Appendix
A.4 Current Transformer Requirements
SIPROTEC, 7SD80, Manual
E50417-G1140-C474-A1, Release date 09.2011
349
A.4.3 Class Conversion
Table A-1 Conversion into other classes
Note
Detailed information on the transformer design is available on the Internet. (www.siprotec.de)
A.4.4 Core Balance Current Transformer
General
The requirements for core balance current transformers are determined by the function „sensitive ground fault
detection“.
The recommendations are given according to the standard IEC 60044-1.
Requirements
British Standard BS 3938
ANSI/IEEE C 57.13, Class C
I
sn
= 5 A (typical value)
with
V
k
Knee-point voltage
R
Ct
Internal burden
R
B
Rated burden
I
sn
Secondary rated transformer current
K
SSC
Rated overcurrent factor
V
s.t.max
sec. terminal voltage at 20 I
pn
Maximum difference of the primary rated currents of the core balance current
transformer:
0.33 ≥ I
pn-local
/ I
pn-remote
≤ 3
Select the primary rated current of the core balance current transformer so that
ground current flowing in the ground fault of the subnetwork is transmitted by the
core balance current transformer.
Transmission ratio, typical
Depending on the specific power system and the magnitude of the maximum
ground fault current, a different ratio may have to be selected.
60 / 1
Overcurrent limiting factor FS = 10
Minimum power 1.2 VA
Maximum connected burden
– For secondary current thresholds ≥ 20 mA
– For secondary current thresholds < 20 mA
≤ 1.2 VA (≤ 1.2 Ω)
≤ 0.4 VA (≤ 0.4 Ω)
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