Siemens 7SJ85 - Current Transformer Requirements

Current Transformer Requirements
The requirements for phase current transformers are usually determined by the overcurrent protection, partic-
ularly by the high-current element settings. Besides, there is a minimum requirement based on experience.
The recommendations are given according to the standard IEC 60044-1.
The standards IEC 60044-6, BS 3938 and ANSI/IEEE C 57.13 are referred to for converting the requirement into
the knee-point voltage and other transformer classes.
Accuracy Limiting Factors
Table A-2 Effective and Rated Accuracy Limiting Factor
Required minimum effective accu-
racy limiting factor
but at least 20
with
K
ALF'
Minimum effective accuracy limiting factor
I>>
PU
Primary pickup value of the high-current
element
I
rated,prim
Primary rated transformer current
Resulting rated accuracy limiting
factor
with
K
ALF
Rated accuracy limiting factor
R
BC
Connected burden (device and cables)
R
BN
Rated burden
R
Ct
Internal burden
Table A-3 Calculation Example According to IEC 60044–1
I
rated,sec
= 1 A
K
ALF'
= 20
R
BC
= 0.6 Ω (device and cables)
R
Ct
= 3 Ω
R
BN
= 5 Ω (5 VA)
K
ALF
set to 10,
so that: 5P10, 5 VA
with
I
rated,sec
= Secondary rated transformer current
Class Conversion
Table A-4
Conversion into Other Classes
British Standard BS 3938
ANSI/IEEE C 57.13, class C
I
rated,sec
= 5 A (typical value)
A.6
Appendix
A.6 Current Transformer Requirements
SIPROTEC 5, Overcurrent Protection, Manual 1591
C53000-G5040-C017-8, Edition 07.2017