Siemens 7SK80 - Class Conversion; Cable Core Balance Current Transformer; General; Requirements

Appendix
A.4 Current Transformer Requirements
SIPROTEC, 7SK80, Manual
E50417-G1140-C344-A5, Release date 11.2012
443
A.4.2 Class conversion
Table A-1 Conversion into other classes
A.4.3 Cable core balance current transformer
General
The requirements to the cable core balance current transformer 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
sNom
= 5 A (typical value)
IEC 60044-6 (transient response), class
TPS
Classes TPX, TPY, TPZ
K≈ 1
K
SSC
≈ K
ALF
Calculation See ChapterA.4.1 Accuracy limiting factors with: K
SSC
≈
K
ALF
T
P
depending on power system and specified closing sequence
with
V
k
Knee-point voltage
R
Ct
Internal burden resistance
R
BN
Nominal burden resistance
I
sNom
Secondary nominal transformer current
K
ALF
Rated accuracy limiting factor
V
s.t.max
Sec. terminal voltage at 20 I
pNom
V
al
Sec. magnetization limit voltage
K Dimensioning factor
K
SSC
Factor symmetr. Rated fault current
T
P
Primary time constant
Transformation ratio, typical
It may be necessary to select a different transformation ratio to suit the specific
power system and thus the amount of the maximum ground fault current.
60 / 1
Accuracy limiting factor FS = 10
Minimum power 1.2 VA
Maximum connected load
– For secondary current threshold values ≥ 20 mA
– For secondary current threshold values < 20 mA
≤ 1.2 VA (≤ 1.2 Ω)
≤ 0.4 VA (≤ 0.4 Ω)