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

Appendix
A.4 Current Transformer Requirements
A.4

Current Transformer Requirements

A.4.1

Current Transformer Ratio:

Maximum ratio between primary currents of the
current transformers at both ends of the protect-
ed object.
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.
A.4.2

Overcurrent Factors

Required minimum effective accuracy limiting
factor
Example:
Current transformer: 5P40, 5 VA
I = 1 A
sn
= 500 A
I
pn
= 5 Ω = 5 VA / I
R
b
= 3 Ω
R
CT
: 0.6 Ω
R'
b
: 20 kA
I
SSC-max(ext. fault)
: Secondary transformer rated current
I
sn
348
2
sn
0.25 ≥ I ≤ 4
/ I
pn-local pn-remote
I : Primary rated transformer current of the local device
pn-local
: Primary rated transformer current of the remote
I
pn-remote
device
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
I
SSC -max(ext. fault)
short-circuit current
: Primary transformer rated current
I
pn
R : Secondary direct current winding resistance of the
CT
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!
E50417-G1140-C474-A1, Release date 09.2011
= 20)
SSC
SIPROTEC, 7SD80, Manual