
=
