Siemens SIPROTEC 4 7SD80

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