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Appendix
A
A - 11
Siemens AG ⋅ May 1998
A.1.3 Basis for selection of the stabilization factor k
Step 1:
The relationship between the point (in time) at which
saturation occurs and the stabilisation factor is estabĆ
lished.
The following condition must be fulfilled in order to
prevent bus zone tripping for an external fault
(refer fig A.9.1)
2
K
sin wT
S
.
K>
K
(c1)
2 sin wT
S
.
K > 1 (c2)
K
T
S
Figure A.9.1 Short-circuit on a feeder with CT saturation
To determine the point in time T
s
at which saturation
occurs, the burden factor u must be considered.
T
S
sat
K
=u
sat
A B
Figure A.9.2 Current/time characteristic
If the CT is burdened with
K
=u
sat
, then it will satuĆ
rate after time T
S
whereby area A
(c3)
u (1-cos wT
S
) = 2 (c4)
If equations c2 and c4 are combined, then:
K > U/4
Uć1
Ǹ
(c5),
The stabilization factor must be chosen according to
condition c5.
i.Ă e.
Ă
wTs
ŕ
0
K
ĂsinĂwtĂ +Ă
p
ŕ
ĂĂĂ
0
sat
ĂsinĂwtĂ
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