Current Transformer Requirements; Accuracy Limiting Factors - Siemens siprotec SJ62 User Manual

A Appendix
A.4

Current Transformer Requirements

A.4.1

Accuracy limiting factors

Effective and Rated
Accuracy Limiting
Factor
Calculation
example according
to IEC 60044–1
627
The requirements for phase current transformers are usually determined by the over-
current time protection, particularly 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 con-
verting the requirement into the knee-point voltage and other transformer classes.
Required minimum effective accu-
racy limiting factor
Resulting rated accuracy limiting
factor
I = 1 A
sNom
K = 20
ALF'
= 0.6 Ω (device and cables)
R
BC
= 3 Ω
R
Ct
= 5 Ω (5 VA)
R
BN
with
= secondary transformer nominal current
I
sNom
but at least 20
with
K Minimum effective accuracy limiting
ALF'
factor
50-2 Primary pickup value of the high-current
PU
element
I Primary nominal transformer current
pNom
with
K Rated accuracy limiting factor
ALF
R Connected burden resistance
BC
(device and cables)
R Nominal burden resistance
BN
R Transformer internal burden resistance
Ct
K set to 10,
ALF
so that: 5P10, 5 VA
SIPROTEC 4, 7SJ62/63/64 Handbuch
C53000-G1140-C147-A, Edition 07.2015