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D.2
Overcurrent Factors
Required minimum effective accuracy limiting
factor
Example:
Current transformer : 5P40, 5 VA
= 1 A
Ι
sn
= 500 A
Ι
pn
R
= 5 Ω = 5 VA / Ι
b
R
= 3 Ω
CT
R'
: 0,6 Ω
b
: 20 kA
Ι
SSC-max(ext. fault)
: Secondary transformer rated current
Ι
sn
SIPROTEC 4, 7SD80, Manual
E50417-G1100-C474-A2, Edition 02.2018
2
sn
and K'
= 30
SSC min
K'
: Effective symmetrical short-circuit current factor
SSC
K
: Specified symmetrical short-circuit current factor
SSC
(example: Current transformer 5P20: K
: maximum through-flowing
Ι
SSC -max(ext. Fehler)
short-circuit current
: Primary transformer rated current
Ι
pn
R
: Secondärer Secondary direct current winding resistance
CT
of the current transformer at 75 °C
R
: Rated burden resistance of the current transformer
b
R'
: Connected burden resistance at the current transformer
b
(usually resistance of the current transformer feed cable +
burden resistance of the protection device)
88,9 > 48 and 88.9 > 30
transformer OK !
Current Transformer Requirements
D.2 Overcurrent Factors
= 20)
SSC
319
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