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Appendix
A.4 Current Transformer Requirements
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
Required minimum effective accuracy limiting
factor
Resulting rated accuracy limiting factor
Calculation example according to IEC 60044–1
= 1 A
I
sNom
K
= 20
ALF'
= 0.6 Ω (device and cables)
R
BC
= 3 Ω
R
Ct
= 5 Ω (5 VA)
R
BN
with
I
= secondary transformer nominal current
sNom
442
but at least 20
with
K
Minimum effective accuracy limiting factor
ALF'
Primary pickup value of the high-current
50-2
PU
element
I
Primary nominal transformer current
pNom
with
K
Rated accuracy limiting factor
ALF
R
Connected burden resistance (device and
BC
cables)
R
Nominal burden resistance
BN
R
Transformer internal burden resistance
Ct
K
set to 10,
ALF
so that: 5P10, 5 VA
E50417-G1140-C344-A5, Release date 11.2012
SIPROTEC, 7SK80, Manual
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