Appendix
A.4 Current Transformer Requirements
SIPROTEC, 7SJ62/64, Manual
C53000-G1140-C207-2, Release date 01.2008
612
A.4
Current Transformer Requirements
The requirements for phase current transformers are usually determined by the overcurrent time protection,
particularly by the high-current element settings. Besides, there is a minimum requirement based on experi-
ence.
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.
A.4.1
Accuracy limiting factors
Effective and Rated Accuracy Limiting Factor
Calculation example according to IEC 60044–1
Required minimum effective accuracy limiting
factor
but at least 20
with
K
ALF'
Minimum effective accuracy limiting factor
50-2
PU
Primary pickup value of the high-current
element
I
pNom
Primary nominal transformer current
Resulting rated accuracy limiting factor
with
K
ALF
Rated accuracy limiting factor
R
BC
Connected burden resistance (device and
cables)
R
BN
Nominal burden resistance
R
Ct
Transformer internal burden resistance
I
sNom
= 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
sNom
= secondary transformer nominal current
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