Siemens SIPROTEC 7SD80 - Current Transformer Requirements; Current Transformer Ratio; Overcurrent Factors

Appendix
A.4 Current Transformer Requirements
SIPROTEC, 7SD80, Manual
E50417-G1140-C474-A1, Release date 09.2011
348
A.4 Current Transformer Requirements
A.4.1 Current Transformer Ratio:
A.4.2 Overcurrent Factors
Maximum ratio between primary currents of the
current transformers at both ends of the protect-
ed object.
0.25 ≥ I
pn-local
/ I
pn-remote
≤ 4
Note:
The maximum ratio can also be selected greater
or smaller. It may be that the sensitivity of the dif-
ferential protection indicated in the technical
data are no longer observed.
I
pn-local
: Primary rated transformer current of the local device
I
pn-remote
: Primary rated transformer current of the remote
device
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)
I
SSC -max(ext. fault)
: maximum through-flowing
short-circuit current
I
pn
: Primary transformer rated current
R
CT
: 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
I
sn
= 1 A
I
pn
= 500 A
R
b
= 5 Ω = 5 VA / I
sn
2
R
CT
= 3 Ω
R’
b
: 0.6 Ω
I
SSC-max(ext. fault)
: 20 kA
88.9 > 48 and 88.9 > 30
transformer OK!
I
sn
: Secondary transformer rated current