Current Transformer Requirements - Siemens 7SJ82 Manual

A.6

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
Required minimum effective accu-
racy limiting factor
Resulting rated accuracy limiting
factor
Table A-3
I = 1 A
rated,sec
K = 20
ALF'
R = 0.6 Ω (device and cables)
BC
R = 3 Ω
Ct
R = 5 Ω (5 VA)
BN
with
I = Secondary rated transformer current
rated,sec
Class Conversion
Table A-4
British Standard BS 3938
ANSI/IEEE C 57.13, class C
SIPROTEC 5, Overcurrent Protection, Manual
C53000-G5040-C017-8, Edition 07.2017
Effective and Rated Accuracy Limiting Factor
but at least 20
with
K
ALF'
I>>
PU
I
rated,prim
with
K
ALF
R
BC
R
BN
R
Ct
Calculation Example According to IEC 60044–1
Conversion into Other Classes
I
rated,sec
Minimum effective accuracy limiting factor
Primary pickup value of the high-current
element
Primary rated transformer current
Rated accuracy limiting factor
Connected burden (device and cables)
Rated burden
Internal burden
K set to 10,
ALF
so that: 5P10, 5 VA
= 5 A (typical value)
Appendix
A.6 Current Transformer Requirements
1591