Functions
2.14 Differential Protection and Its Protected Objects
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
126
2.14.4 Current Transformer Requirements
The differential protection is of decisive importance for the requirements that the current transformers must
meet. The high-speed trip stage (IDiff >>) uses instantaneous values and can therefore reliably trip high-current
internal short-circuits. For determination of the primary CT rated current, in practice the general usual proce-
dures are used. It must be selected equal to or greater than the rated current of the protected object.
2.14.4.1 Functional Description
Design recommendations
The external short circuit determines the requirements that the current transformers must meet because of the
possible DC current component. In case of a short-circuit flowing through the power transformers, a minimum
of 5 ms should elapse before current transformer saturation occurs. The two following tables show the design
specifications. The IEC 60044-1 and IEC 60044-6 standards were used in these tables. The equations for cal-
culating the requirements as knee-point voltages are listed in table 2-7.
Table 2-5 Overcurrent Factors
with
K
td
Transient dimensioning factor
I
pSSC
Primary symmetric short circuit current
I
pN
Primary CT rated current
R
BC
Connected load
R
BN
Rated load
R
Ct
Internal load
Required operational overcurrent factor Resulting rated overcurrent factor
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